csh
CSH(1) BSD General Commands Manual CSH(1)
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), interac-
tive 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 confu-
sion 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 with-
out 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 con-
tinue onto another line.
-V Causes the verbose variable to be set even before .cshrc is exe-
cuted.
-v Causes the verbose variable to be set, with the effect that com-
mand 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 sys-
tems 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 ar-
guments 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 termi-
nal, 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 cur-
rent 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 fol-
lowing 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 pre-
ceded 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 mean-
ing 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 com-
mands 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, respec-
tively. (See Expressions.)
Jobs
The shell associates a job with each pipeline. It keeps a table of cur-
rent 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 an-
other 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 out-
put 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 out-
put 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 cur-
rent 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 nor-
mally 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 im-
mediately 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 ex-
ample, 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 incom-
plete, 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 par-
tial 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 considera-
tion by setting the variable fignore to the list of suffixes to be ig-
nored. 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 comple-
tion possible requires not ignoring these suffixes, then they are not ig-
nored. 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 in-
put 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 se-
quences of words from these saved commands into the input stream. The
size of the history list is controlled by the history variable; the pre-
vious 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 nec-
essary 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 substitu-
tions.
x Like 'q', but break into words at blanks, tabs, and new-
lines.
Unless preceded by a 'g' the change is applied only to the first modifi-
able word. With substitutions, it is an error for no word to be applica-
ble.
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.,
'!$'. Here, the reference is to the previous command unless a previous
history reference occurred on the same line in which case this form re-
peats the previous reference. Thus "!?foo?^ !$" 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 pre-
vented 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 in-
put 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 per-
formed 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 nu-
meric 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 con-
sidered to be zero, and the second and additional words of multiword val-
ues 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 '$' substitu-
tion does not occur there until later, if at all. A '$' is passed un-
changed 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 argu-
ments.
Unless enclosed in '"' or given the ':q' modifier, the results of vari-
able 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 sep-
arated 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 fol-
lowing 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 charac-
ter is considered a letter. If name is not a shell vari-
able, but is set in the environment, then that value is re-
turned (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 de-
faults 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 back-
ground 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 ap-
plied selectively to the arguments of built-in commands. By selectively,
we mean that portions of expressions which are not evaluated are not sub-
jected 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 re-
places 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 sub-
stitution, 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 substitu-
tion 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 charac-
ter.
The sequence "[...]" matches any one of the characters enclosed. Within
"[...]", a pair of characters separated by '-' matches any character lex-
ically 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 ctype(3). A charac-
ter class may not be used as an endpoint of a range.
The character '~' at the beginning of a filename refers to home directo-
ries. Standing alone, i.e., '~', it expands to the invoker's home direc-
tory 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 re-
sults 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 file-
name 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 file-
name substitution, and each input line is compared to word
before any substitutions are done on the input line. Un-
less a quoting '\', '"', ''' or '`' appears in word, vari-
able and command substitution is performed on the interven-
ing lines, allowing '\' to quote '$', '\' and '`'. Com-
mands that are substituted have all blanks, tabs, and new-
lines preserved, except for the final newline which is
dropped. The resultant text is placed in an anonymous tem-
porary 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 acci-
dental 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 un-
less one of the '!' forms is given. Otherwise similar to
'>'.
A command receives the environment in which the shell was invoked as mod-
ified 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; in-
stead they receive the original standard input of the shell. The '<<'
mechanism should be used to present inline data. This permits shell com-
mand scripts to function as components of pipelines and allows the shell
to block read its input. Note that the default standard input for a com-
mand 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 termi-
nal, 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 stan-
dard output. Simply use the form '|&' instead of just '|'.
Expressions
Several of the built-in commands (to be described later) take expres-
sions, in which the operators are similar to those of C, with the same
precedence, but with the opposite grouping: right to left. These expres-
sions appear in the @, exit, if, and while commands. The following oper-
ators 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 miss-
ing 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 signif-
icant to the parser ('&', '|', '<', '>', '(', and ')'), they should be
surrounded by spaces.
Also available in expressions as primitive operands are command execu-
tions 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
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 com-
mand 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 exam-
ined.
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 sin-
gle 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 accom-
plish the rereading implied by the loop. (To the extent that this al-
lows, 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".
alloc Shows the amount of dynamic memory acquired, broken down
into used and free memory. With an argument shows the num-
ber of free and used blocks in each size category. The
categories start at size 8 and double at each step. This
command's output may vary across system types, since sys-
tems other than the VAX may use a different memory alloca-
tor.
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 en-
closing 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 be-
gins with '/', then this is tried to see if it is a direc-
tory.
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 com-
mands generated as the result of command or variable sub-
stitution, 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 vari-
able (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 pro-
grams 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. Execu-
tion 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 sub-
stitution 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 paren-
thesized 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 speci-
fied 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 speci-
fied, 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 cur-
rent 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-ex-
tended 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) func-
tion.
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 inte-
ger) 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 pre-
fixes 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 ef-
fectively 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 auto-
matic 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 inter-
rupts, 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 re-
ceived or a child process terminates because it was inter-
rupted.
In any case, if the shell is running detached and inter-
rupts are being ignored, all forms of onintr have no mean-
ing and interrupts continue to be ignored by the shell and
all invoked commands. Finally, onintr statements are ig-
nored in the system startup files where interrupts are dis-
abled (/etc/csh.cshrc, /etc/csh.login).
popd
popd +n
Pops the directory stack, returning to the new top direc-
tory. 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 num-
bered from the top starting at 0.
rehash Causes the internal hash table of the contents of the di-
rectories 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 direc-
tory.
repeat count command
The specified command, which is subject to the same re-
strictions as the command in the one line if statement
above, is executed count times. I/O redirections occur ex-
actly 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 ex-
ist. The final form sets name to the list of words in
wordlist. The value is always command and filename ex-
panded.
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 dur-
ing 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 speci-
fied 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 speci-
fied simple command is timed and a time summary as de-
scribed 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 ac-
cess 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 be unaliased.
unhash Use of the internal hash table to speed location of exe-
cuted 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 er-
ror for nothing to be unset.
unsetenv pattern
Removes all variables whose names match the specified pat-
tern 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 inter-
rupt, 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 evalu-
ated. break and continue may be used to terminate or con-
tinue 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 vari-
ables. 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 initial-
ization; 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 envi-
ronment 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 im-
port 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., "$1" is re-
placed by "$argv[1]", etc.
cdpath Gives a list of alternate directories searched to find subdi-
rectories 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 exe-
cuted. 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 de-
fault character '!'. The second character of its value re-
places 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 his-
tory 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 al-
ways saved on the history list.
home The home directory of the invoker, initialized from the envi-
ronment. The filename expansion of "~" refers to this vari-
able.
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 de-
fault, 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 ac-
cidentally destroyed, and that '>>' redirections refer to ex-
isting files.
noglob If set, filename expansion is inhibited. This inhibition is
most useful in shell scripts that are not dealing with file-
names, or after a list of filenames has been obtained and fur-
ther 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 re-
turned. 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 speci-
fies 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 preced-
ing '\' 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 descrip-
tion 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 ab-
normally, 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 com-
mand 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 exe-
cute 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 af-
ter 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 (ei-
ther 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 in-
terrupts 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 par-
ent. 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 sub-
stitutions may substitute no more characters than are allowed in an argu-
ment 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), a.out(5), environ(7), script(7)
HISTORY
csh appeared in 3BSD. It was a first implementation of a command lan-
guage 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
William Joy. 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 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 in-
ternally.
Shell built-in functions are not stoppable/restartable. Command se-
quences 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 proce-
dures; 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 any-
where, to be combined with '|', and to be used with '&' and ';' metasyn-
tax.
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.
BSD May 2, 2011 BSD
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