NAME
sh
—
public domain Bourne shell
SYNOPSIS
sh |
[-+abCefhiklmnpruvXx ]
[-+o option]
[-c string | -s | file [argument ...]] |
DESCRIPTION
sh
is a reimplementation of the Bourne
shell, a command interpreter for both interactive and script use.
The options are as follows:
-c
stringsh
will execute the command(s) contained in string.-i
- Interactive shell. A shell is “interactive” if this option
is used or if both standard input and standard error are attached to a
tty(4). An interactive shell has job control enabled, ignores the
SIGINT
,SIGQUIT
, andSIGTERM
signals, and prints prompts before reading input (see thePS1
andPS2
parameters). For non-interactive shells, thetrackall
option is on by default (see theset
command below). -l
- Login shell. If the basename the shell is called with (i.e. argv[0])
starts with ‘
-
’ or if this option is used, the shell is assumed to be a login shell and the shell reads and executes the contents of /etc/profile and $HOME/.profile if they exist and are readable. -p
- Privileged shell. A shell is “privileged” if this option is
used or if the real user ID or group ID does not match the effective user
ID or group ID (see
getuid(2) and
getgid(2)). A privileged shell does not process
$HOME/.profile nor the
ENV
parameter (see below). Instead, the file /etc/suid_profile is processed. Clearing the privileged option causes the shell to set its effective user ID (group ID) to its real user ID (group ID). -r
- Restricted shell. A shell is “restricted” if this option is
used; if the basename the shell was invoked with was “rsh”;
or if the
SHELL
parameter is set to “rsh”. The following restrictions come into effect after the shell processes any profile andENV
files:- The
cd
command is disabled. - The
SHELL
,ENV
, andPATH
parameters cannot be changed. - Command names can't be specified with absolute or relative paths.
- The
-p
option of the built-in commandcommand
can't be used. - Redirections that create files can't be used (i.e.
‘
>
’, ‘>|
’, ‘>>
’, ‘<>
’).
- The
-s
- The shell reads commands from standard input; all non-option arguments are positional parameters.
In addition to the above, the options described in the
set
built-in command can also be used on the command
line: both [-+abCefhkmnuvXx
] and
[-+o
option] can be used for
single letter or long options, respectively.
If neither the -c
nor the
-s
option is specified, the first non-option
argument specifies the name of a file the shell reads commands from. If
there are no non-option arguments, the shell reads commands from the
standard input. The name of the shell (i.e. the contents of $0) is
determined as follows: if the -c
option is used and
there is a non-option argument, it is used as the name; if commands are
being read from a file, the file is used as the name; otherwise, the
basename the shell was called with (i.e. argv[0]) is used.
If the ENV
parameter is set when an
interactive shell starts (or, in the case of login shells, after any
profiles are processed), its value is subjected to parameter, command,
arithmetic, and tilde (‘~’) substitution and the resulting
file (if any) is read and executed.
The exit status of the shell is 127 if the command file specified on the command line could not be opened, or non-zero if a fatal syntax error occurred during the execution of a script. In the absence of fatal errors, the exit status is that of the last command executed, or zero, if no command is executed.
Command syntax
The shell begins parsing its input by breaking it into
words.
Words, which are sequences of characters, are delimited by unquoted
whitespace characters (space, tab, and newline) or meta-characters
(‘<
’,
‘>
’,
‘|
’,
‘;
’,
‘(
’,
‘)
’, and
‘&
’). Aside from delimiting words,
spaces and tabs are ignored, while newlines usually delimit commands. The
meta-characters are used in building the following
tokens:
‘<
’,
‘<&
’,
‘<<
’,
‘>
’,
‘>&
’,
‘>>
’, etc. are used to specify
redirections (see
Input/output redirection
below); ‘|
’ is used to create
pipelines; ‘;
’ is used to separate
commands; ‘&
’ is used to create
asynchronous pipelines; ‘&&
’
and ‘||
’ are used to specify
conditional execution; ‘;;
’ is used in
case
statements; and lastly,
‘( .. )
’ is used to create
subshells.
Whitespace and meta-characters can be quoted individually using a
backslash (‘\’), or in groups using double
(‘"’) or single (‘'’) quotes. Note that the
following characters are also treated specially by the shell and must be
quoted if they are to represent themselves:
‘\
’,
‘"
’,
‘'
’,
‘#
’,
‘$
’,
‘`
’,
‘~
’,
‘{
’,
‘}
’,
‘*
’,
‘?
’, and
‘[
’. The first three of these are the
above mentioned quoting characters (see
Quoting below);
‘#
’, if used at the beginning of a
word, introduces a comment — everything after the
‘#
’ up to the nearest newline is
ignored; ‘$
’ is used to introduce
parameter, command, and arithmetic substitutions (see
Substitution below);
‘`
’ introduces an old-style command
substitution (see Substitution
below); ‘~
’ begins a directory
expansion (see Tilde expansion
below); and finally, ‘*
’,
‘?
’, and
‘[
’ are used in file name generation
(see File name patterns
below).
As words and tokens are parsed, the shell
builds commands, of which there are two basic types:
simple-commands,
typically programs that are executed, and
compound-commands,
such as for
and if
statements, grouping constructs, and function definitions.
A simple-command consists of some combination of parameter
assignments (see Parameters below),
input/output redirections (see
Input/output
redirections below), and command words; the only restriction is that
parameter assignments come before any command words. The command words, if
any, define the command that is to be executed and its arguments. The
command may be a shell built-in command, a function, or an external command
(i.e. a separate executable file that is located using the
PATH
parameter; see
Command execution below). Note
that all command constructs have an exit status: for external commands, this
is related to the status returned by
wait(2) (if the command could not be found, the exit status is 127;
if it could not be executed, the exit status is 126); the exit status of
other command constructs (built-in commands, functions, compound-commands,
pipelines, lists, etc.) are all well-defined and are described where the
construct is described. The exit status of a command consisting only of
parameter assignments is that of the last command substitution performed
during the parameter assignment or 0 if there were no command
substitutions.
Commands can be chained together using the
‘|
’ token to form pipelines, in which
the standard output of each command but the last is piped (see
pipe(2)) to the standard input of the following command. The exit
status of a pipeline is that of its last command. A pipeline may be prefixed
by the ‘!
’ reserved word, which causes
the exit status of the pipeline to be logically complemented: if the
original status was 0, the complemented status will be 1; if the original
status was not 0, the complemented status will be 0.
Lists
of commands can be created by separating pipelines by any of the following
tokens: ‘&&
’,
‘||
’,
‘&
’,
‘|&
’, and
‘;
’. The first two are for conditional
execution: “cmd1
&& cmd2”
executes cmd2 only if the exit status of
cmd1 is zero;
‘||
’ is the opposite —
cmd2 is executed only if the exit status of
cmd1 is non-zero.
‘&&
’ and
‘||
’ have equal precedence which is
higher than that of ‘&
’,
‘|&
’, and
‘;
’, which also have equal precedence.
Note that the ‘&&
’ and
‘||
’ operators are
"left-associative". For example, both of these commands will print
only "bar":
$ false && echo foo || echo bar $ true || echo foo && echo bar
The ‘&
’ token causes the
preceding command to be executed asynchronously; that is, the shell starts
the command but does not wait for it to complete (the shell does keep track
of the status of asynchronous commands; see
Job control below). When an
asynchronous command is started when job control is disabled (i.e. in most
scripts), the command is started with signals SIGINT
and SIGQUIT
ignored and with input redirected from
/dev/null (however, redirections specified in the
asynchronous command have precedence). Note that a command must follow the
‘&&
’ and
‘||
’ operators, while it need not
follow ‘&
’,
‘|&
’, or
‘;
’. The exit status of a list is that
of the last command executed, with the exception of asynchronous lists, for
which the exit status is 0.
Compound commands are created using the following reserved words. These words are only recognized if they are unquoted and if they are used as the first word of a command (i.e. they can't be preceded by parameter assignments or redirections):
case esac in while ]] do fi name ! { done for select ( } elif function then ) else if until [[
Note: Some shells (but not this one) execute
control structure commands in a subshell when one or more of their file
descriptors are redirected, so any environment changes inside them may fail.
To be portable, the exec
statement should be used
instead to redirect file descriptors before the control structure.
In the following compound command descriptions, command lists (denoted as list) that are followed by reserved words must end with a semicolon, a newline, or a (syntactically correct) reserved word. For example, the following are all valid:
$ { echo foo; echo bar; } $ { echo foo; echo bar<newline> } $ { { echo foo; echo bar; } }
This is not valid:
$ { echo foo; echo bar }
- (list)
- Execute list in a subshell. There is no implicit way to pass environment changes from a subshell back to its parent.
- { list; }
- Compound construct; list is executed, but not in a
subshell. Note that ‘
{
’ and ‘}
’ are reserved words, not meta-characters. - case word in [[(] pattern [| pattern] ... ) list ;; ] ... esac
- The
case
statement attempts to match word against a specified pattern; the list associated with the first successfully matched pattern is executed. Patterns used incase
statements are the same as those used for file name patterns except that the restrictions regarding ‘.
’ and ‘/
’ are dropped. Note that any unquoted space before and after a pattern is stripped; any space within a pattern must be quoted. Both the word and the patterns are subject to parameter, command, and arithmetic substitution, as well as tilde substitution. For historical reasons, open and close braces may be used instead ofin
andesac
e.g.case $foo { *) echo bar; }
. The exit status of acase
statement is that of the executed list; if no list is executed, the exit status is zero. - for name [in word ...]; do list; done
- For each word in the specified word list, the
parameter name is set to the word and
list is executed. If
in
is not used to specify a word list, the positional parameters ($1, $2, etc.) are used instead. For historical reasons, open and close braces may be used instead ofdo
anddone
e.g.for i; { echo $i; }
. The exit status of afor
statement is the last exit status of list; if list is never executed, the exit status is zero. - if list; then list; [elif list; then list;] ... [else list;] fi
- If the exit status of the first list is zero, the
second list is executed; otherwise, the
list following the
elif
, if any, is executed with similar consequences. If all the lists following theif
andelif
s fail (i.e. exit with non-zero status), the list following theelse
is executed. The exit status of anif
statement is that of non-conditional list that is executed; if no non-conditional list is executed, the exit status is zero. - select name [in word ...]; do list; done
- The
select
statement provides an automatic method of presenting the user with a menu and selecting from it. An enumerated list of the specified word(s) is printed on standard error, followed by a prompt (PS3: normally
‘#? ’). A number corresponding to one of the enumerated words is then read from standard input, name is set to the selected word (or unset if the selection is not valid),REPLY
is set to what was read (leading/trailing space is stripped), and list is executed. If a blank line (i.e. zero or moreIFS
characters) is entered, the menu is reprinted without executing list.When list completes, the enumerated list is printed if
REPLY
isNULL
, the prompt is printed, and so on. This process continues until an end-of-file is read, an interrupt is received, or abreak
statement is executed inside the loop. If “in word ...” is omitted, the positional parameters are used (i.e. $1, $2, etc.). For historical reasons, open and close braces may be used instead ofdo
anddone
e.g.select i; { echo $i; }
. The exit status of aselect
statement is zero if abreak
statement is used to exit the loop, non-zero otherwise. - until list; do list; done
- This works like
while
, except that the body is executed only while the exit status of the first list is non-zero. - while list; do list; done
- A
while
is a pre-checked loop. Its body is executed as often as the exit status of the first list is zero. The exit status of awhile
statement is the last exit status of the list in the body of the loop; if the body is not executed, the exit status is zero. - function name { list; }
- Defines the function name (see Functions below). Note that redirections specified after a function definition are performed whenever the function is executed, not when the function definition is executed.
- name() command
- Mostly the same as
function
(see Functions below). - [[ expression ]]
- Similar to the
test
and[ ... ]
commands (described later), with the following exceptions:- Field splitting and file name generation are not performed on arguments.
- The
-a
(AND) and-o
(OR) operators are replaced with ‘&&
’ and ‘||
’, respectively. - Operators (e.g. ‘
-f
’, ‘=’, ‘!’) must be unquoted. - The second operand of the ‘!=’ and ‘=’
expressions are patterns (e.g. the comparison
[[ foobar = f*r ]]
succeeds). - There are two additional binary operators,
‘
<
’ and ‘>
’, which return true if their first string operand is less than, or greater than, their second string operand, respectively. - The single argument form of
test
, which tests if the argument has a non-zero length, is not valid; explicit operators must always be used e.g. instead of [ str ] use [[ -n str ]]. - Parameter, command, and arithmetic substitutions are performed as
expressions are evaluated and lazy expression evaluation is used for
the ‘
&&
’ and ‘||
’ operators. This means that in the following statement,$(< foo)
is evaluated if and only if the file foo exists and is readable:$ [[ -r foo && $(< foo) = b*r ]]
Quoting
Quoting is used to prevent the shell from treating characters or
words specially. There are three methods of quoting. First,
‘\
’ quotes the following character,
unless it is at the end of a line, in which case both the
‘\
’ and the newline are stripped.
Second, a single quote (‘'’) quotes everything up to the next
single quote (this may span lines). Third, a double quote
(‘"’) quotes all characters, except
‘$
’,
‘`
’ and
‘\
’, up to the next unquoted double
quote. ‘$
’ and
‘`
’ inside double quotes have their
usual meaning (i.e. parameter, command, or arithmetic substitution) except
no field splitting is carried out on the results of double-quoted
substitutions. If a ‘\
’ inside a
double-quoted string is followed by
‘\
’,
‘$
’,
‘`
’, or
‘"
’, it is replaced by the second
character; if it is followed by a newline, both the
‘\
’ and the newline are stripped;
otherwise, both the ‘\
’ and the
character following are unchanged.
Note: See POSIX mode below for a special rule regarding differences in quoting when the shell is in POSIX mode.
Aliases
There are two types of aliases: normal command aliases and tracked aliases. Command aliases are normally used as a short hand for a long or often used command. The shell expands command aliases (i.e. substitutes the alias name for its value) when it reads the first word of a command. An expanded alias is re-processed to check for more aliases. If a command alias ends in a space or tab, the following word is also checked for alias expansion. The alias expansion process stops when a word that is not an alias is found, when a quoted word is found, or when an alias word that is currently being expanded is found.
The following command aliases are defined automatically by the shell:
hash='alias -t' type='whence -v'
Tracked aliases allow the shell to remember where it found a
particular command. The first time the shell does a path search for a
command that is marked as a tracked alias, it saves the full path of the
command. The next time the command is executed, the shell checks the saved
path to see that it is still valid, and if so, avoids repeating the path
search. Tracked aliases can be listed and created using
alias -t
. Note that changing the
PATH
parameter clears the saved paths for all
tracked aliases. If the trackall
option is set (i.e.
set -o
trackall
or
set -h
), the shell tracks all commands. This option
is set automatically for non-interactive shells. For interactive shells,
only the following commands are automatically tracked:
cat(1), cc(1),
chmod(1), cp(1),
date(1), ed(1),
emacs(1), grep(1),
ls(1),
mail(1), make(1),
mv(1),
pr(1),
rm(1),
sed(1), vi(1), and
who(1).
Substitution
The first step the shell takes in executing a simple-command is to perform substitutions on the words of the command. There are three kinds of substitution: parameter, command, and arithmetic. Parameter substitutions, which are described in detail in the next section, take the form $name or ${...}; command substitutions take the form $(command) or `command`; and arithmetic substitutions take the form $((expression)).
If a substitution appears outside of double quotes, the results of
the substitution are generally subject to word or field splitting according
to the current value of the IFS
parameter. The
IFS
parameter specifies a list of characters which
are used to break a string up into several words; any characters from the
set space, tab, and newline that appear in the IFS
characters are called “IFS whitespace”. Sequences of one or
more IFS
whitespace characters, in combination with
zero or one non-IFS
whitespace characters, delimit a
field. As a special case, leading and trailing IFS
whitespace is stripped (i.e. no leading or trailing empty field is created
by it); leading non-IFS
whitespace does create an
empty field.
Example: If IFS
is set to
“<space>:”, and VAR is set to
“<space>A<space>:<space><space>B::D”,
the substitution for $VAR results in four fields: ‘A’,
‘B’, ‘’ (an empty field), and ‘D’.
Note that if the IFS
parameter is set to the
NULL
string, no field splitting is done; if the
parameter is unset, the default value of space, tab, and newline is
used.
Also, note that the field splitting applies only to the immediate
result of the substitution. Using the previous example, the substitution for
$VAR:E results in the fields: ‘A’, ‘B’,
‘’, and ‘D:E’, not ‘A’,
‘B’, ‘’, ‘D’, and
‘E’. This behavior is POSIX compliant, but incompatible with
some other shell implementations which do field splitting on the word which
contained the substitution or use IFS
as a general
whitespace delimiter.
The results of substitution are, unless otherwise specified, also subject to file name expansion (see the relevant section below).
A command substitution is replaced by the output generated by the
specified command, which is run in a subshell. For
$(command) substitutions, normal quoting rules are
used when command is parsed; however, for the
`command` form, a
‘\
’ followed by any of
‘$
’,
‘`
’, or
‘\
’ is stripped (a
‘\
’ followed by any other character is
unchanged). As a special case in command substitutions, a command of the
form <file is interpreted to mean substitute the
contents of file. Note that $(<
foo)
has the same effect as $(cat foo)
, but
it is carried out more efficiently because no process is started.
Note: $(command) expressions are currently parsed by finding the matching parenthesis, regardless of quoting. This should be fixed soon.
Arithmetic substitutions are replaced by the value of the
specified expression. For example, the command echo
$((2+3*4))
prints 14. See
Arithmetic expressions for
a description of an expression.
Parameters
Parameters are shell variables; they can be assigned values and
their values can be accessed using a parameter substitution. A parameter
name is either one of the special single punctuation or digit character
parameters described below, or a letter followed by zero or more letters or
digits (‘_
’ counts as a letter).
Parameter substitutions take the form $name,
${name}, or
${name[expr]} where
name is a parameter name. If substitution is performed
on a parameter that is not set, a null string is substituted unless the
nounset
option (set
-o
nounset
or
set
-u
) is set, in which
case an error occurs.
Parameters can be assigned values in a number of ways. First, the
shell implicitly sets some parameters like
‘#
’,
‘PWD
’, and
‘$
’; this is the only way the special
single character parameters are set. Second, parameters are imported from
the shell's environment at startup. Third, parameters can be assigned values
on the command line: for example, FOO=bar
sets the
parameter “FOO” to “bar”; multiple parameter
assignments can be given on a single command line and they can be followed
by a simple-command, in which case the assignments are in effect only for
the duration of the command (such assignments are also exported; see below
for the implications of this). Note that both the parameter name and the
‘=
’ must be unquoted for the shell to
recognize a parameter assignment. The fourth way of setting a parameter is
with the export
, readonly
,
and typeset
commands; see their descriptions in the
Command execution section.
Fifth, for
loops set parameters as well as the
getopts
, read
, and
set -A
commands. Lastly, parameters can be assigned
values using assignment operators inside arithmetic expressions (see
Arithmetic expressions
below) or using the
${name=value}
form of the parameter substitution (see below).
Parameters with the export attribute (set using the
export
or typeset
-x
commands, or by parameter assignments followed by
simple commands) are put in the environment (see
environ(7)) of commands run by the shell as
name=value pairs. The order in
which parameters appear in the environment of a command is unspecified. When
the shell starts up, it extracts parameters and their values from its
environment and automatically sets the export attribute for those
parameters.
Modifiers can be applied to the ${name} form of parameter substitution:
- ${name:-word}
- If name is set and not
NULL
, it is substituted; otherwise, word is substituted. - ${name:+word}
- If name is set and not
NULL
, word is substituted; otherwise, nothing is substituted. - ${name:=word}
- If name is set and not
NULL
, it is substituted; otherwise, it is assigned word and the resulting value of name is substituted. - ${name:?word}
- If name is set and not
NULL
, it is substituted; otherwise, word is printed on standard error (preceded by name:) and an error occurs (normally causing termination of a shell script, function, or script sourced using the ‘.’ built-in). If word is omitted, the string “parameter null or not set” is used instead.
In the above modifiers, the
‘:
’ can be omitted, in which case the
conditions only depend on name being set (as opposed
to set and not NULL
). If word
is needed, parameter, command, arithmetic, and tilde substitution are
performed on it; if word is not needed, it is not
evaluated.
The following forms of parameter substitution can also be used:
- ${#name}
- The number of positional parameters if name is
‘
*
’, ‘@
’, or not specified; otherwise the length of the string value of parameter name. - ${#name[*]}
- ${#name[@]}
- The number of elements in the array name.
- ${name#pattern}
- ${name##pattern}
- If pattern matches the beginning of the value of
parameter name, the matched text is deleted from the
result of substitution. A single ‘
#
’ results in the shortest match, and two of them result in the longest match. - ${name%pattern}
- ${name%%pattern}
- Like ${..#..} substitution, but it deletes from the end of the value.
The following special parameters are implicitly set by the shell and cannot be set directly using assignments:
!
- Process ID of the last background process started. If no background processes have been started, the parameter is not set.
#
- The number of positional parameters ($1, $2, etc.).
$
- The PID of the shell, or the PID of the original shell if it is a subshell. Do NOT use this mechanism for generating temporary file names; see mktemp(1) instead.
-
- The concatenation of the current single letter options (see the
set
command below for a list of options). ?
- The exit status of the last non-asynchronous command executed. If the last
command was killed by a signal,
$?
is set to 128 plus the signal number. 0
- The name of the shell, determined as follows: the first argument to
sh
if it was invoked with the-c
option and arguments were given; otherwise the file argument, if it was supplied; or else the basename the shell was invoked with (i.e.argv[0]
).$0
is also set to the name of the current script or the name of the current function, if it was defined with thefunction
keyword (i.e. a Korn shell style function). 1
...9
- The first nine positional parameters that were supplied to the shell, function, or script sourced using the ‘.’ built-in. Further positional parameters may be accessed using ${number}.
*
- All positional parameters (except parameter 0) i.e. $1, $2, $3, ... If
used outside of double quotes, parameters are separate words (which are
subjected to word splitting); if used within double quotes, parameters are
separated by the first character of the
IFS
parameter (or the empty string ifIFS
isNULL
). @
- Same as
$*
, unless it is used inside double quotes, in which case a separate word is generated for each positional parameter. If there are no positional parameters, no word is generated.$@
can be used to access arguments, verbatim, without losingNULL
arguments or splitting arguments with spaces.
The following parameters are set and/or used by the shell:
CDPATH
- Search path for the
cd
built-in command. It works the same way asPATH
for those directories not beginning with ‘/
’ incd
commands. Note that ifCDPATH
is set and does not contain ‘.’ or contains an empty path, the current directory is not searched. Also, thecd
built-in command will display the resulting directory when a match is found in any search path other than the empty path. COLUMNS
- Set to the number of columns on the terminal or window. Currently set to
the “cols” value as reported by
stty(1) if that value is non-zero. This parameter is used by the
set -o
andkill -l
commands to format information columns. ENV
- If this parameter is found to be set after any profile files are executed, the expanded value is used as a shell startup file. It typically contains function and alias definitions.
ERRNO
- Integer value of the shell's errno variable. It indicates the reason the last system call failed. Not yet implemented.
EXECSHELL
- If set, this parameter is assumed to contain the shell that is to be used to execute commands that execve(2) fails to execute and which do not start with a “#!shell” sequence.
FCEDIT
- The editor used by the
fc
command (see below). FPATH
- Like
PATH
, but used when an undefined function is executed to locate the file defining the function. It is also searched when a command can't be found usingPATH
. See Functions below for more information. HOME
- The default directory for the
cd
command and the value substituted for an unqualified~
(see Tilde expansion below). IFS
- Internal field separator, used during substitution and by the
read
command, to split values into distinct arguments; normally set to space, tab, and newline. See Substitution above for details.Note: This parameter is not imported from the environment when the shell is started.
SH_VERSION
- The version of shell and the date the version was created (read-only).
LINENO
- The line number of the function or shell script that is currently being executed.
LINES
- Set to the number of lines on the terminal or window.
OLDPWD
- The previous working directory. Unset if
cd
has not successfully changed directories since the shell started, or if the shell doesn't know where it is. OPTARG
- When using
getopts
, it contains the argument for a parsed option, if it requires one. OPTIND
- The index of the next argument to be processed when using
getopts
. Assigning 1 to this parameter causesgetopts
to process arguments from the beginning the next time it is invoked. PATH
- A colon separated list of directories that are searched when looking for commands and files sourced using the ‘.’ command (see below). An empty string resulting from a leading or trailing colon, or two adjacent colons, is treated as a ‘.’ (the current directory).
POSIXLY_CORRECT
- If set, this parameter causes the
posix
option to be enabled. See POSIX mode below. PPID
- The process ID of the shell's parent (read-only).
PS1
- The primary prompt for interactive shells. Parameter, command, and
arithmetic substitutions are performed.
‘
!
’ is replaced with the current command number (see thefc
command below). A literal ‘!
’ can be put in the prompt by placing ‘!!
’ inPS1
. The default is ‘$ ’ for non-root users, ‘# ’ for root. PS2
- Secondary prompt string, by default ‘> ’, used when more input is needed to complete a command.
PS4
- Used to prefix commands that are printed during execution tracing (see the
set
-x
command below). Parameter, command, and arithmetic substitutions are performed before it is printed. The default is ‘+ ’. PWD
- The current working directory. May be unset or
NULL
if the shell doesn't know where it is. REPLY
- Default parameter for the
read
command if no names are given. TMPDIR
- The directory temporary shell files are created in. If this parameter is not set, or does not contain the absolute path of a writable directory, temporary files are created in /tmp.
Tilde expansion
Tilde expansion, which is done in parallel with parameter
substitution, is done on words starting with an unquoted
‘~
’. The characters following the
tilde, up to the first ‘/
’, if any,
are assumed to be a login name. If the login name is empty,
‘+
’, or
‘-
’, the value of the
HOME
, PWD
, or
OLDPWD
parameter is substituted, respectively.
Otherwise, the password file is searched for the login name, and the tilde
expression is substituted with the user's home directory. If the login name
is not found in the password file or if any quoting or parameter
substitution occurs in the login name, no substitution is performed.
In parameter assignments (such as those preceding a simple-command
or those occurring in the arguments of alias
,
export
, readonly
, and
typeset
), tilde expansion is done after any
assignment (i.e. after the equals sign) or after an unquoted colon
(‘:’); login names are also delimited by colons.
The home directory of previously expanded login names are cached
and re-used. The alias -d
command may be used to
list, change, and add to this cache (e.g. alias -d
fac=/usr/local/facilities; cd ~fac/bin
).
File name patterns
A file name pattern is a word containing one or more unquoted
‘?
’,
‘*
’,
‘+
’,
‘@
’, or
‘!
’ characters or “[..]”
sequences. The shell replaces file name patterns with the sorted names of
all the files that match the pattern (if no files match, the word is left
unchanged). The pattern elements have the following meaning:
- ?
- Matches any single character.
- *
- Matches any sequence of characters.
- [..]
- Matches any of the characters inside the brackets. Ranges of characters
can be specified by separating two characters by a
‘
-
’ (e.g. “[a0-9]” matches the letter ‘a’ or any digit). In order to represent itself, a ‘-
’ must either be quoted or the first or last character in the character list. Similarly, a ‘]
’ must be quoted or the first character in the list if it is to represent itself instead of the end of the list. Also, a ‘!
’ appearing at the start of the list has special meaning (see below), so to represent itself it must be quoted or appear later in the list.Within a bracket expression, the name of a character class enclosed in ‘[:’ and ‘:]’ stands for the list of all characters belonging to that class. Supported character classes:
alnum cntrl lower space alpha digit print upper blank graph punct xdigit
These match characters using the macros specified in ctype(3). A character class may not be used as an endpoint of a range.
- [!..]
- Like [..], except it matches any character not inside the brackets.
- *(pattern| ...| pattern)
- Matches any string of characters that matches zero or more occurrences of
the specified patterns. Example: The pattern
*(foo|bar)
matches the strings “”, “foo”, “bar”, “foobarfoo”, etc. - +(pattern| ...| pattern)
- Matches any string of characters that matches one or more occurrences of
the specified patterns. Example: The pattern
+(foo|bar)
matches the strings “foo”, “bar”, “foobar”, etc. - ?(pattern| ...| pattern)
- Matches the empty string or a string that matches one of the specified
patterns. Example: The pattern
?(foo|bar)
only matches the strings “”, “foo”, and “bar”. - @(pattern| ...| pattern)
- Matches a string that matches one of the specified patterns. Example: The
pattern
@(foo|bar)
only matches the strings “foo” and “bar”. - !(pattern| ...| pattern)
- Matches any string that does not match one of the specified patterns.
Examples: The pattern
!(foo|bar)
matches all strings except “foo” and “bar”; the pattern!(*)
matches no strings; the pattern!(?)*
matches all strings (think about it).
Note that pdksh
currently never matches
‘.’ and ‘..’, but the original
ksh
, Bourne sh
, and bash do,
so this may have to change (too bad).
Note that none of the above pattern elements match either a period (‘.’) at the start of a file name or a slash (‘/’), even if they are explicitly used in a [..] sequence; also, the names ‘.’ and ‘..’ are never matched, even by the pattern ‘.*’.
If the markdirs
option is set, any
directories that result from file name generation are marked with a trailing
‘/
’.
Input/output redirection
When a command is executed, its standard input, standard output, and standard error (file descriptors 0, 1, and 2, respectively) are normally inherited from the shell. Three exceptions to this are commands in pipelines, for which standard input and/or standard output are those set up by the pipeline, asynchronous commands created when job control is disabled, for which standard input is initially set to be from /dev/null, and commands for which any of the following redirections have been specified:
- > file
- Standard output is redirected to file. If
file does not exist, it is created; if it does
exist, is a regular file, and the
noclobber
option is set, an error occurs; otherwise, the file is truncated. Note that this means the commandcmd < foo > foo
will open foo for reading and then truncate it when it opens it for writing, before cmd gets a chance to actually read foo. - >| file
- Same as
>
, except the file is truncated, even if thenoclobber
option is set. - >> file
- Same as
>
, except if file exists it is appended to instead of being truncated. Also, the file is opened in append mode, so writes always go to the end of the file (see open(2)). - < file
- Standard input is redirected from file, which is opened for reading.
- <> file
- Same as
<
, except the file is opened for reading and writing. - << marker
- After reading the command line containing this kind of redirection (called
a “here document”), the shell copies lines from the command
source into a temporary file until a line matching
marker is read. When the command is executed,
standard input is redirected from the temporary file. If
marker contains no quoted characters, the contents
of the temporary file are processed as if enclosed in double quotes each
time the command is executed, so parameter, command, and arithmetic
substitutions are performed, along with backslash (‘\’)
escapes for ‘
$
’, ‘`
’, ‘\
’, and ‘\newline
’. If multiple here documents are used on the same command line, they are saved in order. - <<- marker
- Same as
<<
, except leading tabs are stripped from lines in the here document. - <& fd
- Standard input is duplicated from file descriptor
fd. fd can be a single digit,
indicating the number of an existing file descriptor; the letter
‘
p
’, indicating the file descriptor associated with the output of the current co-process; or the character ‘-
’, indicating standard input is to be closed. - >& fd
- Same as
<&
, except the operation is done on standard output.
In any of the above redirections, the file descriptor that is redirected (i.e. standard input or standard output) can be explicitly given by preceding the redirection with a single digit. Parameter, command, and arithmetic substitutions, tilde substitutions, and (if the shell is interactive) file name generation are all performed on the file, marker, and fd arguments of redirections. Note, however, that the results of any file name generation are only used if a single file is matched; if multiple files match, the word with the expanded file name generation characters is used. Note that in restricted shells, redirections which can create files cannot be used.
For simple-commands, redirections may appear anywhere in the
command; for compound-commands (if
statements,
etc.), any redirections must appear at the end. Redirections are processed
after pipelines are created and in the order they are given, so the
following will print an error with a line number prepended to it:
Arithmetic expressions
Integer arithmetic expressions can be used with the
let
command, inside $((..)) expressions, inside
array references (e.g.
name[expr]), as numeric
arguments to the test
command, and as the value of
an assignment to an integer parameter.
Expressions may contain alpha-numeric parameter identifiers, array references, and integer constants and may be combined with the following C operators (listed and grouped in increasing order of precedence):
Unary operators:
+ - ! ~ ++ --
Binary operators:
, = *= /= %= += -= <<= >>= &= ^= |= || && | ^ & == != < <= >= > << >> + - * / %
Ternary operators:
?: (precedence is immediately higher than assignment)
Grouping operators:
( )
Integer constants may be specified with arbitrary bases using the
notation base#number, where
base is a decimal integer specifying the base, and
number is a number in the specified base.
Additionally, integers may be prefixed with ‘0X’ or
‘0x’ (specifying base 16) or ‘0’ (base 8) in all
forms of arithmetic expressions, except as numeric arguments to the
test
command.
The operators are evaluated as follows:
- unary +
- Result is the argument (included for completeness).
- unary -
- Negation.
- !
- Logical NOT; the result is 1 if argument is zero, 0 if not.
- ~
- Arithmetic (bit-wise) NOT.
- ++
- Increment; must be applied to a parameter (not a literal or other expression). The parameter is incremented by 1. When used as a prefix operator, the result is the incremented value of the parameter; when used as a postfix operator, the result is the original value of the parameter.
- --
- Similar to
++
, except the parameter is decremented by 1. - ,
- Separates two arithmetic expressions; the left-hand side is evaluated first, then the right. The result is the value of the expression on the right-hand side.
- =
- Assignment; the variable on the left is set to the value on the right.
- *= /= += -= <<= >>= &= ^= |=
- Assignment operators. ⟨var⟩⟨op⟩=⟨expr⟩ is the same as ⟨var⟩=⟨var⟩⟨op⟩⟨expr⟩, with any operator precedence in ⟨expr⟩ preserved. For example, “var1 *= 5 + 3” is the same as specifying “var1 = var1 * (5 + 3)”.
- ||
- Logical OR; the result is 1 if either argument is non-zero, 0 if not. The right argument is evaluated only if the left argument is zero.
- &&
- Logical AND; the result is 1 if both arguments are non-zero, 0 if not. The right argument is evaluated only if the left argument is non-zero.
- |
- Arithmetic (bit-wise) OR.
- ^
- Arithmetic (bit-wise) XOR (exclusive-OR).
- &
- Arithmetic (bit-wise) AND.
- ==
- Equal; the result is 1 if both arguments are equal, 0 if not.
- !=
- Not equal; the result is 0 if both arguments are equal, 1 if not.
- <
- Less than; the result is 1 if the left argument is less than the right, 0 if not.
- <= >= >
- Less than or equal, greater than or equal, greater than. See
<
. - << >>
- Shift left (right); the result is the left argument with its bits shifted left (right) by the amount given in the right argument.
- + - * /
- Addition, subtraction, multiplication, and division.
- %
- Remainder; the result is the remainder of the division of the left argument by the right. The sign of the result is unspecified if either argument is negative.
- ⟨arg1⟩?⟨arg2⟩:⟨arg3⟩
- If ⟨arg1⟩ is non-zero, the result is ⟨arg2⟩; otherwise the result is ⟨arg3⟩.
Functions
Functions are defined using either Korn shell
function
function-name syntax
or the Bourne/POSIX shell function-name() syntax (see
below for the difference between the two forms). Functions are like
.-scripts
(i.e. scripts sourced using the
‘.’ built-in) in that they are executed in the current
environment. However, unlike .-scripts
, shell
arguments (i.e. positional parameters $1, $2, etc.) are never visible inside
them. When the shell is determining the location of a command, functions are
searched after special built-in commands, before regular and non-regular
built-ins, and before the PATH
is searched.
An existing function may be deleted using
unset
-f
function-name. A list of functions can be obtained
using typeset +f
and the function definitions can be
listed using typeset -f
. The
autoload
command (which is an alias for
typeset -fu
) may be used to create undefined
functions: when an undefined function is executed, the shell searches the
path specified in the FPATH
parameter for a file
with the same name as the function, which, if found, is read and executed.
If after executing the file the named function is found to be defined, the
function is executed; otherwise, the normal command search is continued
(i.e. the shell searches the regular built-in command table and
PATH
). Note that if a command is not found using
PATH
, an attempt is made to autoload a function
using FPATH
(this is an undocumented feature of the
original Korn shell).
Functions can have two attributes, “trace” and
“export”, which can be set with typeset
-ft
and typeset -fx
, respectively. When a
traced function is executed, the shell's xtrace
option is turned on for the function's duration; otherwise, the
xtrace
option is turned off. The
“export” attribute of functions is currently not used. In the
original Korn shell, exported functions are visible to shell scripts that
are executed.
Since functions are executed in the current shell environment,
parameter assignments made inside functions are visible after the function
completes. If this is not the desired effect, the
typeset
command can be used inside a function to
create a local parameter. Note that special parameters (e.g.
$$
, $!
) can't be scoped in
this way.
The exit status of a function is that of the last command executed
in the function. A function can be made to finish immediately using the
return
command; this may also be used to explicitly
specify the exit status.
Functions defined with the function
reserved word are treated differently in the following ways from functions
defined with the ()
notation:
- The $0 parameter is set to the name of the function (Bourne-style functions leave $0 untouched).
- Parameter assignments preceding function calls are not kept in the shell environment (executing Bourne-style functions will keep assignments).
OPTIND
is saved/reset and restored on entry and exit from the function sogetopts
can be used properly both inside and outside the function (Bourne-style functions leaveOPTIND
untouched, so usinggetopts
inside a function interferes with usinggetopts
outside the function).
In the future, the following differences will also be added:
- A separate trap/signal environment will be used during the execution of functions. This will mean that traps set inside a function will not affect the shell's traps and signals that are not ignored in the shell (but may be trapped) will have their default effect in a function.
- The EXIT trap, if set in a function, will be executed after the function returns.
POSIX mode
The shell is intended to be POSIX compliant; however, in some
cases, POSIX behaviour is contrary either to the original Korn shell
behaviour or to user convenience. How the shell behaves in these cases is
determined by the state of the posix
option
(set -o posix
). If it is on, the POSIX behaviour is
followed; otherwise, it is not. The posix
option is
set automatically when the shell starts up if the environment contains the
POSIXLY_CORRECT
parameter. The shell can also be
compiled so that it is in POSIX mode by default; however, this is usually
not desirable.
The following is a list of things that are affected by the state
of the posix
option:
- Reading of
$ENV
: if not inposix
mode, theENV
parameter is not expanded and included when the shell starts. - Occurrences of
\"
inside double quoted`..`
command substitutions. In POSIX mode, the\"
is interpreted when the command is interpreted; in non-POSIX mode, the backslash is stripped before the command substitution is interpreted. For example,echo "`echo \"hi\"`"
produces “"hi"” in POSIX mode, “hi” in non-POSIX mode. To avoid problems, use the$(...)
form of command substitution. kill -l
output. In POSIX mode, only signal names are listed (in a single line); in non-POSIX mode, signal numbers, names, and descriptions are printed (in columns). In the future, a new option (-v
perhaps) will be added to distinguish the two behaviours.echo
options. In POSIX mode,-e
and-E
are not treated as options, but printed like other arguments; in non-POSIX mode, these options control the interpretation of backslash sequences.fg
exit status. In POSIX mode, the exit status is 0 if no errors occur; in non-POSIX mode, the exit status is that of the last foregrounded job.getopts
. In POSIX mode, options must start with a ‘-
’; in non-POSIX mode, options can start with either ‘-
’ or ‘+
’.set -
. In POSIX mode, this does not clear theverbose
orxtrace
options; in non-POSIX mode, it does.set
exit status. In POSIX mode, the exit status ofset
is 0 if there are no errors; in non-POSIX mode, the exit status is that of any command substitutions performed in generating theset
command. For example,set -- `false`; echo $?
prints 0 in POSIX mode, 1 in non-POSIX mode. This construct is used in most shell scripts that use the old getopt(1) command.- Argument expansion of the
alias
,export
,readonly
, andtypeset
commands. In POSIX mode, normal argument expansion is done; in non-POSIX mode, field splitting, file globbing, and (normal) tilde expansion are turned off, while assignment tilde expansion is turned on. - Signal specification. In POSIX mode, signals can be specified as digits, only if signal numbers match POSIX values (i.e. HUP=1, INT=2, QUIT=3, ABRT=6, KILL=9, ALRM=14, and TERM=15); in non-POSIX mode, signals can always be digits.
- Alias expansion. In POSIX mode, alias expansion is only carried out when
reading command words; in non-POSIX mode, alias expansion is carried out
on any word following an alias that ended in a space. For example, the
following
for
loop uses parameter ‘i’ in POSIX mode and ‘j’ in non-POSIX mode:alias a='for ' i='j' a i in 1 2; do echo i=$i j=$j; done
test
. In POSIX mode, the expression ‘-t
’ (preceded by some number of ‘!’ arguments) is always true as it is a non-zero length string; in non-POSIX mode, it tests if file descriptor 1 is a tty(4) (i.e. the fd argument to the-t
test may be left out and defaults to 1).
Command execution
After evaluation of command-line arguments, redirections, and
parameter assignments, the type of command is determined: a special
built-in, a function, a regular built-in, or the name of a file to execute
found using the PATH
parameter. The checks are made
in the above order. Special built-in commands differ from other commands in
that the PATH
parameter is not used to find them, an
error during their execution can cause a non-interactive shell to exit, and
parameter assignments that are specified before the command are kept after
the command completes. Just to confuse things, if the
posix
option is turned off (see the
set
command below), some special commands are very
special in that no field splitting, file globbing, nor tilde expansion is
performed on arguments that look like assignments. Regular built-in commands
are different only in that the PATH
parameter is not
used to find them.
The original ksh
and POSIX differ somewhat
in which commands are considered special or regular:
POSIX special commands
.
, :
,
break
, continue
,
eval
, exec
,
exit
, export
,
readonly
, return
,
set
, shift
,
trap
, unset
Additional sh
special commands
builtin
, times
,
typeset
Very special commands (non-POSIX)
alias
, readonly
,
set
, typeset
POSIX regular commands
alias
, bg
,
cd
, command
,
false
, fc
,
fg
, getopts
,
jobs
, kill
,
read
, true
,
umask
, unalias
,
wait
Additional sh
regular commands
[
, echo
,
let
, print
,
pwd
, test
,
ulimit
, whence
In the future, the additional sh
special
and regular commands may be treated differently from the POSIX special and
regular commands.
Once the type of command has been determined, any command-line parameter assignments are performed and exported for the duration of the command.
The following describes the special and regular built-in commands:
.
file [arg ...]- Execute the commands in file in the current
environment. The file is searched for in the directories of
PATH
. If arguments are given, the positional parameters may be used to access them while file is being executed. If no arguments are given, the positional parameters are those of the environment the command is used in. :
[...]- The null command. Exit status is set to zero.
alias
[-d
|-t
[-r
] |+-x
] [-p
] [+
] [name [=value] ...]- Without arguments,
alias
lists all aliases. For any name without a value, the existing alias is listed. Any name with a value defines an alias (see Aliases above).When listing aliases, one of two formats is used. Normally, aliases are listed as name=value, where value is quoted. If options were preceded with ‘
+
’, or a lone ‘+
’ is given on the command line, only name is printed.The
-d
option causes directory aliases, which are used in tilde expansion, to be listed or set (see Tilde expansion above).If the
-p
option is used, each alias is prefixed with the string “alias ”.The
-t
option indicates that tracked aliases are to be listed/set (values specified on the command line are ignored for tracked aliases). The-r
option indicates that all tracked aliases are to be reset.The
-x
option sets (+x
clears) the export attribute of an alias, or, if no names are given, lists the aliases with the export attribute (exporting an alias has no effect). bg
[job ...]- Resume the specified stopped job(s) in the background. If no jobs are
specified,
%+
is assumed. See Job control below for more information. break
[level]- Exit the levelth inner-most
for
,until
, orwhile
loop. level defaults to 1. builtin
command [arg ...]- Execute the built-in command command.
cd
[-LP
] [dir]- Set the working directory to dir. If the parameter
CDPATH
is set, it lists the search path for the directory containing dir. ANULL
path means the current directory. If dir is found in any component of theCDPATH
search path other than theNULL
path, the name of the new working directory will be written to standard output. If dir is missing, the home directoryHOME
is used. If dir is ‘-
’, the previous working directory is used (see theOLDPWD
parameter).If the
-L
option (logical path) is used or if thephysical
option isn't set (see theset
command below), references to ‘..’ in dir are relative to the path used to get to the directory. If the-P
option (physical path) is used or if thephysical
option is set, ‘..’ is relative to the filesystem directory tree. ThePWD
andOLDPWD
parameters are updated to reflect the current and old working directory, respectively. cd
[-LP
] old new- The string new is substituted for old in the current directory, and the shell attempts to change to the new directory.
command
[-p
] cmd [arg ...]- cmd is executed exactly as if
command
had not been specified, with two exceptions: firstly, cmd cannot be a shell function; and secondly, special built-in commands lose their specialness (i.e. redirection and utility errors do not cause the shell to exit, and command assignments are not permanent).If the
-p
option is given, a default search path is used instead of the current value ofPATH
(the actual value of the default path is system dependent: on POSIX-ish systems, it is the value returned bygetconf CS_PATH
). continue
[level]- Jumps to the beginning of the levelth inner-most
for
,until
, orwhile
loop. level defaults to 1. echo
[-Een
] [arg ...]- Prints its arguments (separated by spaces) followed by a newline, to the
standard output. The newline is suppressed if any of the arguments contain
the backslash sequence ‘
\c
’. See theprint
command below for a list of other backslash sequences that are recognized.The options are provided for compatibility with BSD shell scripts. The
-n
option suppresses the trailing newline,-e
enables backslash interpretation (a no-op, since this is normally done), and-E
suppresses backslash interpretation. If theposix
option is set, only the first argument is treated as an option, and only if it is exactly “-n”. eval
command ...- The arguments are concatenated (with spaces between them) to form a single string which the shell then parses and executes in the current environment.
exec
[command [arg ...]]- The command is executed without forking, replacing the shell process.
If no command is given except for I/O redirection, the I/O redirection is permanent and the shell is not replaced. Any file descriptors which are opened or dup(2)'d in this way are made available to other executed commands (note that the Korn shell differs here: it does not pass on file descriptors greater than 2).
exit
[status]- The shell exits with the specified exit status. If
status is not specified, the exit status is the
current value of the
$?
parameter. export
[-p
] [parameter[=value]]- Sets the export attribute of the named parameters. Exported parameters are
passed in the environment to executed commands. If values are specified,
the named parameters are also assigned.
If no parameters are specified, the names of all parameters with the export attribute are printed one per line, unless the
-p
option is used, in which caseexport
commands defining all exported parameters, including their values, are printed. false
- A command that exits with a non-zero status.
fc
-e -
|-s
[-g
] [old=new] [prefix]- Re-execute the selected command (the previous command by default) after
performing the optional substitution of old with
new. If
-g
is specified, all occurrences of old are replaced with new. The meaning of-e -
and-s
is identical: re-execute the selected command without invoking an editor. This command is usually accessed with the predefinedalias r='fc -e -'
. fg
[job ...]- Resume the specified job(s) in the foreground. If no jobs are specified,
%+
is assumed. See Job control below for more information. getopts
optstring name [arg ...]- Used by shell procedures to parse the specified arguments (or positional
parameters, if no arguments are given) and to check for legal options.
optstring contains the option letters that
getopts
is to recognize. If a letter is followed by a colon, the option is expected to have an argument. Options that do not take arguments may be grouped in a single argument. If an option takes an argument and the option character is not the last character of the argument it is found in, the remainder of the argument is taken to be the option's argument; otherwise, the next argument is the option's argument.Each time
getopts
is invoked, it places the next option in the shell parameter name and the index of the argument to be processed by the next call togetopts
in the shell parameterOPTIND
. If the option was introduced with a ‘+
’, the option placed in name is prefixed with a ‘+
’. When an option requires an argument,getopts
places it in the shell parameterOPTARG
.When an illegal option or a missing option argument is encountered, a question mark or a colon is placed in name (indicating an illegal option or missing argument, respectively) and
OPTARG
is set to the option character that caused the problem. Furthermore, if optstring does not begin with a colon, a question mark is placed in name,OPTARG
is unset, and an error message is printed to standard error.When the end of the options is encountered,
getopts
exits with a non-zero exit status. Options end at the first (non-option argument) argument that does not start with a ‘-
’, or when a ‘--
’ argument is encountered.Option parsing can be reset by setting
OPTIND
to 1 (this is done automatically whenever the shell or a shell procedure is invoked).Warning: Changing the value of the shell parameter
OPTIND
to a value other than 1, or parsing different sets of arguments without resettingOPTIND
, may lead to unexpected results. hash
[-r
] [name ...]- Without arguments, any hashed executable command pathnames are listed. The
-r
option causes all hashed commands to be removed from the hash table. Each name is searched as if it were a command name and added to the hash table if it is an executable command. jobs
[-lnp
] [job ...]- Display information about the specified job(s); if no jobs are specified,
all jobs are displayed. The
-n
option causes information to be displayed only for jobs that have changed state since the last notification. If the-l
option is used, the process ID of each process in a job is also listed. The-p
option causes only the process group of each job to be printed. See Job control below for the format of job and the displayed job. kill
[-s
signame | -signum | -signame] { job | pid | pgrp } ...- Send the specified signal to the specified jobs, process IDs, or process
groups. If no signal is specified, the
TERM
signal is sent. If a job is specified, the signal is sent to the job's process group. See Job control below for the format of job. kill
-l
[exit-status ...]- Print the signal name corresponding to exit-status. If no arguments are specified, a list of all the signals, their numbers, and a short description of them are printed.
print
[-nrsu
[n] |-R
[-en
]] [argument ...]print
prints its arguments on the standard output, separated by spaces and terminated with a newline. The-n
option suppresses the newline. By default, certain C escapes are translated. These include ‘\b
’, ‘\f
’, ‘\n
’, ‘\r
’, ‘\t
’, ‘\v
’, and ‘\0###
’ (‘#
’ is an octal digit, of which there may be 0 to 3). ‘\c
’ is equivalent to using the-n
option. ‘\
’ expansion may be inhibited with the-r
option. The-s
option prints to the history file instead of standard output; and the-u
option prints to file descriptor n (n defaults to 1 if omitted).The
-R
option is used to emulate, to some degree, the BSD echo(1) command, which does not process ‘\
’ sequences unless the-e
option is given. As above, the-n
option suppresses the trailing newline.pwd
[-LP
]- Print the present working directory. If the
-L
option is used or if thephysical
option isn't set (see theset
command below), the logical path is printed (i.e. the path used tocd
to the current directory). If the-P
option (physical path) is used or if thephysical
option is set, the path determined from the filesystem (by following ‘..’ directories to the root directory) is printed. read
[-rsu
[n]] [parameter ...]- Reads a line of input from the standard input, separates the line into
fields using the
IFS
parameter (see Substitution above), and assigns each field to the specified parameters. If there are more parameters than fields, the extra parameters are set toNULL
, or alternatively, if there are more fields than parameters, the last parameter is assigned the remaining fields (inclusive of any separating spaces). If no parameters are specified, theREPLY
parameter is used. If the input line ends in a backslash and the-r
option was not used, the backslash and the newline are stripped and more input is read. If no input is read,read
exits with a non-zero status.The first parameter may have a question mark and a string appended to it, in which case the string is used as a prompt (printed to standard error before any input is read) if the input is a tty(4) (e.g.
read nfoo?'number of foos: '
).The
-u
n option causes input to be read from file descriptor n (n defaults to 0 if omitted). If the-s
option is used, input is saved to the history file. readonly
[-p
] [parameter [=value] ...]- Sets the read-only attribute of the named parameters. If values are given,
parameters are set to them before setting the attribute. Once a parameter
is made read-only, it cannot be unset and its value cannot be changed.
If no parameters are specified, the names of all parameters with the read-only attribute are printed one per line, unless the
-p
option is used, in which casereadonly
commands defining all read-only parameters, including their values, are printed. return
[status]- Returns from a function or
.
script, with exit status status. If no status is given, the exit status of the last executed command is used. If used outside of a function or.
script, it has the same effect asexit
. Note thatpdksh
treats both profile andENV
files as.
scripts, while the original Korn shell only treats profiles as.
scripts. set
[+-abCefhkmnpsuvXx
] [+-o
option] [+-A
name] [--
] [arg ...]- The
set
command can be used to set (-
) or clear (+
) shell options, set the positional parameters, or set an array parameter. Options can be changed using the+-o
option syntax, where option is the long name of an option, or using the+-
letter syntax, where letter is the option's single letter name (not all options have a single letter name). The following table lists both option letters (if they exist) and long names along with a description of what the option does:-A
name- Sets the elements of the array parameter name to
arg ... If
-A
is used, the array is reset (i.e. emptied) first; if+A
is used, the first N elements are set (where N is the number of arguments); the rest are left untouched. -a
|allexport
- All new parameters are created with the export attribute.
-b
|notify
- Print job notification messages asynchronously, instead of just before
the prompt. Only used if job control is enabled
(
-m
). -C
|noclobber
- Prevent > redirection from overwriting existing files. Instead, >| must be used to force an overwrite.
-e
|errexit
- Exit (after executing the
ERR
trap) as soon as an error occurs or a command fails (i.e. exits with a non-zero status). This does not apply to commands whose exit status is explicitly tested by a shell construct such asif
,until
,while
,&&
,||
, or!
statements. -f
|noglob
- Do not expand file name patterns.
-h
|trackall
- Create tracked aliases for all executed commands (see Aliases above). Enabled by default for non-interactive shells.
-k
|keyword
- Parameter assignments are recognized anywhere in a command.
-m
|monitor
- Enable job control (default for interactive shells).
-n
|noexec
- Do not execute any commands. Useful for checking the syntax of scripts (ignored if interactive).
-p
|privileged
- The shell is a privileged shell. It is set automatically if, when the shell starts, the real UID or GID does not match the effective UID (EUID) or GID (EGID), respectively. See above for a description of what this means.
-s
|stdin
- If used when the shell is invoked, commands are read from standard
input. Set automatically if the shell is invoked with no arguments.
When
-s
is used with theset
command it causes the specified arguments to be sorted before assigning them to the positional parameters (or to array name, if-A
is used). -u
|nounset
- Referencing of an unset parameter is treated as an error, unless one
of the ‘
-
’, ‘+
’, or ‘=
’ modifiers is used. -v
|verbose
- Write shell input to standard error as it is read.
-X
|markdirs
- Mark directories with a trailing
‘
/
’ during file name generation. -x
|xtrace
- Print commands and parameter assignments when they are executed,
preceded by the value of
PS4
. bgnice
- Background jobs are run with lower priority.
ignoreeof
- The shell will not exit when end-of-file is read;
exit
must be used. interactive
- The shell is an interactive shell. This option can only be used when the shell is invoked. See above for a description of what this means.
login
- The shell is a login shell. This option can only be used when the shell is invoked. See above for a description of what this means.
nohup
- Do not kill running jobs with a
SIGHUP
signal when a login shell exits. Currently set by default, but this will change in the future to be compatible with the original Korn shell (which doesn't have this option, but does send theSIGHUP
signal). nolog
- No effect. In the original Korn shell, this prevents function definitions from being stored in the history file.
physical
- Causes the
cd
andpwd
commands to use “physical” (i.e. the filesystem's) ‘..’ directories instead of “logical” directories (i.e. the shell handles ‘..’, which allows the user to be oblivious of symbolic links to directories). Clear by default. Note that setting this option does not affect the current value of thePWD
parameter; only thecd
command changesPWD
. See thecd
andpwd
commands above for more details. posix
- Enable POSIX mode. See POSIX mode above.
restricted
- The shell is a restricted shell. This option can only be used when the shell is invoked. See above for a description of what this means.
vi
- Enable vi(1)-like command-line editing (interactive shells only).
vi-esccomplete
- In vi command-line editing, do command and file name completion when escape (^[) is entered in command mode.
vi-show8
- Prefix characters with the eighth bit set with ‘M-’. If this option is not set, characters in the range 128-160 are printed as is, which may cause problems.
vi-tabcomplete
- In vi command-line editing, do command and file name completion when tab (^I) is entered in insert mode.
viraw
- No effect. In the original Korn shell, unless
viraw
was set, the vi command-line mode would let the tty(4) driver do the work until ESC (^[) was entered.pdksh
is always in viraw mode.
These options can also be used upon invocation of the shell. The current set of options (with single letter names) can be found in the parameter ‘$-’.
set
-o
with no option name will list all the options and whether each is on or off;set +o
will print the long names of all options that are currently on.Remaining arguments, if any, are positional parameters and are assigned, in order, to the positional parameters (i.e. $1, $2, etc.). If options end with ‘
--
’ and there are no remaining arguments, all positional parameters are cleared. If no options or arguments are given, the values of all names are printed. For unknown historical reasons, a lone ‘-
’ option is treated specially - it clears both the-x
and-v
options. shift
[number]- The positional parameters number+1, number+2, etc. are renamed to ‘1’, ‘2’, etc. number defaults to 1.
test
expression[
expression]
test
evaluates the expression and returns zero status if true, 1 if false, or greater than 1 if there was an error. It is normally used as the condition command ofif
andwhile
statements. Symbolic links are followed for all file expressions except-h
and-L
.The following basic expressions are available:
-a
file- file exists.
-b
file- file is a block special device.
-c
file- file is a character special device.
-d
file- file is a directory.
-e
file- file exists.
-f
file- file is a regular file.
-G
file- file's group is the shell's effective group ID.
-g
file- file's mode has the setgid bit set.
-h
file- file is a symbolic link.
-k
file- file's mode has the sticky(8) bit set.
-L
file- file is a symbolic link.
-O
file- file's owner is the shell's effective user ID.
-o
option- Shell option is set (see the
set
command above for a list of options). As a non-standard extension, if the option starts with a ‘!
’, the test is negated; the test always fails if option doesn't exist (so [ -o foo -o -o !foo ] returns true if and only if option foo exists). -p
file- file is a named pipe.
-r
file- file exists and is readable.
-S
file- file is a unix(4)-domain socket.
-s
file- file is not empty.
-t
[fd]- File descriptor fd is a
tty(4) device. If the
posix
option is not set, fd may be left out, in which case it is taken to be 1 (the behaviour differs due to the special POSIX rules described above). -u
file- file's mode has the setuid bit set.
-w
file- file exists and is writable.
-x
file- file exists and is executable.
- file1
-nt
file2 - file1 is newer than file2.
- file1
-ot
file2 - file1 is older than file2.
- file1
-ef
file2 - file1 is the same file as file2.
- string
- string has non-zero length.
-n
string- string is not empty.
-z
string- string is empty.
- string = string
- Strings are equal.
- string != string
- Strings are not equal.
- number
-eq
number - Numbers compare equal.
- number
-ne
number - Numbers compare not equal.
- number
-ge
number - Numbers compare greater than or equal.
- number
-gt
number - Numbers compare greater than.
- number
-le
number - Numbers compare less than or equal.
- number
-lt
number - Numbers compare less than.
The above basic expressions, in which unary operators have precedence over binary operators, may be combined with the following operators (listed in increasing order of precedence):
expr -o expr Logical OR. expr -a expr Logical AND. ! expr Logical NOT. ( expr ) Grouping.
On operating systems not supporting /dev/fd/n devices (where n is a file descriptor number), the
test
command will attempt to fake it for all tests that operate on files (except the-e
test). For example, [ -w /dev/fd/2 ] tests if file descriptor 2 is writable.Note that some special rules are applied (courtesy of POSIX) if the number of arguments to
test
or[ ... ]
is less than five: if leading ‘!
’ arguments can be stripped such that only one argument remains then a string length test is performed (again, even if the argument is a unary operator); if leading ‘!
’ arguments can be stripped such that three arguments remain and the second argument is a binary operator, then the binary operation is performed (even if the first argument is a unary operator, including an unstripped ‘!
’).Note: A common mistake is to use “if [ $foo = bar ]”, which fails if parameter “foo” is
NULL
or unset, if it has embedded spaces (i.e.IFS
characters), or if it is a unary operator like ‘!’ or ‘-n
’. Use tests like “if [ "X$foo" = Xbar ]” instead.times
- Print the accumulated user and system times used both by the shell and by
processes that the shell started which have exited. The format of the
output is:
0m0.00s 0m0.00s 0m0.00s 0m0.00s
trap
[handler signal ...]- Sets a trap handler that is to be executed when any of the specified
signals are received. handler is either a
NULL
string, indicating the signals are to be ignored, a minus sign (‘-’), indicating that the default action is to be taken for the signals (see signal(3)), or a string containing shell commands to be evaluated and executed at the first opportunity (i.e. when the current command completes, or before printing the nextPS1
prompt) after receipt of one of the signals. signal is the name of a signal (e.g.PIPE
orALRM
) or the number of the signal (see thekill -l
command above).There are two special signals:
EXIT
(also known as 0), which is executed when the shell is about to exit, andERR
, which is executed after an error occurs (an error is something that would cause the shell to exit if the-e
orerrexit
option were set - see theset
command above).EXIT
handlers are executed in the environment of the last executed command. Note that for non-interactive shells, the trap handler cannot be changed for signals that were ignored when the shell started.With no arguments,
trap
lists, as a series oftrap
commands, the current state of the traps that have been set since the shell started. Note that the output oftrap
cannot be usefully piped to another process (an artifact of the fact that traps are cleared when subprocesses are created).The original Korn shell's
DEBUG
trap and the handling ofERR
andEXIT
traps in functions are not yet implemented. true
- A command that exits with a zero value.
typeset
[[+-lprtUux
] [-L
[n]] [-R
[n]] [-Z
[n]] [-i
[n]] |-f
[-tux
]] [name [=value] ...]- Display or set parameter attributes. With no name
arguments, parameter attributes are displayed; if no options are used, the
current attributes of all parameters are printed as
typeset
commands; if an option is given (or ‘-
’ with no option letter), all parameters and their values with the specified attributes are printed; if options are introduced with ‘+
’, parameter values are not printed.If name arguments are given, the attributes of the named parameters are set (
-
) or cleared (+
). Values for parameters may optionally be specified. Iftypeset
is used inside a function, any newly created parameters are local to the function.When
-f
is used,typeset
operates on the attributes of functions. As with parameters, if no name arguments are given, functions are listed with their values (i.e. definitions) unless options are introduced with ‘+
’, in which case only the function names are reported.-f
- Function mode. Display or set functions and their attributes, instead of parameters.
-i
[n]- Integer attribute. n specifies the base to use when displaying the integer (if not specified, the base given in the first assignment is used). Parameters with this attribute may be assigned values containing arithmetic expressions.
-L
[n]- Left justify attribute. n specifies the field
width. If n is not specified, the current width
of a parameter (or the width of its first assigned value) is used.
Leading whitespace (and zeros, if used with the
-Z
option) is stripped. If necessary, values are either truncated or space padded to fit the field width. -l
- Lower case attribute. All upper case characters in values are
converted to lower case. (In the original Korn shell, this parameter
meant “long integer” when used with the
-i
option.) -p
- Print complete
typeset
commands that can be used to re-create the attributes (but not the values) of parameters. This is the default action (option exists for ksh93 compatibility). -R
[n]- Right justify attribute. n specifies the field width. If n is not specified, the current width of a parameter (or the width of its first assigned value) is used. Trailing whitespace is stripped. If necessary, values are either stripped of leading characters or space padded to make them fit the field width.
-r
- Read-only attribute. Parameters with this attribute may not be assigned to or unset. Once this attribute is set, it cannot be turned off.
-t
- Tag attribute. Has no meaning to the shell; provided for application
use.
For functions,
-t
is the trace attribute. When functions with the trace attribute are executed, thextrace
(-x
) shell option is temporarily turned on. -U
- Unsigned integer attribute. Integers are printed as unsigned values
(only useful when combined with the
-i
option). This option is not in the original Korn shell. -u
- Upper case attribute. All lower case characters in values are
converted to upper case. (In the original Korn shell, this parameter
meant “unsigned integer” when used with the
-i
option, which meant upper case letters would never be used for bases greater than 10. See the-U
option.)For functions,
-u
is the undefined attribute. See Functions above for the implications of this. -x
- Export attribute. Parameters (or functions) are placed in the environment of any executed commands. Exported functions are not yet implemented.
-Z
[n]- Zero fill attribute. If not combined with
-L
, this is the same as-R
, except zero padding is used instead of space padding.
ulimit
[-acdfHlmnpSst
[value]] ...- Display or set process limits. If no options are used, the file size limit
(
-f
) is assumed. value, if specified, may be either an arithmetic expression starting with a number or the word “unlimited”. The limits affect the shell and any processes created by the shell after a limit is imposed; limits may not be increased once they are set.-a
- Display all limits; unless
-H
is used, soft limits are displayed. -c
n- Impose a size limit of n blocks on the size of core dumps.
-d
n- Impose a size limit of n kilobytes on the size of the data area.
-f
n- Impose a size limit of n blocks on files written by the shell and its child processes (files of any size may be read).
-H
- Set the hard limit only (the default is to set both hard and soft limits).
-l
n- Impose a limit of n kilobytes on the amount of locked (wired) physical memory.
-m
n- Impose a limit of n kilobytes on the amount of physical memory used.
-n
n- Impose a limit of n file descriptors that can be open at once.
-p
n- Impose a limit of n processes that can be run by the user at any one time.
-S
- Set the soft limit only (the default is to set both hard and soft limits).
-s
n- Impose a size limit of n kilobytes on the size of the stack area.
-t
n- Impose a time limit of n CPU seconds spent in user mode to be used by each process.
As far as
ulimit
is concerned, a block is 512 bytes. umask
[-S
] [mask]- Display or set the file permission creation mask, or umask (see
umask(2)). If the
-S
option is used, the mask displayed or set is symbolic; otherwise, it is an octal number.Symbolic masks are like those used by chmod(1). When used, they describe what permissions may be made available (as opposed to octal masks in which a set bit means the corresponding bit is to be cleared). For example, “ug=rwx,o=” sets the mask so files will not be readable, writable, or executable by “others”, and is equivalent (on most systems) to the octal mask “007”.
unalias
[-adt
] [name ...]- The aliases for the given names are removed. If the
-a
option is used, all aliases are removed. If the-t
or-d
options are used, the indicated operations are carried out on tracked or directory aliases, respectively. unset
[-fv
] parameter ...- Unset the named parameters (
-v
, the default) or functions (-f
). The exit status is non-zero if any of the parameters have the read-only attribute set, zero otherwise. wait
[job ...]- Wait for the specified job(s) to finish. The exit status of
wait
is that of the last specified job; if the last job is killed by a signal, the exit status is 128 + the number of the signal (seekill -l
exit-status above); if the last specified job can't be found (because it never existed, or had already finished), the exit status ofwait
is 127. See Job control below for the format of job.wait
will return if a signal for which a trap has been set is received, or if aSIGHUP
,SIGINT
, orSIGQUIT
signal is received.If no jobs are specified,
wait
waits for all currently running jobs (if any) to finish and exits with a zero status. If job monitoring is enabled, the completion status of jobs is printed (this is not the case when jobs are explicitly specified). whence
[-pv
] [name ...]- For each name, the type of command is listed
(reserved word, built-in, alias, function, tracked alias, or executable).
If the
-p
option is used, a path search is performed even if name is a reserved word, alias, etc. Without the-v
option,whence
is similar tocommand
-v
except thatwhence
will find reserved words and won't print aliases as alias commands. With the-v
option,whence
is the same ascommand
-V
. Note that forwhence
, the-p
option does not affect the search path used, as it does forcommand
. If the type of one or more of the names could not be determined, the exit status is non-zero.
Job control
Job control refers to the shell's ability to monitor and control
jobs, which are processes or groups of processes created for commands or
pipelines. At a minimum, the shell keeps track of the status of the
background (i.e. asynchronous) jobs that currently exist; this information
can be displayed using the jobs
commands. If job
control is fully enabled (using set -m
or
set -o monitor
), as it is for interactive shells,
the processes of a job are placed in their own process group. Foreground
jobs can be stopped by typing the suspend character from the terminal
(normally ^Z), jobs can be restarted in either the foreground or background
using the fg
and bg
commands, and the state of the terminal is saved or restored when a
foreground job is stopped or restarted, respectively.
Note that only commands that create processes (e.g. asynchronous
commands, subshell commands, and non-built-in, non-function commands) can be
stopped; commands like read
cannot be.
When a job is created, it is assigned a job number. For
interactive shells, this number is printed inside “[..]”,
followed by the process IDs of the processes in the job when an asynchronous
command is run. A job may be referred to in the bg
,
fg
, jobs
,
kill
, and wait
commands
either by the process ID of the last process in the command pipeline (as
stored in the $!
parameter) or by prefixing the job
number with a percent sign (‘%’). Other percent sequences can
also be used to refer to jobs:
- %+ | %% | %
- The most recently stopped job, or, if there are no stopped jobs, the oldest running job.
- %-
- The job that would be the
%+
job if the latter did not exist. - %n
- The job with job number n.
- %?string
- The job with its command containing the string string (an error occurs if multiple jobs are matched).
- %string
- The job with its command starting with the string string (an error occurs if multiple jobs are matched).
When a job changes state (e.g. a background job finishes or foreground job is stopped), the shell prints the following status information:
[where...
- number
- is the job number of the job;
- flag
- is the ‘
+
’ or ‘-
’ character if the job is the%+
or%-
job, respectively, or space if it is neither; - status
- indicates the current state of the job and can be:
- Done [number]
- The job exited. number is the exit status of the job, which is omitted if the status is zero.
- Running
- The job has neither stopped nor exited (note that running does not necessarily mean consuming CPU time - the process could be blocked waiting for some event).
- Stopped [signal]
- The job was stopped by the indicated signal (if
no signal is given, the job was stopped by
SIGTSTP
). - signal-description [“core dumped”]
- The job was killed by a signal (e.g. memory fault, hangup); use
kill -l
for a list of signal descriptions. The “core dumped” message indicates the process created a core file.
- command
- is the command that created the process. If there are multiple processes in the job, each process will have a line showing its command and possibly its status, if it is different from the status of the previous process.
When an attempt is made to exit the shell while there are jobs in
the stopped state, the shell warns the user that there are stopped jobs and
does not exit. If another attempt is immediately made to exit the shell, the
stopped jobs are sent a SIGHUP
signal and the shell
exits. Similarly, if the nohup
option is not set and
there are running jobs when an attempt is made to exit a login shell, the
shell warns the user and does not exit. If another attempt is immediately
made to exit the shell, the running jobs are sent a
SIGHUP
signal and the shell exits.
FILES
- ~/.profile
- User's login profile.
- /etc/profile
- System login profile.
- /etc/suid_profile
- Privileged shell profile.
- /etc/shells
- Shell database.
SEE ALSO
csh(1), ed(1), ksh(1), mg(1), stty(1), vi(1), shells(5), environ(7), script(7)
Morris Bolsky and David Korn, The KornShell Command and Programming Language, 1983, ISBN 0-13-516972-0.
Stephen G. Kochan and Patrick H. Wood, UNIX Shell Programming, Hayden.
IEEE Inc., IEEE Standard for Information Technology - Portable Operating System Interface (POSIX) - Part 2: Shell and Utilities, 1993, ISBN 1-55937-266-9.
NOTES
sh
is implemented as a run-time option of
pdksh
, with only those sh
features whose syntax or semantics are incompatible with a traditional
Bourne shell disabled. Since this leaves some sh
extensions exposed, caution should be used where backwards compatibility
with traditional Bourne or POSIX compliant shells is an issue.
AUTHORS
This shell is based on the public domain 7th edition Bourne shell
clone by Charles Forsyth and parts of the BRL shell by Doug A. Gwyn, Doug
Kingston, Ron Natalie, Arnold Robbins, Lou Salkind, and others. The first
release of pdksh
was created by Eric Gisin, and it
was subsequently maintained by John R. MacMillan (change!john@sq.sq.com),
Simon J. Gerraty (sjg@zen.void.oz.au), and Michael Rendell
(michael@cs.mun.ca). The CONTRIBUTORS file in the
source distribution contains a more complete list of people and their part
in the shell's development.