NAME
csh —
a shell (command interpreter)
with C-like syntax
SYNOPSIS
csh |
[-bcefinstvVxX]
[arg ...] |
DESCRIPTION
The
csh is a command language interpreter incorporating a
history mechanism (see
History
substitutions), job control facilities (see
Jobs), interactive file name and user name
completion (see
File Name
Completion), and a C-like syntax. It is used both as an interactive login
shell and a shell script command processor.
Argument list processing
If the first argument (argument 0) to the shell is
‘
-
’, then this is a
login shell. A login shell also can be specified by invoking the shell with
the ‘
-l
’ flag as the
only argument.
The rest of the flag arguments are interpreted as follows:
-
-
- -b
- This flag forces a ``break'' from option processing,
causing any further shell arguments to be treated as non-option arguments.
The remaining arguments will not be interpreted as shell options. This may
be used to pass options to a shell script without confusion or possible
subterfuge. The shell will not run a set-user ID script without this
option.
-
-
- -c
- Commands are read from the (single) following argument
which must be present. Any remaining arguments are placed in
argv.
-
-
- -e
- The shell exits if any invoked command terminates
abnormally or yields a non-zero exit status.
-
-
- -f
- The shell will start faster, because it will neither search
for nor execute commands from the file .cshrc in the
invoker's home directory.
-
-
- -i
- The shell is interactive and prompts for its top-level
input, even if it appears not to be a terminal. Shells are interactive
without this option if their inputs and outputs are terminals.
-
-
- -l
- The shell is a login shell (only applicable if
-l is the only flag specified).
-
-
- -m
- Read .cshrc even if not owned by the
user. This flag is normally given only by
su(1).
-
-
- -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 options were given, the first argument is taken as the
name of a file of commands to be executed. The shell opens this file, and
saves its name for possible resubstitution by `$0'. Since many systems use
either the standard version 6 or version 7 shells whose shell scripts are not
compatible with this shell, the shell will execute such a `standard' shell if
the first character of a script is not a `#', i.e., if the script does not
start with a comment. Remaining arguments initialize the variable
argv.
An instance of
csh begins by executing commands from the file
/etc/csh.cshrc and, if this is a login shell,
/etc/csh.login. It then executes commands from
.cshrc in the
home directory of the
invoker, and, if this is a login shell, the file
.login in
the same location. It is typical for users on 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 begin reading commands from the terminal,
prompting with `% '. Processing of arguments and the use of the shell to
process files containing command scripts will be described later.
The shell repeatedly performs the following actions: a line of command input is
read and broken into
words. This sequence of words is
placed on the command history list and parsed. Finally each command in the
current line is executed.
When a login shell terminates it executes commands from the files
.logout in the user's
home directory
and
/etc/csh.logout.
Lexical structure
The shell splits input lines into words at blanks and tabs with the following
exceptions. The characters `&' `|' `;' `<' `>' `(' `)' 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.
Strings enclosed in matched pairs of quotations, `'', ``' or `"', form
parts of a word; metacharacters in these strings, including blanks and tabs,
do not form separate words. These quotations have semantics to be described
later. Within pairs of `´' or `"' characters, a newline preceded by
a `\' gives a true newline character.
When the shell's input is not a terminal, the character `#' introduces a comment
that continues to the end of the input line. 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 (that may be
a component of a pipeline, etc.). It is also possible to separate pipelines
with `||' or `&&' showing, as in the C language, that the second is to
be executed only if the first fails or succeeds respectively. (See
Expressions.)
Jobs
The shell associates a
job with each pipeline. It keeps a
table of current jobs, printed by the
jobs command, and
assigns them small integer numbers. When a job is started asynchronously with
`&', the shell prints a line that looks like:
[1]
1234
showing that the job which was started asynchronously was job number 1 and had
one (top-level) process, whose process id was 1234.
If you are running a job and wish to do something else you may hit the key
^Z (control-Z) which sends a STOP signal to the current job.
The shell will then normally show that the job has been `Stopped', and print
another prompt. You can then manipulate the state of this job, putting it in
the
background with the
bg command, or
run some other commands and eventually bring the job back into the foreground
with the
foreground command
fg. A
^Z takes effect immediately and is like an interrupt in that
pending output and unread input are discarded when it is typed. There is
another special key
^Y that does not generate a STOP signal
until a program attempts to
read(2) it. This request can
usefully be typed ahead when you have prepared some commands for a job that
you wish to stop after it has read them.
A job being run in the background will stop if it tries to read from the
terminal. Background jobs are normally allowed to produce output, but this can
be disabled by giving the command ``stty tostop''. If you set this tty option,
then background jobs will stop when they try to produce output like they do
when they try to read input.
There are several ways to refer to jobs in the shell. The character `%'
introduces a job name. If you wish to refer to job number 1, you can name it
as `%1'. Just naming a job brings it to the foreground; thus `%1' is a synonym
for `fg %1', bringing job number 1 back into the foreground. Similarly saying
`%1 &' resumes job number 1 in the background. Jobs can also be named by
prefixes of the string typed in to start them, if these prefixes are
unambiguous, thus `%ex' would normally restart a suspended
ex(1) job, if there were only one
suspended job whose name began with the string `ex'. It is also possible to
say `%?string' which specifies a job whose text contains
string, if there is only one such job.
The shell maintains a notion of the current and previous jobs. In output about
jobs, the current job is marked with a `+' and the previous job with a `-'.
The abbreviation `%+' refers to the current job and `%-' refers to the
previous job. For close analogy with the syntax of the
history mechanism (described below), `%%' is also a
synonym for the current job.
The job control mechanism requires that the
stty(1) option
new be set. It is an artifact from a
new
implementation of the tty driver that allows generation of interrupt
characters from the keyboard to tell jobs to stop. See
stty(1) for details on setting
options in the new tty driver.
Status reporting
The shell learns immediately whenever a process changes state. It normally
informs you whenever a job becomes blocked so that no further progress is
possible, but only just before it prints a prompt. This is done so that it
does not otherwise disturb your work. If, however, you set the shell variable
notify, the shell will notify you immediately of changes
of status in background jobs. There is also a shell command
notify that marks a single process so that its status
changes will be immediately reported. By default
notify
marks the current process; simply say `notify' after starting a background job
to mark it.
When you try to leave the shell while jobs are stopped, you will be warned that
`You have stopped jobs.' You may use the
jobs command to
see what they are. If you try to exit again immediately, the shell will not
warn you a second time, and the suspended jobs will be terminated.
File Name Completion
When the file name completion feature is enabled by setting the shell variable
filec (see
set),
csh
will interactively complete file names and user names from unique prefixes,
when they are input from the terminal followed by the escape character (the
escape key, or control-[) For example, if the current directory looks like
DSC.OLD bin cmd lib xmpl.c
DSC.NEW chaosnet cmtest mail xmpl.o
bench class dev mbox xmpl.out
and the input is
% vi ch<escape>
csh will complete the prefix ``ch'' to the only matching file
name ``chaosnet'', changing the input line to
% vi chaosnet
However, given
% vi D<escape>
csh will only expand the input to
% vi DSC.
and will sound the terminal bell to indicate that the expansion is incomplete,
since there are two file names matching the prefix ``D''.
If a partial file name is followed by the end-of-file character (usually
control-D), then, instead of completing the name,
csh will
list all file names matching the prefix. For example, the input
% vi D<control-D>
causes all files beginning with ``D'' to be listed:
DSC.NEW DSC.OLD
while the input line remains unchanged.
The same system of escape and end-of-file can also be used to expand partial
user names, if the word to be completed (or listed) begins with the character
``~''. For example, typing
cd ~ro<escape>
may produce the expansion
cd ~root
The use of the terminal bell to signal errors or multiple matches can be
inhibited by setting the variable
nobeep.
Normally, all files in the particular directory are candidates for name
completion. Files with certain suffixes can be excluded from consideration by
setting the variable
fignore to the list of suffixes to
be ignored. Thus, if
fignore is set by the command
% set fignore = (.o .out)
then typing
% vi x<escape>
would result in the completion to
% vi xmpl.c
ignoring the files "xmpl.o" and "xmpl.out". However, if the
only completion possible requires not ignoring these suffixes, then they are
not ignored. In addition,
fignore does not affect the
listing of file names by control-D. All files are listed regardless of their
suffixes.
Substitutions
We now describe the various transformations the shell performs on the input in
the order in which they occur.
History substitutions
History substitutions place words from previous command input as portions of new
commands, making it easy to repeat commands, repeat arguments of a previous
command in the current command, or fix spelling mistakes in the previous
command with little typing and a high degree of confidence. History
substitutions begin with the character `!' and may begin
anywhere in the input stream (with the proviso that they
do not nest.) This `!' may be preceded by a `\' to prevent
its special meaning; for convenience, an `!' is passed unchanged when it is
followed by a blank, tab, newline, `=' or `('. (History substitutions also
occur when an input line begins with `↑'. This special abbreviation will
be described later.) Any input line that contains history substitution is
echoed on the terminal before it is executed as it would have been typed
without history substitution.
Commands input from the terminal that consist of one or more words are saved on
the history list. The history substitutions reintroduce sequences of words
from these saved commands into the input stream. The size of the history list
is controlled by the
history variable; the previous
command is always retained, regardless of the value of the history variable.
Commands are numbered sequentially from 1.
For example, consider the following output from the
history command:
09 write michael
10 ex write.c
11 cat oldwrite.c
12 diff *write.c
The commands are shown with their event numbers. It is not usually necessary to
use event numbers, but the current event number can be made part of the
prompt by placing 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 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 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/
- Substitute l for
r
- t
- Remove all leading pathname components, leaving the
tail.
- &
- Repeat the previous substitution.
- g
- Apply the change once on each word, prefixing the above,
e.g., `g&'.
- a
- Apply the change as many times as possible on a single
word, prefixing the above. It can be used together with `g' to apply a
substitution globally.
- p
- Print the new command line but do not execute it.
- q
- Quote the substituted words, preventing further
substitutions.
- x
- Like q, but break into words at blanks, tabs and
newlines.
Unless preceded by a `g' the change is applied only to the first modifiable
word. With substitutions, it is an error for no word to be applicable.
The left hand side of substitutions are not regular expressions in the sense of
the editors, but instead strings. Any character may be used as the delimiter
in place of `/'; a `\' quotes the delimiter into the
l
and
r strings. The character `&' in the right hand
side is replaced by the text from the left. A `\' also quotes `&'. A null
l (`//') uses the previous string either from an
l or from a contextual scan string
s in
`!?s\?'. The trailing delimiter
in the substitution may be omitted if a newline follows immediately as may the
trailing `?' in a contextual scan.
A history reference may be given without an event specification, e.g., `!$'.
Here, the reference is to the previous command unless a previous history
reference occurred on the same line in which case this form repeats the
previous reference. Thus `!?foo?↑ !$' gives the first and last arguments
from the command matching `?foo?'.
A special abbreviation of a history reference occurs when the first non-blank
character of an input line is a `↑'. This is equivalent to `!:s↑'
providing a convenient shorthand for substitutions on the text of the previous
line. Thus `↑lb↑lib' fixes the spelling of `lib' in the previous
command. Finally, a history substitution may be surrounded with `{' and `}' if
necessary to insulate it from the characters that follow. Thus, after `ls -ld
~paul' we might do `!{l}a' to do `ls -ld ~paula', while `!la' would look for a
command starting with `la'.
Quotations with ´ and
"
The quotation of strings by `´' and `"' can be used to prevent all or
some of the remaining substitutions. Strings enclosed in `´' are
prevented any further interpretation. Strings enclosed in `"' may be
expanded as described below.
In both cases the resulting text becomes (all or part of) a single word; only in
one special case (see
Command Substitution below) does a
`"' quoted string yield parts of more than one word; `´' quoted
strings never do.
Alias substitution
The shell maintains a list of aliases that can be established, displayed and
modified by the
alias and
unalias
commands. After a command line is scanned, it is parsed into distinct commands
and the first word of each command, left-to-right, is checked to see if it has
an alias. If it does, then the text that is the alias for that command is
reread with the history mechanism available as though that command were the
previous input line. The resulting words replace the command and argument
list. If no reference is made to the history list, then the argument list is
left unchanged.
Thus if the alias for `ls' is `ls -l' the command `ls /usr' would map to `ls -l
/usr', the argument list here being undisturbed. Similarly if the alias for
`lookup' was `grep !↑ /etc/passwd' then `lookup bill' would map to `grep
bill /etc/passwd'.
If an alias is found, the word transformation of the input text is performed and
the aliasing process begins again on the reformed input line. Looping is
prevented if the first word of the new text is the same as the old by flagging
it to prevent further aliasing. Other loops are detected and cause an error.
Note that the mechanism allows aliases to introduce parser metasyntax. Thus, we
can `alias print ´pr \!* | lpr´' to make a command that
pr's its arguments to the line printer.
Variable substitution
The shell maintains a set of variables, each of which has as value a list of
zero or more words. Some of these variables are set by the shell or referred
to by it. For instance, the
argv variable is an image of
the shell's argument list, and words of this variable's value are referred to
in special ways.
The values of variables may be displayed and changed by using the
set and
unset commands. Of the
variables referred to by the shell a number are toggles; the shell does not
care what their value is, only whether they are set or not. For instance, the
verbose variable is a toggle that causes command input
to be echoed. The setting of this variable results from the
-v command line option.
Other operations treat variables numerically. The `@' command permits numeric
calculations to be performed and the result assigned to a variable. Variable
values are, however, always represented as (zero or more) strings. For the
purposes of numeric operations, the null string is considered to be zero, and
the second and additional words of multiword values are ignored.
After the input line is aliased and parsed, and before each command is executed,
variable substitution is performed keyed by `$' characters. This expansion can
be prevented by preceding the `$' with a `\' except within `"'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 variable's value separated
by blanks. When the `:q' modifier is applied to a substitution the variable
will expand to multiple words with each word separated by a blank and quoted
to prevent later command or filename substitution.
The following metasequences are provided for introducing variable values into
the shell input. Except as noted, it is an error to reference a variable that
is not set.
- $name
-
- ${name}
- Are replaced by the words of the value of variable
name, each separated by a blank. Braces insulate
name from following characters that would otherwise
be part of it. Shell variables have names consisting of up to 20 letters
and digits starting with a letter. The underscore character is considered
a letter. If name is not a shell variable, but is
set in the environment, then that value is returned (but `:' modifiers and
the other forms given below are not available here).
- $name[selector]
-
- ${name[selector]}
- May be used to select only some of the words from the value
of name. The selector is subjected to `$'
substitution and may consist of a single number or two numbers separated
by a `-'. The first word of a variable's value is numbered `1'. If the
first number of a range is omitted it defaults to `1'. If the last number
of a range is omitted it defaults to `$#name'. The selector `*' selects
all words. It is not an error for a range to be empty if the second
argument is omitted or in range.
- $#name
-
- ${#name}
- Gives the number of words in the variable. This is useful
for later use in a `$argv[selector]'.
- $0
- Substitutes the name of the file from which command input
is being read. An error occurs if the name is not known.
- $number
-
- ${number}
- Equivalent to `$argv[number]'.
- $*
- Equivalent to `$argv[*]'.
The modifiers `:e', `:h', `:t', `:r', `:q' and `:x' may be applied to the
substitutions above as may `:gh', `:gt' and `:gr'. If braces `{' '}' appear in
the command form then the modifiers must appear within the braces. The current
implementation allows only one `:' modifier on each `$' expansion.
The following substitutions may not be modified with `:' modifiers.
- $?name
-
- ${?name}
- Substitutes the string `1' if name is set, `0' if it is
not.
- $?0
- Substitutes `1' if the current input filename is known, `0'
if it is not.
- $$
- Substitute the (decimal) process number of the (parent)
shell.
- $!
- Substitute the (decimal) process number of the last
background process started by this shell.
- $<
- Substitutes a line from the standard input, with no further
interpretation. It can be used to read from the keyboard in a shell
script.
Command and filename
substitution
The remaining substitutions, command and filename substitution, are applied
selectively to the arguments of builtin commands. By selectively, we mean that
portions of expressions which are not evaluated are not subjected to these
expansions. For commands that are not internal to the shell, the command name
is substituted separately from the argument list. This occurs very late, after
input-output redirection is performed, and in a child of the main shell.
Command substitution
Command substitution is shown by a command enclosed in ``'. The output from such
a command is normally broken into separate words at blanks, tabs and newlines,
with null words being discarded; this text then replaces the original string.
Within `"'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 that match the pattern. In a
list of words specifying filename substitution it is an error for no pattern
to match an existing file name, but it is not required for each pattern to
match. Only the metacharacters `*', `?' and `[' imply pattern matching, the
characters `~' and `{' being more akin to abbreviations.
In matching filenames, the character `.' at the beginning of a filename or
immediately following a `/', as well as the character `/' must be matched
explicitly. The character `*' matches any string of characters, including the
null string. The character `?' matches any single character. The sequence
‘[
...]’ matches any one of the characters
enclosed. Within ‘[
...]’, a pair of
characters separated by `-' matches any character lexically between the two
(inclusive).
The character `~' at the beginning of a filename refers to home directories.
Standing alone, i.e., `~' it expands to the invoker's home directory as
reflected in the value of the variable
home. When
followed by a name consisting of letters, digits and `-' characters, the shell
searches for a user with that name and substitutes their home directory; thus
`~ken' might expand to `/usr/ken' and `~ken/chmach' to `/usr/ken/chmach'. If
the character `~' is followed by a character other than a letter or `/' or
does not appear at the beginning of a word, it is left undisturbed.
The metanotation `a{b,c,d}e' is a shorthand for `abe ace ade'. Left to right
order is preserved, with results of matches being sorted separately at a low
level to preserve this order. This construct may be nested. Thus,
`~source/s1/{oldls,ls}.c' expands to `/usr/source/s1/oldls.c
/usr/source/s1/ls.c' without chance of error if the home directory for
`source' is `/usr/source'. Similarly `../{memo,*box}' might expand to `../memo
../box ../mbox'. (Note that `memo' was not sorted with the results of the
match to `*box'.) As a special case `{', `}' and `{}' are passed undisturbed.
The standard input and the standard output of a command may be redirected with
the following syntax:
- < name
- Open file name (which is first
variable, command and filename expanded) as the standard input.
- << word
- Read the shell input up to a line that is identical to
word. Word is not subjected to
variable, filename or command substitution, and each input line is
compared to word before any substitutions are done
on the input line. Unless a quoting `\', `"', `´' or ``' appears
in word, variable and command substitution is
performed on the intervening lines, allowing `\' to quote `$', `\' and
``'. Commands that are substituted have all blanks, tabs, and newlines
preserved, except for the final newline which is dropped. The resultant
text is placed in an anonymous temporary file that is given to the command
as its standard input.
- > name
-
- >! name
-
- >& name
-
- >&! name
- The file name is used as the standard
output. If the file does not exist then it is created; if the file exists,
it is truncated; its previous contents are lost.
If the variable noclobber is set, then the file must
not exist or be a character special file (e.g., a terminal or `/dev/null')
or an error results. This helps prevent accidental destruction of files.
Here, the `!' forms can be used to suppress this check.
The forms involving `&' route the standard error output into the
specified file as well as the standard output. Name
is expanded in the same way as `<' input filenames are.
- >> name
-
- >>& name
-
- >>! name
-
- >>&! name
- Uses file name as the standard
output; like `>' but places output at the end of the file. If the
variable noclobber is set, then it is an error for
the file not to exist unless one of the `!' forms is given. Otherwise
similar to `>'.
A command receives the environment in which the shell was invoked as modified by
the input-output parameters and the presence of the command in a pipeline.
Thus, unlike some previous shells, commands run from a file of shell commands
have no access to the text of the commands by default; instead they receive
the original standard input of the shell. The `<<' mechanism should be
used to present inline data. This permits shell command scripts to function as
components of pipelines and allows the shell to block read its input. Note
that the default standard input for a command run detached is
not modified to be the empty file
/dev/null; instead the standard input remains as the
original standard input of the shell. If this is a terminal and if the process
attempts to read from the terminal, then the process will block and the user
will be notified (see
Jobs above).
The standard error output may be directed through a pipe with the standard
output. Simply use the form `|&' instead of just `|'.
Expressions
Several of the builtin commands (to be described later) take expressions, in
which the operators are similar to those of C, with the same precedence, but
with the
opposite grouping: right to left. These expressions
appear in the
@,
exit,
if, and
while commands. The
following operators are available:
|| && | ↑ & == !=
=~ !~ ≤ ≥ < > << >> + - * / % ! ~ ( )
Here the precedence increases to the right, `==' `!=' `=~' and `!~', `≤'
`≥' `<' and `>', `<<' and `>>', `+' and `-', `*' `/'
and `%' being, in groups, at the same level. The `==' `!=' `=~' and `!~'
operators compare their arguments as strings; all others operate on numbers.
The operators `=~' and `!~' are like `!=' and `==' except that the right hand
side is a
pattern (containing, e.g., `*'s, `?'s and
instances of `[...]') against which the left hand operand is matched. This
reduces the need for use of the
switch statement in
shell scripts when all that is really needed is pattern matching.
Strings that begin with `0' are considered octal numbers. Null or missing
arguments are considered `0'. The 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 that 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
succeed, returning true, i.e., `1', if the command exits with status 0,
otherwise they fail, returning false, i.e., `0'. If more detailed status
information is required then the command should be executed outside an
expression and the variable
status examined.
Control flow
The shell contains several commands that can be used to regulate the flow of
control in command files (shell scripts) and (in limited but useful ways) from
terminal input. These commands all operate by forcing the shell to reread or
skip in its input and, because of the implementation, restrict the placement
of some of the commands.
The
foreach,
switch, and
while statements, as well as the
if-then-else form of the
if statement
require that the major keywords appear in a single simple command on an input
line as shown below.
If the shell's input is not seekable, the shell buffers up input whenever a loop
is being read and performs seeks in this internal buffer to accomplish the
rereading implied by the loop. (To the extent that this allows, backward
goto's will succeed on non-seekable inputs.)
Builtin commands
Builtin 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.
- bg
-
- bg
%job ...
- Puts the current or specified jobs into the background,
continuing them if they were stopped.
- break
- Causes execution to resume after the end
of the nearest enclosing foreach or
while. The remaining commands on the current line are
executed. Multi-level breaks are thus possible by writing them all on one
line.
- breaksw
- Causes a break from a switch, resuming
after the endsw.
- case
label:
- A label in a switch statement as
discussed below.
- cd
-
- cd
name
-
- chdir
-
- chdir
name
- Change the shell's working directory to directory
name. If no argument is given then change to the
home directory of the user. If name is not found as
a subdirectory of the current directory (and does not begin with `/', `./'
or `../'), then each component of the variable cdpath is
checked to see if it has a subdirectory name.
Finally, if all else fails but name is a shell
variable whose value begins with `/', then this is tried to see if it is a
directory.
- continue
- Continue execution of the nearest enclosing
while or foreach. The rest of the
commands on the current line are executed.
- default:
- Labels the default case in a switch
statement. The default should come after all case
labels.
- dirs
- Prints the directory stack; the top of the stack is at the
left, the first directory in the stack being the current directory.
- echo
wordlist
-
- echo
-n wordlist
- The specified words are written to the shell's standard
output, separated by spaces, and terminated with a newline unless the
-n option is specified.
- else
-
- end
-
- endif
-
- endsw
- See the description of the foreach,
if, switch, and
while statements below.
- eval
arg ...
- (As in sh(1).)
The arguments are read as input to the shell and the resulting command(s)
executed in the context of the current shell. This is usually used to
execute commands generated as the result of command or variable
substitution, since parsing occurs before these substitutions. See
tset(1) for an example of
using eval.
- exec
command
- The specified command is executed in place of the current
shell.
- exit
-
- exit
(expr)
- The shell exits either with the value of the
status variable (first form) or with the value of the
specified expr (second form).
- fg
-
- fg
%job ...
- Brings the current or specified jobs into the foreground,
continuing them if they were stopped.
- foreach
name (wordlist)
-
- ...
-
- end
- The variable name is successively set to
each member of wordlist and the sequence of commands
between this command and the matching end are executed.
(Both foreach and end must appear
alone on separate lines.) The builtin command continue
may be used to continue the loop prematurely and the builtin 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 null characters in the output.
Useful for programs that wish to use the shell to filename expand a list
of words.
- goto
word
- The specified word is filename and
command expanded to yield a string of the form `label'. The shell rewinds
its input as much as possible and searches for a line of the form `label:'
possibly preceded by blanks or tabs. Execution continues after the
specified line.
- hashstat
- Print a statistics line showing how effective the internal
hash table has been at locating commands (and avoiding
exec´s). An exec is attempted for
each component of the path where the hash function
indicates a possible hit, and in each component that does not begin with a
`/'.
- history
-
- history
n
-
- history
-r n
-
- history
-h n
- Displays the history event list; if n
is given only the n most recent events are printed.
The -r option reverses the order of printout to be most
recent first instead of oldest first. 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.
- 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, 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 id's in addition to the normal information.
- kill
%job
-
- kill
pid ...
-
- kill
-l
[exit_status]
-
- kill
-s signal_name pid ...
-
- kill
-signal_name pid
...
-
- kill
-signal_number pid
...
- 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
<signal.h>, stripped of the
prefix ``SIG''). The signal names are listed by ``kill -l''; if an
exit_status is specified, only the corresponding
signal name will be written. There is no default, just saying `kill' does
not send a signal to the current job. If the signal being sent is TERM
(terminate) or HUP (hangup), then the job or process will be sent a CONT
(continue) signal as well.
- limit
-
- limit
resource
-
- limit
resource maximum-use
-
- limit
-h
-
- limit
-h resource
-
- limit
-h resource maximum-use
- Manipulates per-process system resource limits via the
getrlimit(2) and
setrlimit(2) system
calls; this limits the consumption by the current process and each process
it creates to not individually exceed maximum-use on
the specified resource. If no
maximum-use is given, then the current limit is
printed; if no resource is given, then all
limitations are given.
If the -h flag is given, the hard limits are used instead
of the current limits. The hard limits impose a ceiling on the values of
the current limits. Only the super-user may raise the hard limits, but a
user may lower or raise the current limits within the legal range.
Resources controllable currently include:
-
-
- cputime
- The maximum number of CPU-seconds to be used by each
process.
-
-
- filesize
- The largest single file (in bytes) that can be
created.
-
-
- datasize
- The maximum growth of the data+stack region via
sbrk(2) beyond the end of
the program text.
-
-
- stacksize
- The maximum size of the automatically-extended stack
region.
-
-
- coredumpsize
- The size of the largest core dump (in bytes) that will
be created.
-
-
- memoryuse
- The maximum size (in bytes) to which a process's
resident set size (RSS) may grow.
-
-
- memorylocked
- The maximum size (in bytes) which a process may lock
into memory using the
mlock(2) function.
-
-
- maxproc
- The maximum number of simultaneous processes for this
user id.
-
-
- openfiles
- The maximum number of simultaneous open files for this
user id.
-
-
- sbsize
- The maximum socket buffer size of a process (in
bytes).
-
-
- vmemoryuse
- The maximum size (in bytes) which a process can
obtain.
The maximum-use may be given as a (floating point or
integer) number followed by a scale factor. For all limits other than
cputime the default scale is `k' or `kilobytes'
(1024 bytes); a scale factor of `m' or `megabytes' may also be used. For
cputime the default scale is `seconds'; a scale
factor of `m' for minutes or `h' for hours, or a time of the form `mm:ss'
giving minutes and seconds also may be used.
For both resource names and scale factors, unambiguous
prefixes of the names suffice.
Limits of an arbitrary process can be displayed or set using the
sysctl(8) utility. See the
getrlimit(2) and
setrlimit(2) man pages
for an additional description of system resource limits.
- 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 super-user may specify negative
priority by using `nice -number ...'. Command is
always executed in a sub-shell, and the restrictions placed on commands in
simple if statements apply.
- nohup
-
- nohup
command
- The first form can be used in shell scripts to cause
hangups to be ignored for the remainder of the script. The second form
causes the specified command to be run with hangups ignored. All processes
detached with `&' are effectively nohup´ed.
- notify
-
- notify
%job ...
- Causes the shell to notify the user asynchronously when the
status of the current or specified jobs change; normally notification is
presented before a prompt. This is automatic if the shell variable
notify is set.
- onintr
-
- onintr
-
-
- onintr
label
- Control the action of the shell on interrupts. The first
form restores the default action of the shell on interrupts which is to
terminate shell scripts or to return to the terminal command input level.
The second form `onintr -' causes all interrupts to be ignored. The final
form causes the shell to execute a `goto label' when an interrupt is
received or a child process terminates because it was interrupted.
In any case, if the shell is running detached and interrupts are being
ignored, all forms of onintr have no meaning and
interrupts continue to be ignored by the shell and all invoked commands.
Finally onintr statements are ignored in the system
startup files where interrupts are disabled (/etc/csh.cshrc,
/etc/csh.login).
- popd
-
- popd
+n
- Pops the directory stack, returning to the new top
directory. With an argument `+ n´ discards the
n´th entry in the stack. The members of the
directory stack are numbered from the top starting at 0.
- pushd
-
- pushd
name
-
- pushd
+n
- With no arguments, pushd exchanges the
top two elements of the directory stack. Given a
name argument, pushd changes to
the new directory (ala cd) and pushes the old current
working directory (as in cwd) onto the directory stack.
With a numeric argument, pushd rotates the
n´th argument of the directory stack around to
be the top element and changes to it. The members of the directory stack
are numbered from the top starting at 0.
- rehash
- Causes the internal hash table of the contents of the
directories in the path variable to be recomputed. This
is needed if new commands are added to directories in the
path while you are logged in. This should only be
necessary if you add commands to one of your own directories, or if a
systems programmer changes the contents of a system directory.
- repeat
count command
- The specified command, which is
subject to the same restrictions as the command in
the one line if statement above, is executed
count times. I/O redirections occur exactly once,
even if count is 0.
- set
-
- set
name
-
- set
name=word
-
- set
name[index]=word
-
- set
name=(wordlist)
- The first form of the command shows the value of all shell
variables. Variables that have other than a single word as their value
print as a parenthesized word list. The second form sets
name to the null string. The third form sets
name to the single word. The
fourth form sets the index'th component of
name to word; this component
must already exist. The final form sets name to the
list of words in wordlist. The value is always
command and filename expanded.
These arguments may be repeated to set multiple values in a single set
command. Note however, that variable expansion happens for all arguments
before any setting occurs.
- setenv
-
- setenv
name
-
- setenv
name value
- The first form lists all current environment variables. It
is equivalent to
printenv(1). The last form
sets the value of environment variable name to be
value, a single string. The second form sets
name to an empty string. The most commonly used
environment variables
USER
,
TERM
, and PATH
are
automatically imported to and exported from the csh
variables user, term, and
path; there is no need to use
setenv for these.
- shift
-
- shift
variable
- The members of argv are shifted to the
left, discarding argv[1]. It is an error for
argv not to be set or to have less than one word as
value. The second form performs the same function on the specified
variable.
- source
name
-
- source
-h name
- The shell reads commands from name.
Source commands may be nested; if they are nested too
deeply the shell may run out of file descriptors. An error in a
source at any level terminates all nested
source commands. Normally input during
source commands is not placed on the history list; the
-h option causes the commands to be placed on the history list without
being executed.
- stop
-
- stop
%job ...
- Stops the current or specified jobs that are executing in
the background.
- suspend
- Causes the shell to stop in its tracks, much as if it had
been sent a stop signal with ^Z. This is most often used
to stop shells started by su(1).
- switch
(string)
-
- case
str1:
-
- ...
-
- breaksw
-
- ...
-
- default:
-
- ...
-
- breaksw
-
- endsw
- Each case label is successively matched against the
specified string which is first command and filename
expanded. The file metacharacters `*', `?' and `[...]' may be used in the
case labels, which are variable expanded. If none of the labels match
before the `default' label is found, then the execution begins after the
default label. Each case label and the default label must appear at the
beginning of a line. The command breaksw causes
execution to continue after the endsw. Otherwise control
may fall through case labels and the default label as in C. If no label
matches and there is no default, execution continues after the
endsw.
- time
-
- time
command
- With no argument, a summary of time used by this shell and
its children is printed. If arguments are given the specified simple
command is timed and a time summary as described under the
time variable is printed. If necessary, an extra shell
is created to print the time statistic when the command completes.
- umask
-
- umask
value
- The file creation mask is displayed (first form) or set to
the specified value (second form). The mask is given in octal. Common
values for the mask are 002 giving all access to the group and read and
execute access to others or 022 giving all access except write access for
users in the group or others.
- unalias
pattern
- All aliases whose names match the specified pattern are
discarded. Thus all aliases are removed by `unalias *'. It is not an error
for nothing to be unaliased.
- unhash
- Use of the internal hash table to speed location of
executed programs is disabled.
- unlimit
-
- unlimit
resource
-
- unlimit
-h
-
- unlimit
-h resource
- Removes the limitation on resource.
If no resource is specified, then all
resource limitations are removed. If
-h is given, the corresponding hard limits are removed.
Only the super-user may do this.
- unset
pattern
- All variables whose names match the specified pattern are
removed. Thus all variables are removed by `unset *'; this has noticeably
distasteful side-effects. It is not an error for nothing to be
unset.
- unsetenv
pattern
- Removes all variables whose name match the specified
pattern from the environment. See also the setenv
command above and
printenv(1).
- wait
- Wait for all background jobs. If the shell is interactive,
then an interrupt can disrupt the wait. After the interrupt, the shell
prints names and job numbers of all jobs known to be outstanding.
- which
command
- Displays the resolved command that will be executed by the
shell.
- while
(expr)
-
- ...
-
- end
- While the specified expression evaluates non-zero, the
commands between the while and the matching
end are evaluated. Break and
continue may be used to terminate or continue the loop
prematurely. (The while and end must
appear alone on their input lines.) Prompting occurs here the first time
through the loop as for the foreach statement if the
input is a terminal.
- %job
- Brings the specified job into the foreground.
- %job
&
- Continues the specified job in the background.
- @
-
- @
name= expr
-
- @
name[index]= expr
- The first form prints the values of all the shell
variables. The second form sets the specified name
to the value of expr. If the expression contains
`<', `>', `&' or `|' then at least this part of the expression
must be placed within `(' `)'. The third form assigns the value of
expr to the index'th argument
of name. Both name and its
index'th component must already exist.
The operators `*=', `+=', etc are available as in C. The space separating the
name from the assignment operator is optional. Spaces are, however, mandatory
in separating components of
expr which would otherwise
be single words.
Special postfix `++' and `--' operators increment and decrement
name respectively, i.e., `@ i++'.
Pre-defined and
environment variables
The following variables have special meaning to the shell. Of these,
argv,
cwd,
home,
path,
prompt,
shell and
status are always set by the shell. Except for
cwd and
status, this setting
occurs only at initialization; these variables will not then be modified
unless done explicitly by the user.
The shell copies the environment variable
USER
into the
variable
user,
TERM
into
term, and
HOME
into
home, and copies these back into the environment
whenever the normal shell variables are reset. The environment variable
PATH
is likewise handled; it is not necessary to worry
about its setting other than in the file
.cshrc as
inferior
csh processes will import the definition of
path from the environment, and re-export it if you then
change it.
-
-
- argv
- Set to the arguments to the shell, it is from this variable
that positional parameters are substituted, i.e., `$1' is replaced by
`$argv[1]', etc.
-
-
- cdpath
- Gives a list of alternative directories searched to find
subdirectories in chdir commands.
-
-
- cwd
- The full pathname of the current directory.
-
-
- echo
- Set when the -x command line option is
given. Causes each command and its arguments to be echoed just before it
is executed. For non-builtin commands all expansions occur before echoing.
Builtin commands are echoed before command and filename substitution,
since these substitutions are then done selectively.
-
-
- filec
- Enable file name completion.
-
-
- histchars
- Can be given a string value to change the characters used
in history substitution. The first character of its value is used as the
history substitution character, replacing the default character `!'. The
second character of its value replaces the character `^' in quick
substitutions.
-
-
- histfile
- Can be set to the pathname where history is going to be
saved/restored.
-
-
- history
- Can be given a numeric value to control the size of the
history list. Any command that has been referenced in this many events
will not be discarded. Too large values of history
may run the shell out of memory. The last executed command is always saved
on the history list.
-
-
- home
- The home directory of the invoker, initialized from the
environment. The filename expansion of ‘~’
refers to this variable.
-
-
- ignoreeof
- If set the shell ignores end-of-file from input devices
which are terminals. This prevents shells from accidentally being killed
by control-D's.
-
-
- mail
- The files where the shell checks for mail. This checking is
done after each command completion that will result in a prompt, if a
specified interval has elapsed. The shell says `You have new mail.' if the
file exists with an access time not greater than its modify time.
If the first word of the value of mail is numeric it
specifies a different mail checking interval, in seconds, than the
default, which is 10 minutes.
If multiple mail files are specified, then the shell says `New mail in
name' when there is mail in the file
name.
-
-
- noclobber
- As described in the section on
Input/output, restrictions are
placed on output redirection to ensure that files are not accidentally
destroyed, and that `>>' redirections refer to existing files.
-
-
- noglob
- If set, filename expansion is inhibited. This inhibition is
most useful in shell scripts that
are not dealing with filenames, or after a list of filenames has been
obtained and further expansions are not desirable.
-
-
- nonomatch
- If set, it is not an error for a filename expansion to not
match any existing files; instead the primitive pattern is returned. It is
still an error for the primitive pattern to be malformed, i.e., `echo ['
still gives an error.
-
-
- notify
- If set, the shell notifies asynchronously of job
completions; the default is to present job completions just before
printing a prompt.
-
-
- path
- Each word of the path variable specifies a directory in
which commands are to be sought for execution. A null word specifies the
current directory. If there is no path variable then
only full path names will execute. The usual search path is `.', `/bin'
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 that is given neither the -c nor the
-t option will normally hash the contents of the
directories in the path variable after reading
.cshrc, and each time the path
variable is reset. If new commands are added to these directories while
the shell is active, it may be necessary to do a rehash
or the commands may not be found.
-
-
- prompt
- The string that is printed before each command is read from
an interactive terminal input. If a `!' appears in the string it will be
replaced by the current event number unless a preceding `\' is given.
Default is `% ', 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 that has been referenced in this many events will be saved.
During start up the shell sources ~/.history into the history list
enabling history to be saved across logins. Too large values of
savehist will slow down the shell during start up.
If savehist is just set, the shell will use the
value of history.
-
-
- shell
- The file in which the shell resides. This variable is used
in forking shells to interpret files that have execute bits set, but which
are not executable by the system. (See the description of
Non-builtin 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. Builtin commands that fail
return exit status `1', all other builtin commands set status to `0'.
-
-
- time
- Controls automatic timing of commands. This setting allows
two parameters. The first specifies the CPU time threshold at which
reporting should be done for a process, and the optional second specifies
the output format. The following format strings are available:
%c
- Number of involuntary context switches.
%D
- Average unshared data size.
%E
- Elapsed (wall-clock) time.
%F
- Page faults.
%I
- Filesystem blocks in.
%K
- Average total data memory used.
%k
- Number of signals received.
%M
- Maximum Resident Set Size.
%O
- Filesystem blocks out.
%P
- Total percent time spent running.
%R
- Page reclaims.
%r
- Socket messages received.
%S
- Total system CPU time used.
%s
- Socket messages sent.
%U
- Total user CPU time used.
%W
- Number of swaps.
%w
- Number of voluntary context switches (waits).
%X
- Average shared text size.
The default summary is "%Uu %Ss %E %P %X+%Dk %I+%Oio
%Fpf+%Ww"
-
-
- verbose
- Set by the -v command line option, causes
the words of each command to be printed after history substitution.
Non-builtin command
execution
When a command to be executed is found to not be a builtin command the shell
attempts to execute the command via
execve(2). Each word in the
variable
path names a directory from which the shell
will attempt to execute the command. If it is given neither a
-c nor a
-t option, the shell will hash
the names in these directories into an internal table so that it will only try
an
exec in a directory if there is a possibility that the
command resides there. This shortcut greatly speeds command location when many
directories are present in the search path. If this mechanism has been turned
off (via
unhash), or if the shell was given a
-c or
-t argument, and in any case for
each directory component of
path that does not begin
with a `/', the shell concatenates with the given command name to form a path
name of a file which it then attempts to execute.
Parenthesized commands are always executed in a 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 change.
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 shell's 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. Interrupts are
not allowed when a login shell is reading the file
.logout.
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'.
LIMITATIONS
Word lengths - Words can be no longer than 1024 characters. The system limits
argument lists to 10240 characters. The number of arguments to a command that
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),
execve(2),
fork(2),
pipe(2),
setrlimit(2),
sigaction(2),
umask(2),
wait(2),
killpg(3),
tty(4),
a.out(5),
environ(7),
sysctl(8)
An introduction to the C shell.
HISTORY
csh appeared in
2BSD. It was a first
implementation of a command language interpreter incorporating a history
mechanism (see
History
substitutions), job control facilities (see
Jobs), interactive file name and user name
completion (see
File Name
Completion), and a C-like syntax. There are now many shells that also have
these mechanisms, plus a few more (and maybe some bugs too), which are
available through the usenet.
AUTHORS
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 internally.
Shell builtin functions are not stoppable/restartable. Command sequences of the
form `a ; b ; c' are also not handled gracefully when stopping is attempted.
If you suspend `b', the shell will immediately execute `c'. This is especially
noticeable if this expansion results from an
alias. It
suffices to place the sequence of commands in ()'s to force it to a 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 instead of aliases.
Commands within loops, prompted for by `?', are not placed on the
history list. Control structure should be parsed instead of
being recognized as built-in commands. This would allow control commands to be
placed anywhere, to be combined with `|', and to be used with `&' and `;'
metasyntax.
It should be possible to use the `:' modifiers on the output of command
substitutions.
The way the
filec facility is implemented is ugly and
expensive.