NAME
csh
—
a shell (command interpreter) with
C-like syntax
SYNOPSIS
csh |
[-bcefimnstVvXx ] [argument
...] |
csh |
[-l ] |
DESCRIPTION
csh
is a command language interpreter
incorporating a history mechanism (see
History substitutions), job
control facilities (see Jobs), interactive
file name and user name completion (see
File name completion), and a
C-like syntax. It is used both as an interactive login shell and a shell
script command processor.
Argument list processing
If the first argument (argument 0) to the shell is a dash
(‘-’), then this is a login shell. A login shell also can be
specified by invoking the shell with the -l
flag as
the only argument.
The rest of the flag arguments are interpreted as follows:
-b
- This flag forces a “break” from option processing, causing any further shell arguments to be treated as non-option arguments. The remaining arguments will not be interpreted as shell options. This may be used to pass options to a shell script without confusion or possible subterfuge. The shell will not run a set-user-ID script without this option.
-c
- Commands are read from the (single) following argument which must be present. Any remaining arguments are placed in argv.
-e
- The shell exits if any invoked command terminates abnormally or yields a non-zero exit status.
-f
- The shell will start faster, because it will neither search for nor
execute commands from the file .cshrc in the
invoker's home directory. Note: if the environment variable
HOME
is not set, fast startup is the default. -i
- The shell is interactive and prompts for its top-level input, even if it appears not to be a terminal. Shells are interactive without this option if their inputs and outputs are terminals.
-l
- The shell is a login shell (only applicable if
-l
is the only flag specified). -m
- Read .cshrc, regardless of its owner and group. This option is dangerous and should only be used by su(1).
-n
- Commands are parsed, but not executed. This aids in syntactic checking of
shell scripts. When used interactively, the shell can be terminated by
pressing control-D (end-of-file character), since
exit
will not work. -s
- Command input is taken from the standard input.
-t
- A single line of input is read and executed. A backslash
(‘
\
’) may be used to escape the newline at the end of this line and continue onto another line. -V
- Causes the verbose variable to be set even before .cshrc is executed.
-v
- Causes the verbose variable to be set, with the effect that command input is echoed after history substitution.
-X
- Causes the echo variable to be set even before .cshrc is executed.
-x
- Causes the echo variable to be set, so that commands are echoed immediately before execution.
After processing of flag arguments, if arguments remain but none
of the -c
, -i
,
-s
, or -t
options were
given, the first argument is taken as the name of a file of commands to be
executed. The shell opens this file, and saves its name for possible
resubstitution by ‘$0’. Since many systems use either the
standard version 6 or version 7 shells whose shell scripts are not
compatible with this shell, the shell will execute such a
“standard” shell if the first character of a script is not a
hash mark (‘#
’); i.e., if the script
does not start with a comment. Remaining arguments initialize the variable
argv.
An instance of csh
begins by executing
commands from the file /etc/csh.cshrc and, if this
is a login shell, /etc/csh.login. It then executes
commands from .cshrc in the home directory of the
invoker, and, if this is a login shell, the file
.login in the same location. It is typical for users
on CRTs to put the command stty crt
in their
.login file, and to also invoke
tset(1) there.
In the normal case, the shell will begin reading commands from the terminal, prompting with ‘% .’ Processing of arguments and the use of the shell to process files containing command scripts will be described later.
The shell repeatedly performs the following actions: a line of command input is read and broken into “words”. This sequence of words is placed on the command history list and parsed. Finally each command in the current line is executed.
When a login shell terminates it executes commands from the files .logout in the user's home directory and /etc/csh.logout.
Lexical structure
The shell splits input lines into words at blanks and tabs with
the following exceptions. The characters
‘&
’,
‘|
’,
‘;
’,
‘<
’,
‘>
’,
‘(
’, and
‘)
’ form separate words. If doubled in
‘&&
’,
‘||
’,
‘<<
’, or
‘>>
’, these pairs form single
words. These parser metacharacters may be made part of other words, or have
their special meaning prevented, by preceding them with a backslash
(‘\
’). A newline preceded by a
‘\
’ is equivalent to a blank.
Strings enclosed in matched pairs of quotations,
‘'
’,
‘`
’, or
‘"
’, form parts of a word;
metacharacters in these strings, including blanks and tabs, do not form
separate words. These quotations have semantics to be described later.
Within pairs of ‘'
’ or
‘"
’ characters, a newline
preceded by a ‘\
’ gives a true newline
character.
When the shell's input is not a terminal, the character
‘#
’ introduces a comment that
continues to the end of the input line. This special meaning is prevented
when preceded by ‘\
’ and in quotations
using ‘`
’,
‘'
’, and
‘"
’.
Commands
A simple command is a sequence of words, the first of which
specifies the command to be executed. A simple command or a sequence of
simple commands separated by ‘|
’
characters forms a pipeline. The output of each command in a pipeline is
connected to the input of the next. Sequences of pipelines may be separated
by ‘;
’, and are then executed
sequentially. A sequence of pipelines may be executed without immediately
waiting for it to terminate by following it with a
‘&
’.
Any of the above may be placed in
‘(
’
‘)
’ to form a simple command (that may
be a component of a pipeline, for example). It is also possible to separate
pipelines with ‘||
’ or
‘&&
’ showing, as in the C
language, that the second is to be executed only if the first fails or
succeeds, respectively. (See Expressions.)
Jobs
The shell associates a
job with
each pipeline. It keeps a table of current jobs, printed by the
jobs
command, and assigns them small integer
numbers. When a job is started asynchronously with
‘&
’, the shell prints a line that
looks like:
showing that the job which was started asynchronously was job number 1 and had one (top-level) process, whose process ID was 1234.
If you are running a job and wish to do something
else you may hit ^Z
(control-Z), which sends a
SIGSTOP
signal to the current job. The shell will
then normally show that the job has been “Stopped”, and print
another prompt. You can then manipulate the state of this job, putting it in
the background
with the bg
command, or run some other commands and
eventually bring the job back into the
foreground
with the fg
command. A ^Z
takes effect immediately and is like an interrupt in that pending output and
unread input are discarded when it is typed. There is another special key
^Y
that does not generate a
SIGSTOP
signal until a program attempts to
read(2) it. This request can usefully be typed ahead when you have
prepared some commands for a job that you wish to stop after it has read
them.
A job being run in the background will stop if it tries to read
from the terminal. Background jobs are normally allowed to produce output,
but this can be disabled by giving the command stty
tostop
. If you set this tty option, then background jobs will stop
when they try to produce output like they do when they try to read
input.
There are several ways to refer to jobs in the shell. The
character ‘%
’ introduces a job name.
If you wish to refer to job number 1, you can name it as
‘%1
’. Just naming a job brings it to
the foreground; thus %1
is a synonym for
fg %1
, bringing job number 1 back into the
foreground. Similarly, saying %1 &
resumes job
number 1 in the background. Jobs can also be named by prefixes of the string
typed in to start them, if these prefixes are unambiguous; thus
%ex
would normally restart a suspended
ex(1)
job, if there were only one suspended job whose name began with the string
"ex". It is also possible to say %?string
,
which specifies a job whose text contains string, if
there is only one such job.
The shell maintains a notion of the current and previous jobs. In
output about jobs, the current job is marked with a
‘+
’ and the previous job with a
‘-
’. The abbreviation
‘%+
’ refers to the current job and
‘%-
’ refers to the previous job. For
close analogy with the syntax of the history
mechanism (described below), ‘%%
’ is
also a synonym for the current job.
The job control mechanism requires that the
stty(1) option new
be set. It is an artifact
from a new
implementation of the tty driver that allows generation of interrupt
characters from the keyboard to tell jobs to stop. See
stty(1) for details on setting options in the new tty driver.
Status reporting
The shell learns immediately whenever a process changes state. It
normally informs you whenever a job becomes blocked so that no further
progress is possible, but only just before it prints a prompt. This is done
so that it does not otherwise disturb your work. If, however, you set the
shell variable notify, the shell will notify you
immediately of changes of status in background jobs. There is also a shell
command notify
that marks a single process so that
its status changes will be immediately reported. By default
notify
marks the current process; simply say
notify
after starting a background job to mark
it.
When you try to leave the shell while jobs are stopped, you will
be warned that “You have stopped jobs”. You may use the
jobs
command to see what they are. If you try to
exit again immediately, the shell will not warn you a second time, and the
suspended jobs will be terminated.
File name completion
When the file name completion feature is enabled by setting the
shell variable filec (see
set
), csh
will interactively
complete file names and user names from unique prefixes when they are input
from the terminal followed by the escape character (the escape key, or
control-[). For example, if the current directory looks like
DSC.OLD bin cmd lib xmpl.c DSC.NEW chaosnet cmtest mail xmpl.o bench class dev mbox xmpl.out
and the input is
% vi ch<escape>
csh
will complete the prefix
“ch” to the only matching file name “chaosnet”,
changing the input line to
% vi chaosnet
However, given
% vi D<escape>
csh
will only expand the input to
% vi DSC.
and will sound the terminal bell to indicate that the expansion is
incomplete, since there are two file names matching the prefix
‘D
’.
If a partial file name is followed by the end-of-file character
(usually control-D), then, instead of completing the name,
csh
will list all file names matching the prefix.
For example, the input
% vi D<control-D>
causes all files beginning with
‘D
’ to be listed:
DSC.NEW DSC.OLD
while the input line remains unchanged.
The same system of escape and end-of-file can also be used to
expand partial user names, if the word to be completed (or listed) begins
with the tilde character (‘~
’). For
example, typing
cd ~ro<escape>
may produce the expansion
cd ~root
The use of the terminal bell to signal errors or multiple matches can be inhibited by setting the variable nobeep.
Normally, all files in the particular directory are candidates for name completion. Files with certain suffixes can be excluded from consideration by setting the variable fignore to the list of suffixes to be ignored. Thus, if fignore is set by the command
% set fignore = (.o
.out)
then typing
% vi x<escape>
would result in the completion to
% vi xmpl.c
ignoring the files "xmpl.o" and "xmpl.out". However, if the only completion possible requires not ignoring these suffixes, then they are not ignored. In addition, fignore does not affect the listing of file names by control-D. All files are listed regardless of their suffixes.
Substitutions
We now describe the various transformations the shell performs on the input in the order in which they occur.
History substitutions
History substitutions place words from previous command input as
portions of new commands, making it easy to repeat commands, repeat
arguments of a previous command in the current command, or fix spelling
mistakes in the previous command with little typing and a high degree of
confidence. History substitutions begin with the character
‘!
’ and may begin
anywhere
in the input stream (with the proviso that they do not
nest). This ‘!
’ may be preceded by a
‘\
’ to prevent its special meaning;
for convenience, a ‘!
’ character is
passed unchanged when it is followed by a blank, tab, newline,
‘=
’ or
‘(
’. (History substitutions also occur
when an input line begins with ‘^
’.
This special abbreviation will be described later.) Any input line that
contains history substitution is echoed on the terminal before it is
executed as it would have been typed without history substitution.
Commands input from the terminal that consist of one or more words are saved on the history list. The history substitutions reintroduce sequences of words from these saved commands into the input stream. The size of the history list is controlled by the history variable; the previous command is always retained, regardless of the value of the history variable. Commands are numbered sequentially from 1.
For definiteness, consider the following output from the
history
command:
09 write michael 10 ex write.c 11 cat oldwrite.c 12 diff *write.c
The commands are shown with their event numbers. It is not usually
necessary to use event numbers, but the current event number can be made
part of the prompt by placing a ‘!
’ in
the prompt string.
With the current event 13 we can refer to previous
events by event number ‘!11
’,
relatively as in ‘!-2
’ (referring to
the same event), by a prefix of a command word as in
‘!d
’ for event 12 or
‘!wri
’ for event 9, or by a string
contained in a word in the command as in
‘!?mic?
’ also referring to event 9.
These forms, without further change, simply reintroduce the words of the
specified events, each separated by a single blank. As a special case,
‘!!
’ refers to the previous command;
thus ‘!!
’ alone is a
redo.
To select words from an event we can follow the event
specification by a ‘:
’ and a
designator for the desired words. The words of an input line are numbered
from 0, the first (usually command) word being 0, the second word (first
argument) being 1, etc. The basic word designators are:
- 0
- first (command) word
- n
- n'th argument
- ^
- first argument; i.e., ‘
1
’ - $
- last argument
- %
- word matched by (immediately preceding) ?s? search
- x-y
- range of words
- -y
- abbreviates ‘
0-y
’ - *
- abbreviates ‘
^-$
’, or nothing if only 1 word in event - x*
- abbreviates ‘
x-$
’ - x-
- like ‘
x*
’ but omitting word ‘$
’
The ‘:
’ separating the event
specification from the word designator can be omitted if the argument
selector begins with a ‘^
’,
‘$
’,
‘*
’,
‘-
’, or
‘%
’. After the optional word
designator, a sequence of modifiers can be placed, each preceded by a
‘:
’. The following modifiers are
defined:
- h
- Remove a trailing pathname component, leaving the head.
- r
- Remove a trailing ‘
.xxx
’ component, leaving the root name. - e
- Remove all but the extension ‘
.xxx
’ part. - s/l/r/
- Substitute l for r.
- t
- Remove all leading pathname components, leaving the tail.
- &
- Repeat the previous substitution.
- g
- Apply the change once on each word, prefixing the above; e.g.,
‘
g&
’. - a
- Apply the change as many times as possible on a single word, prefixing the
above. It can be used together with
‘
g
’ to apply a substitution globally. - p
- Print the new command line but do not execute it.
- q
- Quote the substituted words, preventing further substitutions.
- x
- Like ‘
q
’, but break into words at blanks, tabs, and newlines.
Unless preceded by a ‘g
’ the
change is applied only to the first modifiable word. With substitutions, it
is an error for no word to be applicable.
The left-hand side of substitutions are not regular expressions in
the sense of the editors, but instead strings. Any character may be used as
the delimiter in place of ‘/
’; a
‘\
’ quotes the delimiter into the
l and r strings. The character
‘&
’ in the right-hand side is
replaced by the text from the left. A
‘\
’ also quotes
‘&
’. A
NULL
l
(‘//
’) uses the previous string either
from an l or from a contextual scan string
s in
‘!?s\?
’. The
trailing delimiter in the substitution may be omitted if a newline follows
immediately as may the trailing ‘?
’ in
a contextual scan.
A history reference may be given without an event specification;
e.g., ‘!$
’. Here, the reference is to
the previous command unless a previous history reference occurred on the
same line in which case this form repeats the previous reference. Thus
“!?foo?^ !$” gives the first and last arguments from the
command matching “?foo?”.
A special abbreviation of a history reference occurs when the
first non-blank character of an input line is a
‘^
’. This is equivalent to
“!:s^” providing a convenient shorthand for substitutions on
the text of the previous line. Thus ^lb^lib
fixes
the spelling of “lib” in the previous command. Finally, a
history substitution may be surrounded with
‘{
’ and
‘}
’ if necessary to insulate it from
the characters that follow. Thus, after ls -ld ~paul
we might do !{l}a
to do ls -ld
~paula
, while !la
would look for a command
starting with “la”.
Quotations with ´ and "
The quotation of strings by
‘'
’ and
‘"
’ can be used to prevent all or
some of the remaining substitutions. Strings enclosed in
‘'
’ are prevented from any further
interpretation. Strings enclosed in
‘"
’ may be expanded as described
below.
In both cases the resulting text becomes (all or part
of) a single word; only in one special case (see
Command
Substitution below) does a
‘"
’ quoted string yield parts of
more than one word; ‘'
’ quoted strings
never do.
Alias substitution
The shell maintains a list of aliases that can be established,
displayed and modified by the alias
and
unalias
commands. After a command line is scanned,
it is parsed into distinct commands and the first word of each command,
left-to-right, is checked to see if it has an alias. If it does, then the
text that is the alias for that command is reread with the history mechanism
available as though that command were the previous input line. The resulting
words replace the command and argument list. If no reference is made to the
history list, then the argument list is left unchanged.
Thus if the alias for “ls” is “ls -l”,
the command ls /usr
would map to ls
-l /usr
, the argument list here being undisturbed. Similarly, if the
alias for “lookup” was “grep !^ /etc/passwd”
then lookup bill
would map to grep
bill /etc/passwd
.
If an alias is found, the word transformation of the input text is performed and the aliasing process begins again on the reformed input line. Looping is prevented if the first word of the new text is the same as the old by flagging it to prevent further aliasing. Other loops are detected and cause an error.
Note that the mechanism allows aliases to introduce parser
metasyntax. Thus, we can alias print 'pr \!* | lpr'
to make a command that pr
's its arguments to the
line printer.
Variable substitution
The shell maintains a set of variables, each of which has as value a list of zero or more words. Some of these variables are set by the shell or referred to by it. For instance, the argv variable is an image of the shell's argument list, and words of this variable's value are referred to in special ways.
The values of variables may be displayed and changed by using the
set
and unset
commands. Of
the variables referred to by the shell a number are toggles; the shell does
not care what their value is, only whether they are set or not. For
instance, the verbose variable is a toggle that causes
command input to be echoed. The setting of this variable results from the
-v
command-line option.
Other operations treat variables numerically. The
@
command permits numeric calculations to be
performed and the result assigned to a variable. Variable values are,
however, always represented as (zero or more) strings. For the purposes of
numeric operations, the null string is considered to be zero, and the second
and additional words of multiword values are ignored.
After the input line is aliased and parsed, and before
each command is executed, variable substitution is performed, keyed by
‘$
’ characters. This expansion can be
prevented by preceding the ‘$
’ with a
‘\
’ except within double quotes
(`"'), where it
always occurs, and
within single quotes (`''), where it
never
occurs. Strings quoted by backticks (` `) are interpreted later (see
Command substitution below),
so ‘$
’ substitution does not occur
there until later, if at all. A ‘$
’ is
passed unchanged if followed by a blank, tab, or end-of-line.
Input/output redirections are recognized before variable expansion, and are variable expanded separately. Otherwise, the command name and entire argument list are expanded together. It is thus possible for the first (command) word (to this point) to generate more than one word, the first of which becomes the command name, and the rest of which become arguments.
Unless enclosed in ‘"
’
or given the ‘:q
’ modifier, the
results of variable substitution may eventually be command and filename
substituted. Within ‘"
’, a
variable whose value consists of multiple words expands to (a portion of) a
single word, with the words of the variable's value separated by blanks.
When the ‘:q
’ modifier is applied to a
substitution the variable will expand to multiple words with each word
separated by a blank and quoted to prevent later command or filename
substitution.
The following metasequences are provided for introducing variable values into the shell input. Except as noted, it is an error to reference a variable that is not set.
- $name
- ${name}
- Are replaced by the words of the value of variable
name, each separated by a blank. Braces insulate
name from following characters that would otherwise
be part of it. Shell variables have names consisting of up to 20 letters
and digits starting with a letter. The underscore character is considered
a letter. If name is not a shell variable, but is
set in the environment, then that value is returned (but
‘
:
’ modifiers and the other forms given below are not available here). - $name[selector]
- ${name[selector]}
- May be used to select only some of the words from the value of
name. The selector is subjected to
‘
$
’ substitution and may consist of a single number or two numbers separated by a ‘-
’. The first word of a variable's value is numbered ‘1
’. If the first number of a range is omitted it defaults to ‘1
’. If the last number of a range is omitted it defaults to ‘$#name
’. The selector ‘*
’ selects all words. It is not an error for a range to be empty if the second argument is omitted or in range. - $#name
- ${#name}
- Gives the number of words in the variable. This is useful for later use in a “$argv[selector]”.
- $0
- Substitutes the name of the file from which command input is being read. An error occurs if the name is not known.
- $number
- ${number}
- Equivalent to “$argv[number]”.
- $*
- Equivalent to “$argv[*]”.
The modifiers ‘:e
’,
‘:h
’,
‘:t
’,
‘:r
’,
‘:q
’, and
‘:x
’ may be applied to the
substitutions above as may ‘:gh
’,
‘:gt
’, and
‘:gr
’. If braces
‘{
’
‘}
’ appear in the command form then
the modifiers must appear within the braces. The current implementation
allows only one ‘:
’ modifier on each
‘$
’ expansion.
The following substitutions may not be modified with
‘:
’ modifiers.
- $?name
- ${?name}
- Substitutes the string “1” if name is set, “0” if it is not.
- $?0
- Substitutes ‘
1
’ if the current input filename is known, ‘0
’ if it is not. - $$
- Substitute the (decimal) process number of the (parent) shell. Do NOT use this mechanism for generating temporary file names; see mktemp(1) instead.
- $!
- Substitute the (decimal) process number of the last background process started by this shell.
- $<
- Substitutes a line from the standard input, with no further interpretation. It can be used to read from the keyboard in a shell script.
Command and filename substitution
The remaining substitutions, command and filename substitution, are applied selectively to the arguments of built-in commands. By selectively, we mean that portions of expressions which are not evaluated are not subjected to these expansions. For commands that are not internal to the shell, the command name is substituted separately from the argument list. This occurs very late, after input-output redirection is performed, and in a child of the main shell.
Command substitution
Command substitution is shown by a command enclosed in
‘`
’. The output from such a command is
normally broken into separate words at blanks, tabs, and newlines, with null
words being discarded; this text then replaces the original string. Within
double quotes (`"'), only newlines force new words; blanks and tabs are
preserved.
In any case, the single final newline does not force a new word. Note that it is thus possible for a command substitution to yield only part of a word, even if the command outputs a complete line.
Filename substitution
If a word contains any of the characters
‘*
’,
‘?
’,
‘[
’, or
‘{
’, or begins with the character
‘~
’, then that word is a candidate for
filename substitution, also known as “globbing”. This word is
then regarded as a pattern, and replaced with an alphabetically sorted list
of file names that match the pattern. In a list of words specifying filename
substitution it is an error for no pattern to match an existing file name,
but it is not required for each pattern to match. Only the metacharacters
‘*
’,
‘?
’, and
‘[
’ imply pattern matching, the
characters ‘~
’ and
‘{
’ being more akin to
abbreviations.
In matching filenames, the character
‘.
’ at the beginning of a filename or
immediately following a ‘/
’, as well
as the character ‘/
’ must be matched
explicitly. The character ‘*
’ matches
any string of characters, including the null string. The character
‘?
’ matches any single character.
The sequence “[...]” matches any one
of the characters enclosed. Within “[...]”, a pair of
characters separated by ‘-
’ matches
any character lexically between the two (inclusive). Within
“[...]”, 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 isalnum(3), isalpha(3), and so on. A character class may not be used as an endpoint of a range.
The character ‘~
’ at the
beginning of a filename refers to home directories. Standing alone, i.e.,
‘~
’, it expands to the invoker's home
directory as reflected in the value of the variable
home. When followed by a name consisting of letters,
digits, and ‘-
’ characters, the shell
searches for a user with that name and substitutes their home directory;
thus “~ken” might expand to “/usr/ken” and
“~ken/chmach” to “/usr/ken/chmach”. If the
character ‘~
’ is followed by a
character other than a letter or ‘/
’,
or does not appear at the beginning of a word, it is left undisturbed.
The metanotation “a{b,c,d}e” is a shorthand for
“abe ace ade”. Left to right order is preserved, with results
of matches being sorted separately at a low level to preserve this order.
This construct may be nested. Thus, “~source/s1/{oldls,ls}.c”
expands to “/usr/source/s1/oldls.c /usr/source/s1/ls.c”
without chance of error if the home directory for “source” is
“/usr/source”. Similarly “../{memo,*box}” might
expand to “../memo ../box ../mbox”. (Note that
“memo” was not sorted with the results of the match to
“*box”.) As a special case
‘{
’,
‘}
’, and
‘{}
’ are passed undisturbed.
Input/output
The standard input and the standard output of a command may be redirected with the following syntax:
- < name
- Open file name (which is first variable, command, and filename expanded) as the standard input.
- << word
- Read the shell input up to a line that is identical to
word. word is not subjected to
variable, command, or filename substitution, and each input line is
compared to word before any substitutions are done
on the input line. Unless a quoting
‘
\
’, ‘"
’, ‘'
’ or ‘`
’ appears in word, variable and command substitution is performed on the intervening lines, allowing ‘\
’ to quote ‘$
’, ‘\
’ and ‘`
’. Commands that are substituted have all blanks, tabs, and newlines preserved, except for the final newline which is dropped. The resultant text is placed in an anonymous temporary file that is given to the command as its standard input. - > name
- >! name
- >& name
- >&! name
- The file name is used as the standard output. If the
file does not exist then it is created; if the file exists, it is
truncated; its previous contents are lost.
If the variable noclobber is set, then the file must not exist or be a character special file (e.g., a terminal or /dev/null) or an error results. This helps prevent accidental destruction of files. Here, the ‘
!
’ forms can be used to suppress this check.The forms involving ‘
&
’ route the standard error output into the specified file as well as the standard output. name is expanded in the same way as ‘<
’ input filenames are. - >> name
- >>& name
- >>! name
- >>&! name
- Uses file name as the standard output; like
‘
>
’ but places output at the end of the file. If the variable noclobber is set, then it is an error for the file not to exist unless one of the ‘!
’ forms is given. Otherwise similar to ‘>
’.
A command receives the environment in which the shell was invoked
as modified by the input-output parameters and the presence of the command
in a pipeline. Thus, unlike some previous shells, commands run from a file
of shell commands have no access to the text of the commands by default;
instead they receive the original standard input of the shell. The
‘<<
’ mechanism should be used to
present inline data. This permits shell command scripts to function as
components of pipelines and allows the shell to block read its input. Note
that the default standard input for a command run detached is
not modified to be the empty file
/dev/null; instead the standard input remains as the
original standard input of the shell. If this is a terminal and if the
process attempts to read from the terminal, then the process will block and
the user will be notified (see Jobs
above).
The standard error output may be directed through a pipe with the
standard output. Simply use the form
‘|&
’ instead of just
‘|
’.
Expressions
Several of the built-in commands (to be described later) take
expressions, in which the operators are similar to those of C, with the same
precedence, but with the
opposite
grouping: right to left. These expressions appear in the
@
, exit
,
if
, and while
commands. The
following operators are available:
Here the precedence increases to the right,
‘==
’
‘!=
’
‘=~
’ and
‘!~
’,
‘<=
’
‘>=
’
‘<
’ and
‘>
’,
‘<<
’ and
‘>>
’,
‘+
’ and
‘-
’,
‘*
’
‘/
’ and
‘%
’ being, in groups, at the same
level. The ‘==
’
‘!=
’
‘=~
’ and
‘!~
’ operators compare their arguments
as strings; all others operate on numbers. The operators
‘=~
’ and
‘!~
’ are like
‘!=
’ and
‘==
’ except that the right hand side
is a pattern (containing, e.g., *'s, ?'s, and
instances of “[...]”) against which the left-hand operand is
matched. This reduces the need for use of the switch
statement in shell scripts when all that is really needed is pattern
matching.
Strings that begin with ‘0
’
are considered octal numbers. Null or missing arguments are considered
‘0
’. The results of all expressions
are strings, which represent decimal numbers. It is important to note that
no two components of an expression can appear in the same word; except when
adjacent to components of expressions that are syntactically significant to
the parser (‘&
’,
‘|
’,
‘<
’,
‘>
’,
‘(
’, and
‘)
’), they should be surrounded by
spaces.
Also available in expressions as primitive operands are command
executions enclosed in ‘{
’ and
‘}
’ and file enquiries of the form
-l
name where
l
is one of:
r read access w write access x execute access e existence o ownership z zero size f plain file d directory p named pipe (FIFO) l symbolic link
The specified name is command and filename expanded and then
tested to see if it has the specified relationship to the real user. If the
file does not exist or is inaccessible then all enquiries return false,
i.e., ‘0
’. Command executions succeed,
returning true, i.e., ‘1
’, if the
command exits with status 0, otherwise they fail, returning false, i.e.,
‘0
’. If more detailed status
information is required then the command should be executed outside an
expression and the variable status examined.
Control flow
The shell contains several commands that can be used to regulate the flow of control in command files (shell scripts) and (in limited but useful ways) from terminal input. These commands all operate by forcing the shell to reread or skip in its input and, because of the implementation, restrict the placement of some of the commands.
The foreach
,
switch
, and while
statements, as well as the if-then-else
form of the
if
statement require that the major keywords appear
in a single simple command on an input line as shown below.
If the shell's input is not seekable, the shell buffers up input whenever a loop is being read and performs seeks in this internal buffer to accomplish the rereading implied by the loop. (To the extent that this allows, backward goto's will succeed on non-seekable inputs.)
Built-in commands
Built-in commands are executed within the shell. If a built-in command occurs as any component of a pipeline except the last then it is executed in a sub-shell.
alias
alias
namealias
name wordlist- The first form prints all aliases. The second form prints the alias for name. The final form assigns the specified wordlist as the alias of name; wordlist is command and filename substituted. name is not allowed to be “alias” or “unalias”.
bg
bg %
job ...- Puts the current or specified jobs into the background, continuing them if they were stopped.
break
- Causes execution to resume after the
end
of the nearest enclosingforeach
orwhile
. The remaining commands on the current line are executed. Multi-level breaks are thus possible by writing them all on one line. breaksw
- Causes a break from a
switch
, resuming after theendsw
. case
label:- A label in a
switch
statement as discussed below. cd
cd
namechdir
chdir
name- Change the shell's working directory to directory
name. If no argument is given then change to the
home directory of the user. If name is not found as
a subdirectory of the current directory (and does not begin with
‘
/
’, ‘./
’ or ‘../
’), then each component of the variable cdpath is checked to see if it has a subdirectory name. Finally, if all else fails but name is a shell variable whose value begins with ‘/
’, then this is tried to see if it is a directory. continue
- Continue execution of the nearest enclosing
while
orforeach
. The rest of the commands on the current line are executed. default
:- Labels the default case in a
switch
statement. The default should come after allcase
labels. dirs
- Prints the directory stack; the top of the stack is at the left, the first directory in the stack being the current directory.
echo
wordlistecho
-n
wordlist- The specified words are written to the shell's standard output, separated
by spaces, and terminated with a newline unless the
-n
option is specified. else
end
endif
endsw
- See the description of the
foreach
,if
,switch
, andwhile
statements below. eval
arg ...- (As in sh(1).) The arguments are read as input to the shell and the
resulting command(s) executed in the context of the current shell. This is
usually used to execute commands generated as the result of command or
variable substitution, since parsing occurs before these substitutions.
See tset(1) for an example of using
eval
. exec
command- The specified command is executed in place of the current shell.
exit
exit
(expr)- The shell exits either with the value of the
status
variable (first form) or with the value of the specifiedexpr
(second form). fg
fg %
job ...- Brings the current or specified jobs into the foreground, continuing them if they were stopped.
foreach
name (wordlist)- ...
end
- The variable name is successively set to each member
of wordlist and the sequence of commands between
this command and the matching
end
are executed. (Bothforeach
andend
must appear alone on separate lines.) The built-in commandcontinue
may be used to continue the loop prematurely and the built-in commandbreak
to terminate it prematurely. When this command is read from the terminal, the loop is read once prompting with ‘?
’ before any statements in the loop are executed. If you make a mistake typing in a loop at the terminal you can rub it out. glob
wordlist- Like
echo
but no ‘\
’ escapes are recognized and words are delimited by NUL characters in the output. Useful for programs that wish to use the shell to filename expand a list of words. goto
word- The specified word is filename and command expanded
to yield a string of the form
‘
label
’. The shell rewinds its input as much as possible and searches for a line of the form “label:”, possibly preceded by blanks or tabs. Execution continues after the specified line. hashstat
- Print a statistics line showing how effective the internal hash table has
been at locating commands (and avoiding
exec
´s). Anexec
is attempted for each component of the path where the hash function indicates a possible hit, and in each component that does not begin with a ‘/
’. history
history
nhistory
-h
nhistory
-r
n- Displays the history event list; if n is given, only
the n most recent events are printed. The
-h
option causes the history list to be printed without leading numbers. This format produces files suitable for sourcing using the-h
option tosource
. The-r
option reverses the order of printout to be most recent first instead of oldest first. if
(expr) command- If the specified expression evaluates to true, then the single
command with arguments is executed. Variable
substitution on command happens early, at the same
time it does for the rest of the
if
command. command must be a simple command, not a pipeline, a command list, or a parenthesized command list. Input/output redirection occurs even if expr is false, i.e., when command is not executed (this is a bug). if
(expr)then
- ...
else if
(expr2)then
- ...
else
- ...
endif
- If the specified expr is true then the commands up
to the first
else
are executed; otherwise if expr2 is true then the commands up to the secondelse
are executed, etc. Any number ofelse-if
pairs are possible; only oneendif
is needed. Theelse
part is likewise optional. (The wordselse
andendif
must appear at the beginning of input lines; theif
must appear alone on its input line or after anelse
.) jobs
jobs
-l
- Lists the active jobs; the
-l
option lists process IDs in addition to the normal information. kill %
jobkill
- [
-s
signal_name] pid kill
-sig
pid ...kill
-l
[exit_status]- Sends either the
SIGTERM
(terminate) signal or the specified signal to the specified jobs or processes. Signals are either given by number or by names (as given in<signal.h>
, stripped of the prefix “SIG”). The signal names are listed by “kill -l”; if an exit_status is specified, only the corresponding signal name will be written. There is no default; just saying “kill” does not send a signal to the current job. If the signal being sent isSIGTERM
(terminate) orSIGHUP
(hangup), then the job or process will be sent aSIGCONT
(continue) signal as well. limit
limit
resourcelimit
resource maximum-uselimit
-h
limit
-h
resourcelimit
-h
resource maximum-use- Limits the consumption by the current process and each process it creates
to not individually exceed maximum-use on the
specified resource. If no
maximum-use is given, then the current limit is
printed; if no resource is given, then all
limitations are given. If the
-h
flag is given, the hard limits are used instead of the current limits. The hard limits impose a ceiling on the values of the current limits. Only the superuser may raise the hard limits, but a user may lower or raise the current limits within the legal range.Resources controllable currently include:
- cputime
- the maximum number of CPU-seconds to be used by each process.
- filesize
- the largest single file (in bytes) that can be created.
- datasize
- the maximum growth of the data+stack region via sbrk(2) beyond the end of the program text.
- stacksize
- the maximum size of the automatically-extended stack region.
- coredumpsize
- the size of the largest core dump (in bytes) that will be created.
- memoryuse
- the maximum size (in bytes) to which a process's resident set size (RSS) may grow.
- memorylocked
- The maximum size (in bytes) which a process may lock into memory using the mlock(2) function.
- maxproc
- The maximum number of simultaneous processes for this user ID.
- openfiles
- The maximum number of simultaneous open files for this user ID.
- vmemoryuse
- the maximum size (in bytes) to which a process's total size may grow.
The maximum-use may be given as a (floating point or integer) number followed by a scale factor. For all limits other than cputime the default scale is ‘
k
’ or “kilobytes” (1024 bytes); a scale factor of ‘m
’ or “megabytes” may also be used. For cputime the default scale is “seconds”; a scale factor of ‘m
’ for minutes or ‘h
’ for hours, or a time of the form “mm:ss” giving minutes and seconds also may be used.For both resource names and scale factors, unambiguous prefixes of the names suffice.
login
- Terminate a login shell, replacing it with an instance of /usr/bin/login. This is one way to log off, included for compatibility with sh(1).
logout
- Terminate a login shell. Especially useful if ignoreeof is set.
nice
nice
+numbernice
commandnice
+number command- The first form sets the scheduling priority for this shell to 4. The
second form sets the priority to the given number.
The final two forms run command at priority 4 and
number respectively. The greater the number, the
less CPU the process will get. The superuser may specify negative priority
by using “nice -number ...”. command
is always executed in a sub-shell, and the restrictions placed on commands
in simple
if
statements apply. nohup
nohup
command- The first form can be used in shell scripts to cause hangups to be ignored
for the remainder of the script. The second form causes the specified
command to be run with hangups ignored. All processes detached with
‘
&
’ are effectivelynohup
´ed. notify
notify %
job ...- Causes the shell to notify the user asynchronously when the status of the current or specified jobs change; normally notification is presented before a prompt. This is automatic if the shell variable notify is set.
onintr
onintr
-
onintr
label- Control the action of the shell on interrupts. The first form restores the
default action of the shell on interrupts, which is to terminate shell
scripts or to return to the terminal command input level. The second form
onintr -
causes all interrupts to be ignored. The final form causes the shell to execute agoto label
when an interrupt is received or a child process terminates because it was interrupted.In any case, if the shell is running detached and interrupts are being ignored, all forms of
onintr
have no meaning and interrupts continue to be ignored by the shell and all invoked commands. Finally,onintr
statements are ignored in the system startup files where interrupts are disabled (/etc/csh.cshrc, /etc/csh.login). popd
popd
+n- Pops the directory stack, returning to the new top directory. With an argument “+n” discards the n´th entry in the stack. The members of the directory stack are numbered from the top starting at 0.
pushd
pushd
namepushd
+n- With no arguments,
pushd
exchanges the top two elements of the directory stack. Given a name argument,pushd
changes to the new directory (alacd
) and pushes the old current working directory (as incwd
) onto the directory stack. With a numeric argument,pushd
rotates the n´th argument of the directory stack around to be the top element and changes to it. The members of the directory stack are numbered from the top starting at 0. rehash
- Causes the internal hash table of the contents of the directories in the
path variable to be recomputed. This is needed if
new commands are added to directories in the
path
while you are logged in. This should only be necessary if you add commands to one of your own directories, or if a systems programmer changes the contents of a system directory. repeat
count command- The specified command, which is subject to the same
restrictions as the command in the one line
if
statement above, is executed count times. I/O redirections occur exactly once, even if count is 0. set
set
nameset
name=wordset
name[index]=wordset
name=(wordlist)- The first form of the command shows the value of all shell variables.
Variables that have other than a single word as their value print as a
parenthesized word list. The second form sets name
to the null string. The third form sets name to the
single word. The fourth form sets the
index'th component of name to
word; this component must already exist. The final
form sets name to the list of words in
wordlist. The value is always command and filename
expanded.
These arguments may be repeated to set multiple values in a single set command. Note however, that variable expansion happens for all arguments before any setting occurs.
setenv
setenv
namesetenv
name value- The first form lists all current environment variables. It is equivalent
to printenv(1). The last form sets the value of environment
variable name to be value, a
single string. The second form sets name to an empty
string. The most commonly used environment variables
USER
,TERM
, andPATH
are automatically imported to and exported from thecsh
variables user, term, and path; there is no need to usesetenv
for these. shift
shift
variable- The members of
argv
are shifted to the left, discardingargv
[1]. It is an error forargv
not to be set or to have less than one word as value. The second form performs the same function on the specified variable. source
namesource
-h
name- The shell reads commands from name.
source
commands may be nested; if they are nested too deeply the shell may run out of file descriptors. An error in asource
at any level terminates all nestedsource
commands. Normally input duringsource
commands is not placed on the history list; the-h
option causes the commands to be placed on the history list without being executed. stop
stop %
job ...- Stops the current or specified jobs that are executing in the background.
suspend
- Causes the shell to stop in its tracks, much as if it had been sent a stop
signal with
^Z
. This is most often used to stop shells started by su(1). switch
(string)case
str1:- ...
breaksw
- ...
default
:- ...
breaksw
endsw
- Each case label is successively matched against the specified
string, which is first command and filename
expanded. The file metacharacters
‘
*
’, ‘?
’ and “[...]” may be used in the case labels, which are variable expanded. If none of the labels match before the “default” label is found, then the execution begins after the default label. Each case label and the default label must appear at the beginning of a line. The commandbreaksw
causes execution to continue after theendsw
. Otherwise control may fall through case labels and the default label as in C. If no label matches and there is no default, execution continues after theendsw
. time
time
command- With no argument, a summary of time used by this shell and its children is
printed. If arguments are given the specified simple command is timed and
a time summary as described under the
time
variable is printed. If necessary, an extra shell is created to print the time statistic when the command completes. umask
umask
value- The file creation mask is displayed (first form) or set to the specified value (second form). The mask is given in octal. Common values for the mask are 002 giving all access to the group and read and execute access to others or 022 giving all access except write access for users in the group or others.
unalias
pattern- All aliases whose names match the specified pattern are discarded. Thus
all aliases are removed by
unalias *
. It is not an error for nothing to beunalias
ed. unhash
- Use of the internal hash table to speed location of executed programs is disabled.
unlimit
unlimit
resourceunlimit
-h
unlimit
-h
resource- Removes the limitation on resource. If no
resource is specified, then all
resource limitations are removed. If
-h
is given, the corresponding hard limits are removed. Only the superuser may do this. unset
pattern- All variables whose names match the specified pattern are removed. Thus
all variables are removed by
unset *
; this has noticeably distasteful side-effects. It is not an error for nothing to beunset
. unsetenv
pattern- Removes all variables whose names match the specified pattern from the
environment. See also the
setenv
command above and printenv(1). wait
- Wait for all background jobs. If the shell is interactive, then an interrupt can disrupt the wait. After the interrupt, the shell prints names and job numbers of all jobs known to be outstanding.
which
command- Displays the resolved command that will be executed by the shell.
while
(expr)- ...
end
- While the specified expression evaluates to non-zero, the commands between
the
while
and the matchingend
are evaluated.break
andcontinue
may be used to terminate or continue the loop prematurely. (Thewhile
andend
must appear alone on their input lines.) Prompting occurs here the first time through the loop as for theforeach
statement if the input is a terminal. %
job- Brings the specified job into the foreground.
%
job&
- Continues the specified job in the background.
@
@
name= expr@
name[index]= expr- The first form prints the values of all the shell variables. The second
form sets the specified name to the value of
expr. If the expression contains
‘
<
’, ‘>
’, ‘&
’ or ‘|
’ then at least this part of the expression must be placed within ‘(
’ ‘)
’. The third form assigns the value of expr to the index'th argument of name. Both name and its index'th component must already exist.The operators ‘
*=
’, ‘+=
’, etc. are available as in C. The space separating the name from the assignment operator is optional. Spaces are, however, mandatory in separating components of expr, which would otherwise be single words.Special postfix ‘
++
’ and ‘--
’ operators increment and decrement name respectively; i.e., “@ i++”.
Pre-defined and environment variables
The following variables have special meaning to the shell. Of these, argv, cwd, home, path, prompt, shell and status are always set by the shell. Except for cwd and status, this setting occurs only at initialization; these variables will not then be modified unless done explicitly by the user.
The shell copies the environment variable
USER
into the variable user,
TERM
into term, and
HOME
into home, and copies
these back into the environment whenever the normal shell variables are
reset. The environment variable PATH
is likewise
handled; it is not necessary to worry about its setting other than in the
file .cshrc as inferior csh
processes will import the definition of path from the
environment, and re-export it if you then change it.
argv
- Set to the arguments to the shell, it is from this variable that positional parameters are substituted; i.e., “$1” is replaced by “$argv[1]”, etc.
cdpath
- Gives a list of alternate directories searched to find subdirectories in
chdir
commands. cwd
- The full pathname of the current directory.
echo
- Set when the
-x
command-line option is given. Causes each command and its arguments to be echoed just before it is executed. For non-built-in commands all expansions occur before echoing. Built-in commands are echoed before command and filename substitution, since these substitutions are then done selectively. filec
- Enable file name completion.
histchars
- Can be given a string value to change the characters used in history
substitution. The first character of its value is used as the history
substitution character, replacing the default character
‘
!
’. The second character of its value replaces the character ‘^
’ in quick substitutions. histfile
- Can be set to the pathname where history is going to be saved/restored.
history
- Can be given a numeric value to control the size of the history list. Any command that has been referenced in this many events will not be discarded. Too large values of history may run the shell out of memory. The last executed command is always saved on the history list.
home
- The home directory of the invoker, initialized from the environment. The filename expansion of “~” refers to this variable.
ignoreeof
- If set the shell ignores end-of-file from input devices which are terminals. This prevents shells from accidentally being killed by control-Ds.
mail
- The files where the shell checks for mail. This checking is done after
each command completion that will result in a prompt, if a specified
interval has elapsed. The shell says “You have new mail.” if
the file exists with an access time not greater than its modify time.
If the first word of the value of mail is numeric it specifies a different mail checking interval, in seconds, than the default, which is 10 minutes.
If multiple mail files are specified, then the shell says “New mail in name” when there is mail in the file name.
noclobber
- As described in the section on
Input/output, restrictions are
placed on output redirection to ensure that files are not accidentally
destroyed, and that ‘
>>
’ redirections refer to existing files. noglob
- If set, filename expansion is inhibited. This inhibition is most useful in shell scripts that are not dealing with filenames, or after a list of filenames has been obtained and further expansions are not desirable.
nonomatch
- If set, it is not an error for a filename expansion to not match any existing files; instead the primitive pattern is returned. It is still an error for the primitive pattern to be malformed; i.e., “echo [” still gives an error.
notify
- If set, the shell notifies asynchronously of job completions; the default is to present job completions just before printing a prompt.
path
- Each word of the path variable specifies a directory
in which commands are to be sought for execution. A null word specifies
the current directory. If there is no path variable
then only full path names will execute. The usual search path is
“.”, “/bin”, “/usr/bin”,
“/sbin” and “/usr/sbin”, but this may vary
from system to system. For the superuser the default search path is
“/bin”, “/usr/bin”, “/sbin”, and
“/usr/sbin”. A shell that is given neither the
-c
nor the-t
option will normally hash the contents of the directories in the path variable after reading .cshrc, and each time the path variable is reset. If new commands are added to these directories while the shell is active, it may be necessary to do arehash
or the commands may not be found. prompt
- The string that is printed before each command is read from an interactive
terminal input. If a ‘
!
’ appears in the string it will be replaced by the current event number unless a preceding ‘\
’ is given. Default is “%”, or “#” for the superuser. savehist
- Is given a numeric value to control the number of entries of the history list that are saved in ~/.history when the user logs out. Any command that has been referenced in this many events will be saved. During start up the shell sources ~/.history into the history list enabling history to be saved across logins. Too large values of savehist will slow down the shell during start up. If savehist is just set, the shell will use the value of history.
shell
- The file in which the shell resides. This variable is used in forking shells to interpret files that have execute bits set, but which are not executable by the system. (See the description of Non-built-in command execution below.) Initialized to the (system-dependent) home of the shell.
status
- The status returned by the last command. If it terminated abnormally, then 0200 is added to the status. Built-in commands that fail return exit status 1, all other built-in commands set status to 0.
time
- Controls automatic timing of commands. If set, then any command that takes more than this many CPU seconds will cause a line giving user, system, and real times, and a utilization percentage which is the ratio of user plus system times to real time to be printed when it terminates.
verbose
- Set by the
-v
command-line option, causes the words of each command to be printed after history substitution.
Non-built-in command execution
When a command to be executed is found to not be a built-in
command the shell attempts to execute the command via
execve(2). Each word in the variable path names
a directory from which the shell will attempt to execute the command. If it
is given neither a -c
nor a
-t
option, the shell will hash the names in these
directories into an internal table so that it will only try an
exec
in a directory if there is a possibility that
the command resides there. This shortcut greatly speeds command location
when many directories are present in the search path. If this mechanism has
been turned off (via unhash
), or if the shell was
given a -c
or -t
argument,
and in any case for each directory component of path
that does not begin with a ‘/
’, the
shell concatenates with the given command name to form a path name of a file
which it then attempts to execute.
Parenthesized commands are always executed in a sub-shell. Thus
(cd; pwd); pwd
prints the home directory; leaving you where you were (printing this after the home directory), while
cd; pwd
leaves you in the home directory.
Parenthesized commands are most often used to prevent
chdir
from affecting the current shell.
If the file has execute permissions but is not an executable binary to the system, then it is assumed to be a file containing shell commands and a new shell is spawned to read it.
If there is an alias for shell
then the
words of the alias will be prepended to the argument list to form the shell
command. The first word of the alias should be the full path name of the
shell (e.g., “$shell”). Note that this is a special, late
occurring, case of alias
substitution, and only
allows words to be prepended to the argument list without change.
Signal handling
The shell normally ignores SIGQUIT
signals. Jobs running detached (either by &
or
the bg
or %... &
commands) are immune to signals generated from the keyboard, including
hangups. Other signals have the values which the shell inherited from its
parent. The shell's handling of interrupts and terminate signals in shell
scripts can be controlled by onintr
. Login shells
catch the SIGTERM
(terminate) signal; otherwise this
signal is passed on to children from the state in the shell's parent.
Interrupts are not allowed when a login shell is reading the file
.logout.
LIMITATIONS
Word lengths - Words can be no longer than 1024 characters. The
number of arguments to a command that involves filename expansion is limited
to 1/6th the number of characters allowed in an argument list. Command
substitutions may substitute no more characters than are allowed in an
argument list. To detect looping, the shell restricts the number of
alias
substitutions on a single line to 20.
FILES
- ~/.cshrc
- read at beginning of execution by each shell
- ~/.login
- read by login shell, after .cshrc at login
- ~/.logout
- read by login shell, at logout
- /bin/sh
- standard shell, for shell scripts not starting with a
‘
#
’ - /tmp/sh.*
- temporary file for ‘
<<
’ - /etc/passwd
- source of home directories for “~name”
SEE ALSO
sh(1), access(2), execve(2), fork(2), pipe(2), setrlimit(2), umask(2), wait(2), killpg(3), sigvec(3), tty(4), environ(7), script(7)
HISTORY
csh
first appeared in
2BSD. It was a first implementation of a command
language interpreter incorporating a history mechanism (see
History substitutions), job
control facilities (see Jobs), interactive
file name and user name completion (see
File name completion), and a
C-like syntax. There are now many shells that also have these mechanisms,
plus a few more (and maybe some bugs too), which are available through the
usenet.
AUTHORS
Bill Joy, 1978. Job control and directory stack features first implemented by J.E. Kulp of IIASA, Laxenburg, Austria, with different syntax than that used now. File name completion code written by Ken Greer, HP Labs. Eight-bit implementation by Christos S. Zoulas, Cornell University.
BUGS
When a command is restarted from a stop, the shell prints the directory it started in if this is different from the current directory; this can be misleading (i.e., wrong) as the job may have changed directories internally.
Shell built-in functions are not stoppable/restartable. Command
sequences of the form “a ; b ; c” are also not handled
gracefully when stopping is attempted. If you suspend
‘b
’, the shell will immediately
execute ‘c
’. This is especially
noticeable if this expansion results from an alias. It suffices to place the
sequence of commands in ()'s to force it to a sub-shell; i.e., “(a ;
b ; c)”.
Control over tty output after processes are started is primitive; perhaps this will inspire someone to work on a good virtual terminal interface. In a virtual terminal interface much more interesting things could be done with output control.
Alias substitution is most often used to clumsily simulate shell procedures; shell procedures should be provided instead of aliases.
Commands within loops, prompted for by
‘?
’, are not placed on the
history
list. Control structure should be parsed
instead of being recognized as built-in commands. This would allow control
commands to be placed anywhere, to be combined with
‘|
’, and to be used with
‘&
’ and
‘;
’ metasyntax.
It should be possible to use the
‘:
’ modifiers on the output of command
substitutions.
The way the filec facility is implemented is ugly and expensive.