NAME

csh − a shell (command interpreter) with C-like syntax

SYNOPSIS

csh [ −cefinstvVxX ] [ arg ...  ]

DESCRIPTION

Csh is a command language interpreter incorporating a history mechanism (see History Substitutions) and a C-like syntax. 

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 CRTs to 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 user’s 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 their special meaning may be prevented, by preceding them with a backslash (\). A newline preceded by a \ is equivalent to a blank. It is usually necessary to use the backslash to escape the parser metacharacters when you want to use them literally rather than as metacharacters. 

Strings enclosed in matched pairs of quotation marks, either single or double quotation marks, ’, ‘ or ", form parts of a word. Metacharacters in these strings, including blanks and tabs, do not form separate words. Such quotations have semantics to be described subsequently.

Within pairs of single or double quotation marks, a newline (carriage return) preceded by a \ gives a true newline character.  This is used to set up a file of strings separated by newlines, as for fgrep.

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 from having this special meaning when preceded by \ or if bracketed by a pair of single or double quotation marks.

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 &, which means "run it in background".

Parentheses ( and ) around a pipeline or sequence of pipelines cause the whole series to be treated as a simple command, which may in turn 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.)

Process ID Numbers

When a process is run in background with &, the shell prints a line which looks like:

1234

indicating that the process which was started asynchronously was number 1234. 

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. 

To check on the status of a process, use the ps (process status) command. 

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 turn off its special meaning; for convenience, a ! is also passed unchanged when it is followed by a blank, tab, newline, = or (.

Therefore, do not put a space after the ! and the command reference when you are invoking the shell’s history mechanism. (History substitutions also occur when an input line begins with ↑. This special abbreviation will be described later.)

An input line which invokes history substitution is echoed on the terminal before it is executed, as it would look if typed out in full. 

The shell’s history list, which may be seen by typing the history command, contains all commands input from the terminal which consist of one or more words.  History substitutions reintroduce sequences of words from these saved commands into the input stream.  The history variable controls the size of the input stream.  The previous command is always retained, regardless of its value.  Commands are numbered sequentially from 1. 

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.  This is done by SETting Prompt = ! and the prompt character of your choice.

For example, if the current event is number 13, we can call up the command recorded as event 11 in several ways: !-2 [i.e., 13-2]; by the first letter of one of its command words, such as !c referring to the "c" in cat; 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 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.

Words are selected from a command event and acted upon according to the following formula:

event:position:action

The event is the command you wish to retrieve.  As mentioned above, it may be summoned up by event number and in several other ways.  All that the event notation does is to tell the shell which command you have in mind. 

Position picks out the words from the command event on which you want the action to take place.  The position notation can do anything from altering the command completely to making some very minor substitution, depending on which words from the command event you specify with the position notation. 

To select words from a command event, follow the event specification with a : and a designator (by position) for the desired words.

The words of a command event are picked out by their position in the input line.  Positions are numbered from 0, the first word (usually command) being position 0, the second word having position 1, and so forth.  If you designate a word from the command event by stating its position, means you want to include it in your revised command.  All the words that you want to include in a revised command must be designated by position notation in order to be included. 

The basic position designators are:

0 first (command) word

n nth argument

↑ first argument, i.e., 1

$ last argument

% matches the word of an ?s?  search which immediately precedes it; used to strip one word out of a command event for use in another command.  Example: !?four?:%:p prints four. 

x−y range of words (e.g., 1-3 means "from position 1 to position 3"). 

−y abbreviates "0−y

∗ stands for "↑−$", or indicates position 1 if only one word in event. 

x∗abbreviates "x−$" where
x is a position number. 

x− like "x∗" but omitting last word "$"

The : separating the event specification from the word designator can be omitted if the argument selector begins with a ↑, $, ∗, − or %.

Modifiers, each preceded by a :, may be used to act on the designated words in the specified command event. 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/old/new/ Substitute new for old

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 (but only 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., !$.  In this case the reference is to the previous command.  If a previous history reference occurred on the same line, this form repeats the previous reference.  Thus !?foo?↑ !$ gives the first and last arguments from the command matching ?foo?. 

You can quickly make substitutions to the previous command line by using the ↑ character as the first non-blank character of an input line. 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 "

The quotation of strings by ’ and " can be used to prevent all or some of the remaining substitutions which would otherwise take place if these characters were interpreted as "metacharacters" or "wild card matching characters". Strings enclosed in single quotes, ’ are prevented any further interpretation or expansion. Strings enclosed in " may still be variable and command 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 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 prs 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 which 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 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 double quotes (") where it always occurs, and within single quotes (’) 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 double quotes or given the :q modifier, the results of variable substitution may eventually be command and filename substituted. Within double quotes, 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.

Metasequences for variable 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 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. 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.

$<
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 builtin 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 double quotes ("), only newlines force new words; blanks and tabs are preserved.

In any case, the single final newline does not force a new word.  Note that it is thus possible for a command substitution to yield only part of a word, even if the command outputs a complete line. 

Filename substitution

If a word contains any of the characters ∗, ?, [ or { or begins with the character ~, then that word is a candidate for filename substitution, also known as "globbing". This word is then regarded as a pattern, and replaced with an alphabetically sorted list of file names 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.

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 abeaceade.  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 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, it 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.

Diagnostic output may be directed through a pipe with the standard output.  Simply use the form |& rather than just |. To redirect standard output and standard error to separate files, use (cmd > file1) >& file2; /dev/tty may be used to redirect input or output to or from your terminal.

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 (which may contain ∗, ? 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

c character special file

b block special file

p named pipe (fifo)

u set-user- ID bit is set

g set-group- ID bit is set

k sticky bit is set

s size greater than zero

t open file descriptor for terminal device

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 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 gotos 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.

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

dirs −l
Prints the directory stack; the top of the stack is at the left, the first directory in the stack being the current directory. In the first form the user’s home directory is represented by ~.

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 or the \c escape is specified.  The following C-like escape sequences are available:

\bbackspace
\cprint line without new-line
\fform-feed
\nnew-line
\rcarriage return
\ttab
\\backslash

\nthe character whose ASCII code is the 1-, 2- or 3-digit octal number n.

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). 

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 up 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 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. 

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 rather than oldest first.  The −h option causes the history list to be printed without leading numbers.  This is used to produce files suitable for sourceing 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, when command is not executed (this is a bug). 

if (expr) then

    ... 

else

    ... 

else

    ... 

endif
If the specified expr is true, then the commands to the first else are executed; else if 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.)

kill pid

kill −sig pid ... 
Sends either the TERM (terminate) signal or the specified signal to the specified processes.  Signals are either given by number or by names (as given in /usr/include/signal.h, stripped of the prefix SIG).  There is no default, saying just "kill" does not send a signal to the current process.  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 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 nohuped.

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. 

popd

popd +n
Pops the directory stack, returning to the new top directory. With an 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 form sets the indexth 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 variable PATH is automatically imported to and exported from the csh variable 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 in the history list without being executed. 

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 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.

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 env(1).

wait
All background jobs are waited for. If 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. 

%

% user
The first form toggles the user ID and group ID between that of root and user for all executed commands (except builtins).  The prompt is automatically toggled between # and #%.  The second form specifies a user name, listed in /etc/passwd, that should be toggled to and from. 

@

@ 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 indexth argument of name. Both name and its indexth 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 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 non-builtin 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. 

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 control-Ds. 

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 on 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. 

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 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. The sequence \\ is replaced with a single \. The prompt should only be set by the user if it is already defined so that it will not be printed when processing shell scripts by using the statement

if ( $?prompt ) set prompt=’\!% ’

If the sequence \@x appears, where x is one of the characters listed below, then it will be replaced by the current time and date in the indicated format. 

Rtime as HH:MM AM/PM, e.g. 8:40PM

rtime as HH:MM:SS AM/PM, e.g. 08:40:25 PM

mmonth of year − 01 to 12

dday of month − 01 to 31

ylast 2 digits of year − 00 to 99

Ddate as mm/dd/yy

Hhour − 00 to 23

Mminute − 00 to 59

Ssecond − 00 to 59

Ttime as HH:MM:SS

jday of year − 001 to 366

wday of week − Sunday = 0

aabbreviated weekday − Sun to Sat

habbreviated month − Jan to Dec

ninsert a new-line character

tinsert a tab character

The default prompt 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 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 which fail return exit status "1", all other builtin 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 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 not to be a builtin command, the shell attempts to execute the command via exec(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 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 an 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. 

Argument List Processing

If argument 0 to the shell is −, then this is a login shell. The flag arguments 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 may aid 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 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".  Remaining arguments initialize the variable argv. Csh scripts should always start with

#! /bin/csh -f
which causes the kernel to fork off /bin/csh to process them even if invoked by a Bourne shell user and inhibits processing of the .cshrc file to prevent interference by the user’s differing aliases.

Signal Handling

The shell normally ignores quit signals.  Processes running in background (by &) 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.  In no case are interrupts allowed when a login shell is reading the file ".logout". 

EXAMPLE

csh

creates a new shell which will accept shell commands with Berkeley extensions. 

FILES

~/.cshrcRead at beginning of execution by each shell.
~/.loginRead by login shell, after ".cshrc" at login.
~/.logoutRead by login shell, at logout.
/bin/shStandard shell, for shell scripts not starting with a #.
/tmp/sh∗Temporary file for <<.
/etc/passwdSource of home directories for "~name".

LIMITATIONS

Words can be no longer than 1024 characters.  The system limits argument lists to 5120 characters.  The number of arguments to a command which involves filename expansion is limited to 1/6th the number of characters allowed in an argument list. Command substitutions may substitute no more characters than are allowed in an argument list. To detect looping, the shell restricts the number of alias substitutions on a single line to 20. 

SEE ALSO

sh(1), access(2), exec(2), fork(2), pipe(2), signal(2), umask(2), wait(2), a.out(4), environ(4)
An Introduction to the C Shell, by William Joy.

BUGS

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. 

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.

Bourne shell scripts which start with # will be executed by csh unless they use the kernel’s #! facility, e.g. 

#! /bin/sh

AUTHOR

William Joy. 

Version 2.3  —  July 04, 1985