csh(1) csh(1)
NAME
csh - a shell (command interpreter) with C-like syntax
SYNOPSIS
csh [-c] [-e] [-f] [-i] [-n] [-s] [-t] [-v] [-V] [-x] [-X]
[arg... ]
DESCRIPTION
csh is a first implementation of a command language
interpreter incorporating a history mechanism (see ``History
Substitutions'') job control facilities (see ``Jobs'') and a
C-like syntax. So as to be able to use its job control
facilities, users of csh must (and automatically) use the
new tty driver fully described in tty(4). This new tty
driver 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.
An instance of csh begins by executing commands from the
file .cshrc in the home directory of the invoker. If this
is a login shell then it also executes commands from the
file .login there. It is typical for users on crt's to put
the command ``stty crt'' in their .login file, and to also
invoke tset(1) there.
In the normal case, the shell will then 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 then 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
then parsed. Finally each command in the current line is
executed.
When a login shell terminates it executes commands from the
file .logout in the users home directory.
Lexical structure
The shell splits input lines into words at blanks and tabs
with the following exceptions. The characters & | ; < > ( )
form separate words. If doubled in &&, ||, << or >> these
pairs form single words. These parser metacharacters may be
made part of other words, or prevented their special
meaning, by preceding them with \. A newline preceded by a
\ is equivalent to a blank.
In addition strings enclosed in matched pairs of quotations,
form parts of a word; metacharacters in these strings,
including blanks and tabs, do not form separate words.
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These quotations have semantics to be described
subsequently. 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 which continues to the end of the input
line. It is prevented this special meaning 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 an &.
Any of the above may be placed in ( ) to form a simple
command (which may be a component of a pipeline, etc.) It is
also possible to separate pipelines with || or &&
indicating, 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 which looks
like:
[1] 1234
indicating that the jobs 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 the key CONTROL-z which sends a stop signal to the
current job. The shell will then normally indicate 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 then eventually bring the job back into the foreground
with the foreground command fg. A CONTROL-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 .ft3 -Y which does not generate a stop
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signal until a program attempts to read(2) it. This can
usefully be typed ahead when you have prepared some commands
for a job which 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 1 back into the foreground.
Similarly saying ``%1&'' resumes job 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 pertaining to 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.
Status reporting
This 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 which 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
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do this or immediately try to exit again, the shell will not
warn you a second time, and the suspended jobs will be
terminated.
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 an \ to prevent
its special meaning; for convenience, a ! 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 which contains history substitution
is echoed on the terminal before it is executed as it could
have been typed without history substitution.
Commands input from the terminal which 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 which is
controlled by the history variable; the previous command is
always retained, regardless of its value. Commands are
numbered sequentially from 1.
For definiteness, consider the following output from the
history command:
9 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 an !
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
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word in the command as in !?mic? also referring to event 9.
These forms, without further modification, 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 essentially 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 a 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 ^, $, * - or %. After the optional word designator
can be placed a sequence of modifiers, each preceded by a :.
The following modifiers are defined:
h Remove a trailing pathname component, leaving the head.
r Remove a trailing .xxx component, leaving the root
name.
e Remove all but the extension .xxx part.
s/l/r/
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Substitute l for r
t Remove all leading pathname components, leaving the
tail.
& Repeat the previous substitution.
g Apply the change globally, prefixing the above, e.g.
g&.
p Print the new command but do not execute it.
q Quote the substituted words, preventing further
substitutions.
x Like q, but break into words at blanks, tabs and
newlines.
Unless preceded by a g the modification is applied only to
the first modifiable word. With substitutions, it is an
error for no word to be applicable.
The left hand side of substitutions are not regular
expressions in the sense of the editors, but rather strings.
Any character may be used as the delimiter in place of /; a
\ quotes the delimiter into the l and r strings. The
character & in the right hand side is replaced by the text
from the left. A \ quotes & also. A null l uses the
previous string either from a l or from a contextual scan
string s in !?s?. The trailing delimiter in the
substitution may be omitted if a newline follows immediately
as may the trailing ? in a contextual scan.
A history reference may be given without an event
specification, e.g. !$. In this case 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 which
follow. Thus, after ``ls -ld ~paul'' we might do !{l}a to
do ``ls -ld ~paula'', while !la would look for a command
starting la.
Quotations with ' and "
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The quotation of strings by ' and " can be used to prevent
all or some of the remaining substitutions. Strings
enclosed in ' are prevented 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
Substitition'' 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 which 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 which 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 which 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
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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 which causes
command input to be echoed. The setting of this variable
results from the -v flag 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 subsequent 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 "s where it always
occurs, and within 's where it never occurs. Strings quoted
by ` 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 variables 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 which 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 which would otherwise be part
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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 in this case).
$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
variables value is numbered 1. If the first number of
a range is omitted it defaults to 1. If the last
member 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 [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 :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.
NOTE: The current implementation allows only one : modifier
on each $ expansion.
The following substitutions may not be modified with :
modifiers.
$?name
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${?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.
$< Substitutes a line from the standard input, with no
further interpretation thereafter. 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. This means that portions of expressions
which are not evaluated are not subjected to these
expansions. For commands which 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 indicated 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 replacing the original
string. Within "s, 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 which 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.
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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.
The character ~ at the beginning of a filename is used to
refer to home directories. Standing alone, i.e. ~ it
expands to the invokers 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 appears
not 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'' whether or not these files exist
without any 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 matching *box.) As a special case {, }
and {} are passed undisturbed.
Input/output
The standard input and 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 which is identical to
word. word is not subjected to variable, filename or
command substitution, and each input line is compared
to word before any substitutions are done on this input
line. Unless a quoting \, ", ' or ` appears in word
variable and command substitution is performed on the
intervening lines, allowing \ to quote $, \ and `.
Commands which are substituted have all blanks, tabs,
and newlines preserved, except for the final newline
which is dropped. The resultant text is placed in an
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anonymous temporary file which is given to the command
as standard input.
> name
>! name
>& name
>&! name
The file name is used as standard output. If the file
does not exist then it is created; if the file exists,
its is truncated, its previous contents being 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. In this case the !
forms can be used and suppress this check. The forms
involving & route the diagnostic 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 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;
rather 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; rather 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.)
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Diagnostic output may be directed through a pipe with the
standard output. Simply use the form |& rather than just |.
Expressions
A number of the builtin commands (to be described
subsequently) take expressions, in which the operators are
similar to those of C, with the same precedence. 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 which begin with 0 are considered octal numbers.
Null or missing arguments are considered 0. The result 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 which are
syntactically significant to the parser (& | < > ( )) 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
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
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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 of an expression and the
variable status examined.
Control flow
The shell contains a number of commands which 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, due to 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
builtin command occurs as any component of a pipeline except
the last then it is executed in a subshell.
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 core in use, broken down
into used and free core, and address of the last
location in the heap. With an argument shows each used
and free block on the internal dynamic memory chain
indicating its address, size, and whether it is used or
free. This is a debugging command and may not work in
production versions of the shell; it requires a
modified version of the system memory allocator.
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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 shells 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 shells standard
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output, separated by spaces, and terminated with a
newline unless the -n flag 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 up once prompting
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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 null characters in the output. Useful for
programs which 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 indicating 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 which does not
begin with a /.
history
history n
history -r n
history -h n
Displays the history event list; if n is given only the
n most recent events are printed. The -r flag option
reverses the order of printout to be most recent first
rather than oldest first. The -h flag option causes
the history list to be printed without leading numbers.
This is used to produce files suitable for sourceing
using the -h flag option to source.
if (expr) command
If the specified expression evaluates 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, when command is not
executed (this is a bug).
if (expr) then
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...
else if (expr2) then
...
else
...
endif
If the specified expr is true then the commands to the
first else are executed; expr2 is true then the
commands 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 flag option lists process
ID's in addition to the normal information.
kill %job
kill -sig %job ...
kill pid
kill -sig pid ...
kill -l
Sends either the TERM (terminate) signal or the
specified signal to the specified jobs or processes.
Signals are either given by number or by names (as
given in /usr/include/signal.h, stripped of the prefix
``SIG''). The signal names are listed by ``kill -l''.
There is no default, saying just kill does not send a
signal to the current job. If the signal being sent is
TERM (terminate) or HUP (hangup), then the job or
process will be sent a CONT (continue) signal as well.
login
Terminate a login shell, replacing it with an instance
of /bin/login. This is one way to log off, included for
compatibility with sh(1).
logout
Terminate a login shell. Especially useful if
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ignoreeof is set.
nice
nice +number
nice command
nice +number command
The first form sets the nice for this shell to 4. The
second form sets the nice to the given number. The
final two forms run command at priority 4 and number
respectively. The super-user may specify negative
niceness by using ``nice -number ...''. Command is
always executed in a sub-shell, and the restrictions
place 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 changes;
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.
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popd
popd +n
Pops the directory stack, returning to the new top
directory. With a argument +n discards the nth entry
in the stack. The elements of the directory stack are
numbered from 0 starting at the top.
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 csw) onto the
directory stack. With a numeric argument, rotates the
nth 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 one of the system directories.
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 which have other than a
single word as 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
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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. In all cases the
value is 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 name value
Sets the value of environment variable name to be
value, a single string. The most commonly used
environment variable 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 flag option causes the
commands to be placed in the history list without being
executed.
stop
stop %job ...
Stops the current or specified job which is executing
in the background.
suspend
Causes the shell to stop in its tracks, much as if it
had been sent a stop signal with CONTROL-z. This is
most often used to stop shells started by su(1).
switch (string)
case str1:
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...
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 a 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
default labels 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 no
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.
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unhash
Use of the internal hash table to speed location of
executed programs is disabled.
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 name match the specified
pattern from the environment. See also the setenv
command above and printenv(1).
wait All background jobs are waited for. It the shell is
interactive, then an interrupt can disrupt the wait, at
which time the shell prints names and job numbers of
all jobs known to be outstanding.
while (expr)
...
end While the specified expression evaluates 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
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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 this is done explicitly by the
user.
This 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
nonbuilt-in commands all expansions occur
before echoing. Builtin commands are echoed
before command and filename substitution,
since these substitutions are then done
selectively.
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.
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history Can be given a numeric value to control the
size of the history list. Any command which
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 -d's
mail The files where the shell checks for mail.
This is done after each command completion
which 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 ``Input/output'',
restrictions are placed on output redirection
to insure that files are not accidentally
destroyed, and that >> redirections refer to
existing files.
noglob If set, filename expansion is inhibited.
This is most useful in shell scripts which
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;
rather the primitive pattern is returned. It
is still an error for the primitive pattern
to be malformed, i.e. ``echo ['' still gives
an error.
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notify If set, the shell notifies asynchronously of
job completions. The default is to rather
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
and /usr/bin, but this may vary from system
to system. For the super-user the default
search path is /etc, /bin and /usr/bin. A
shell which is given neither the -c nor the
-t flag 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 give
the rehash or the commands may not be found.
prompt The string which is printed before each
command is read from an interactive terminal
input. If a ! appears in the string it will
be replaced by the current event number
unless a preceding \ is given. Default is
``%'', or ``#'' for the super-user.
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 which 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.
shell The file in which the shell resides. This is
used in forking shells to interpret files
which have execute bits set, but which are
not executable by the system. (See the
description of ``Nonbuilt-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 which fail
return exit status 1, all other builtin
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commands set status 0.
time Controls automatic timing of commands. If
set, then any command which 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 flag option, causes the words
of each command to be printed after history
substitution.
Nonbuilt-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 flag 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
greatly speeds command location when a large number of
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 which 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 subshell.
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 modification.
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Argument list processing
If argument 0 to the shell is - then this is a login shell.
The flag options are interpreted as follows:
-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 invokers home directory.
-i The shell is interactive and prompts for its top-level
input, even if it appears to not be a terminal. Shells
are interactive without this option if their inputs and
outputs are terminals.
-n Commands are parsed, but not executed. This aids in
syntactic checking of shell scripts.
-s Command input is taken from the standard input.
-t A single line of input is read and executed. A \ 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, with the effect
that command input is echoed after history
substitution.
-x Causes the echo variable to be set, so that commands
are echoed immediately before execution.
-V Causes the verbose variable to be set even before
.cshrc is executed.
-X Is to -x as -V is to -v.
After processing of flag arguments if arguments remain but
none of the -c, -i, -s, or -t flag options was 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 Bourne shell (/bin/sh) 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 #, i.e. if the script does not start with a
comment. Remaining arguments initialize the variable argv.
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Signal handling
The shell normally ignores quit 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 shells handling of
interrupts and terminate signals in shell scripts can be
controlled by onintr. Login shells catch the terminate
signal; otherwise this signal is passed on to children from
the state in the shell's parent. In no case are interrupts
allowed when a login shell is reading the file .logout.
FILES
~/.cshrc Read at beginning of execution by each shell.
/etc/cshrc
Global file read by login shell before ~/.cshrc
~/.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.
LIMITATIONS
Words can be no longer than 1024 characters. The system
limits argument lists to 10240 characters. The number of
arguments to a command which involves filename expansion is
limited to 1/6'th 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.
SEE ALSO
sh(1), access(2), exec(2), fork(2), killpg(2), pipe(2),
sigvec(2), umask(2), wait(2), tty(4), a.out(5), environ(5),
``C Shell Reference'' in Oreo User Interface.
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.
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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 then 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 subshell, 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 rather
than aliases.
Commands within loops, prompted for by ?, are not placed in
the history list. Control structure should be parsed rather
than 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. All and more than one : modifier
should be allowed on $ substitutions.
Symbolic links fool the shell. In particular, dirs and ``cd
..'' don't work properly once you've crossed through a
symbolic link.
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