Provided by: csh_20230828-1_amd64 
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 not execute user or system startup files, load saved
history or perform command hashing. 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:
[1] 1234
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 name
alias 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 enclosing foreach or while. 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 the endsw.
case label:
A label in a switch statement as discussed below.
cd
cd name
chdir
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 or foreach. 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 all case
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 wordlist
echo -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, and while 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 specified expr (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. (Both foreach and end
must appear alone on separate lines.) The built-in command continue may be used to
continue the loop prematurely and the built-in command break 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). An exec 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 n
history -h n
history -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 to source. 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 second else are executed, etc. Any
number of else-if pairs are possible; only one endif is needed. The else part is likewise
optional. (The words else and endif must appear at the beginning of input lines; the if
must appear alone on its input line or after an else.)
jobs
jobs -l
Lists the active jobs; the -l option lists process IDs in addition to the normal
information.
kill %job
kill [-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 is
SIGTERM (terminate) or SIGHUP (hangup), then the job or process will be sent a SIGCONT
(continue) signal as well.
limit
limit resource
limit resource maximum-use
limit -h
limit -h resource
limit -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 (in bytes) of the data segment.
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. Not enforced.
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 +number
nice command
nice +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 effectively nohup'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 a goto 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 name
pushd +n
With no arguments, pushd exchanges the top two elements of the directory stack. Given a
name argument, pushd changes to the new directory (ala cd) and pushes the old current
working directory (as in cwd) 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 name
set name=word
set name[index]=word
set 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 name
setenv 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, and PATH are automatically imported to and exported from the csh variables
user, term, and path; there is no need to use setenv for these.
shift
shift variable
The members of argv are shifted to the left, discarding argv[1]. It is an error for argv
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 name
source -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 a source at any level
terminates all nested source commands. Normally input during source 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 command breaksw
causes execution to continue after the endsw. 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 the endsw.
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. The chmod(2) manual page provides the complete
list of bits that can be set.
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 be unaliased.
unhash Use of the internal hash table to speed location of executed programs is disabled.
unlimit
unlimit resource
unlimit -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 be unset.
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 matching end are evaluated. break and continue may be used to terminate or continue
the loop prematurely. (The while and end must appear alone on their input lines.)
Prompting occurs here the first time through the loop as for the foreach 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 a rehash 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 the first part of the hostname, followed by “%”, 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
ksh(1), sh(1), tty(4), environ(7), script(7)
W. Joy, An Introduction to the C shell.
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.
Debian March 30, 2023 CSH(1)