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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:

(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 in case 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 of in and esac e.g. case $foo { *) echo bar; }. The exit status of a case 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 of do and done e.g. for i; { echo $i; }. The exit status of a for 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 the if and elifs fail (i.e. exit with non-zero status), the list following the else is executed. The exit status of an if 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 more IFS characters) is entered, the menu is reprinted without executing list.

When list completes, the enumerated list is printed if REPLY is NULL, the prompt is printed, and so on. This process continues until an end-of-file is read, an interrupt is received, or a break 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 of do and done e.g. select i; { echo $i; }. The exit status of a select statement is zero if a break 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 a while 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 the function 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 if IFS is NULL).

@

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 losing NULL 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 as PATH for those directories not beginning with ‘/’ in cd commands. Note that if CDPATH is set and does not contain ‘.’ or contains an empty path, the current directory is not searched. Also, the cd 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 and kill -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 using PATH. 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 causes getopts 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 the fc command below). A literal ‘!’ can be put in the prompt by placing ‘!!’ in PS1. 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 command cmd < 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 the noclobber 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:

( )

A parameter that is NULL or unset evaluates to 0. 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:

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 so getopts can be used properly both inside and outside the function (Bourne-style functions leave OPTIND untouched, so using getopts inside a function interferes with using getopts 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 in posix mode, the ENV 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 the verbose or xtrace options; in non-POSIX mode, it does.
• set exit status. In POSIX mode, the exit status of set is 0 if there are no errors; in non-POSIX mode, the exit status is that of any command substitutions performed in generating the set 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, and typeset 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, or while 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. A NULL path means the current directory. If dir is found in any component of the CDPATH search path other than the NULL path, the name of the new working directory will be written to standard output. If dir is missing, the home directory HOME is used. If dir is ‘-’, the previous working directory is used (see the OLDPWD parameter).

If the -L option (logical path) is used or if the physical option isn't set (see the set 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 the physical option is set, ‘..’ is relative to the filesystem directory tree. The PWD and OLDPWD 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 of PATH (the actual value of the default path is system dependent: on POSIX-ish systems, it is the value returned by getconf CS_PATH).

continue [level]

Jumps to the beginning of the levelth inner-most for, until, or while 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 the print 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 the posix 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 case export 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 predefined alias 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 to getopts in the shell parameter OPTIND. 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 parameter OPTARG.

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 resetting OPTIND, 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 the physical option isn't set (see the set command below), the logical path is printed (i.e. the path used to cd to the current directory). If the -P option (physical path) is used or if the physical 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 to NULL, 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, the REPLY 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 -un 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 case readonly 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 as exit. Note that pdksh treats both profile and ENV 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 as if, until, while, or ! statements. For && or ||, only the status of the last command is tested.

-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 the set 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 the SIGHUP signal).

nolog

No effect. In the original Korn shell, this prevents function definitions from being stored in the history file.

physical

Causes the cd and pwd 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 the PWD parameter; only the cd command changes PWD. See the cd and pwd 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 of if and while 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 next PS1 prompt) after receipt of one of the signals. signal is the name of a signal (e.g. PIPE or ALRM) or the number of the signal (see the kill -l command above).

There are two special signals: EXIT (also known as 0), which is executed when the shell is about to exit, and ERR, which is executed after an error occurs (an error is something that would cause the shell to exit if the -e or errexit option were set - see the set 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 of trap commands, the current state of the traps that have been set since the shell started. Note that the output of trap 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 of ERR and EXIT 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. If typeset 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, the xtrace (-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 (see kill -l exit-status above); if the last specified job can't be found (because it never existed, or had already finished), the exit status of wait 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 a SIGHUP, SIGINT, or SIGQUIT 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 to command -v except that whence will find reserved words and won't print aliases as alias commands. With the -v option, whence is the same as command -V. Note that for whence, the -p option does not affect the search path used, as it does for command. 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.