NAME

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

SYNOPSIS

csh [ −cefinstvVxX ] [ arg ...  ]

DESCRIPTION

Csh is a first implementation of a command language interpreter incorporating a history mechanism (see History Substitutions) job control facilities (see Jobs) and a C-like syntax.  To use chs job control facilities, users must automatically use the new tty driver summarized in newtty(4) and fully described in tty(4). This new tty driver allows generation of interrupt characters from the keyboard to inform jobs to stop. (See stty(1) for details on setting options in the new tty driver.)

An instance of csh begins by executing commands from the file ‘.cshrc’ in the home directory of the invoker.  If this is a file login shell, then csh also executes commands from the file ‘.login’.  It is typical for users on crt’s to place the command “stty crt” in their .login file, and also invoke tset(1) at this area.

Normally, the shell then begins reading commands from the terminal, prompting with ‘% ’.  Processing of arguments and the use of the shell to process files containing command scripts are 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 parsed.  Finally, each command in the current line is executed. 

When a login shell terminates, it executes commands from the file ‘.logout’ in the users home directory. 

Lexical structure

The shell splits input lines into words at blanks and tabs with the following exceptions: the characters ‘&’ ‘|’ ‘;’ ‘<’ ‘>’ ‘(’ ‘)’ form separate words.  If doubled in ‘&&’, ‘||’, ‘<<’ or ‘>>’, these pairs form single words. These parser metacharacters may be appended to other words, or prevented their special meaning, by preceding them with ‘\’. A newline preceded by a ‘\’ is equivalent to a blank.

In addition, strings enclosed in matched pairs of quotations, ‘´’, ‘`’ 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 subsequently.  Within pairs of ‘´’ or ‘"’ characters, a newline preceded by a ‘\’ gives a true newline character. 

When the shell’s input is not a terminal, the character ‘#’ introduces a comment which continues to the end of the input line.  The special meaning of this comment is prevented when it is preceded by ‘\’ and placed 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 form a pipeline.  The output of each command in a pipeline is connected to the input of the next  command.  Sequences of pipelines may be separated by ‘;’, and are then executed sequentially.  A sequence of pipelines may be executed without immediately waiting for it to terminate by following it with an ‘&’. 

Any of the above may be placed in ‘(’ ‘)’ to form a simple command (which may be a component of a pipeline, etc.)  It is also possible to separate pipelines with ‘||’ or ‘&&’ indicating, as with C language, that the second pipeline is to be executed only if the first fails or succeeds, respectively. (See Expressions.) 

Jobs

The shell associates a job with each pipeline. The shell maintains 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 as follows:

[1] 1234

This line indicates the jobs started asynchronously was job number 1 and had one (top-level) process, with the process id 1234. 

If when running a job and executing another operation is desired, depress the key ^Z (control-Z), which sends a STOP signal to the current job.  The shell then normally indicates that the job has been ‘Stopped’, and prints another prompt.  The state of this job can then be manipulated by placing it in the background with the bg command.  It is also possible to run some other commands and then eventually bring the job back into the foreground with the foreground command, fg.  A ^Z takes effect immediately and is resembles an interrupt in that pending output and unread input are discarded when ^Z is typed.  Another special key is ^Y which does not generate a STOP signal until a program attempts to read(2) it. ^Y can conveniently be typed ahead when some commands have been prepared for a job you wish to stop after it has read them. 

A job being run in the background stops if it attempts 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 this tty option is set, then background jobs stop when they attempt to produce output when attempting 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’.  Only naming a job brings it to the foreground; thus, ‘%1’ is a synonym for ‘fg %1’, and brings job 1 back into the foreground.  Similarly, saying ‘%1 &’ resumes job 1 in the background.  Jobs can also be named by prefixes of the string typed in to start them, if these prefixes are unambiguous; thus, ‘%ex’ normally restarts a suspended ex(1) job, if there is only one suspended job whose name begins with the string ‘ex’.  It is also possible to say ‘%?string’, which specifies a job whose text contains string, if there is only one such job. 

The shell maintains a notion of the current and previous jobs.  In output pertaining to jobs, the current job is marked with a ‘+’ and the previous job with a ‘−’.  The abbreviation ‘%+’ refers to the current job and ‘%−’ refers to the previous job.  For close analogy with the syntax of the history mechanism (described below), ‘%%’ is also a synonym for the current job. 

Status reporting

This shell learns immediately whenever a process changes state.  The shell normally informs you whenever a job becomes blocked so that no further progress is possible, but only just before it prints a prompt.  The purpose of this delay is to otherwise disturb your work.  If, however, you set the shell variable notify, the shell immediately notifies you of changes of status in background jobs.  There is also a shell command notify which marks a single process so that its status changes are immediately reported.  By default, notify marks the current process; simply say ‘notify’ after starting a background job to mark it. 

When attempting to leave the shell while jobs are stopped, you are warned that ‘You have stopped jobs.’  You may use the jobs command to verify which jobs are stopped.  If you use this procedure or immediately try to exit again, the shell does not warn you a second time, and the suspended jobs are terminated. 

Substitutions

The following describes 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 correct spelling errors in the previous command with minimal typing and a high degree of reliability.  History substitutions begin with the character ‘!’ and may begin anywhere in the input stream, with the provison they do not nest.  This ‘!’ may be preceded by an ‘\’ to prevent its special meaning; for convenience, a ‘!’ is passed unchanged when it is followed by a blank, tab, newline, ‘=’ or ‘(’.  (History substitutions also occur when an input line begins with ‘↑’.  This special abbreviation is described later.)  Any input line which contains history substitution is echoed on the terminal before it is executed, as it could have been typed without history substitution. 

Command input from the terminal which consist of one or more words are saved on the history list.  The history substitutions reintroduce sequences of words from these saved commands into the input stream.  The size of this input stream is controlled by the history variable.  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. 

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 modification, simply reintroduce the words of the specified events, each separated by a single blank.  As a special case ‘!!’ refers to the previous command; thus ‘!!’ alone is essentially a redo. 

To select words from an event, follow the event specification by a ‘:’ and a designator for the desired words.  The words of an input line are numbered from 0, with the first (usually command) word being 0, the second word (first argument) being 1, etc.  The basic word designators are as follows:

0first (command) word
nn’th argument
↑first argument,  i.e. ‘1’
$last argument
%word matched by (immediately preceding) ?s? search
x−yrange of words
−yabbreviates ‘0−y’
∗abbreviates ‘↑−$’, or nothing if only 1 word in event
x∗abbreviates ‘x−$’
x−like ‘x∗’ but omitting word ‘$’

The ‘:’ separating the event specification from the word designator can be omitted if the argument selector begins with a ‘↑’, ‘$’, ‘∗’ ‘−’ or ‘%’, after the optional word designator a sequence of modifiers can be placed, each preceded by a ‘:’.  The following modifiers are defined:

hRemove a trailing pathname component, leaving the head.
rRemove a trailing ‘.xxx’ component, leaving the root name.
eRemove all but the extension ‘.xxx’ part.
s/l/r/Substitute l for r
tRemove all leading pathname components, leaving the tail.
&Repeat the previous substitution.
gApply the change globally, prefixing the above, (e.g. ‘g&’).
pPrint the new command but do not execute it.
qQuote the substituted words, preventing further substitutions.
xLike 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 considered an error for no word to be applicable. 

The left-hand side of substitutions are not regular expressions in terms 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 ‘\’ also quotes ‘&’.  A null l uses the previous string, either from a l or from a contextual scan string s in ‘!?s?’. The trailing delimiter in the substitution may be omitted if a newline immediately follows, as may the trailing ‘?’ in a contextual scan.

A history reference may be given without an event specification, (e.g. ‘!$’).  The reference is to the previous command, unless a previous history reference occurred on the same line; in this case, this form repeats the previous reference.  Thus ‘!?foo?↑ !$’ 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 command is equivalent to ‘!:s↑’ and provides a convenient shorthand for substitutions on the text of the previous line; thus, ‘↑lb↑lib’ corrects 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.  For example, after ‘ls −ld ~paul’ we might enter ‘!{l}a’ to do ‘ls −ld ~paula’, while ‘!la’ would search 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 ‘"’ are yet 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 Substitition below) does a ‘"’ quoted string yield parts of more than one word; ‘´’ quoted strings never perform this function. 

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 the word has an alias, 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. 

For example, if the alias for ‘ls’ is ‘ls −l’ the command ‘ls /usr’ maps to ‘ls −l /usr’, the argument list is undisturbed.  Similarly, if the alias for ‘lookup’ is ‘grep !↑ /etc/passwd’, then ‘lookup bill’ maps 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.  If the first word of the new text is the same as the previously used word by flagging the word to prevent further aliasing.  Other loops are detected and resulting an error. 

Note that the mechanism allows aliases to introduce parser metasyntax; Therefore, we can ‘alias print ´pr \!∗ | lpr´’ to form a command which pr’s its arguments to the line printer. 

Variable substitution

The shell maintains a set of variables, each of which has as value a list of zero or more words.  Some of these variables are either set or referred to by the shell.  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.  A number of the variables referred to by the shell are toggles; the shell does not care what their value is, only whether or not they are set.  For instance, the verbose The −v command line option specifies the setting of this variable. 

Other operations treat variables numerically.  The ‘@’ command permits numeric calculations to be performed and the result assigned to a variable.  Variable values, however, are 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 and keyed by ‘$’ characters.  This expansion can be prevented by preceding the ‘$’ with a ‘\’, except within ‘"’s, where expansion always occurs, and within ‘´’s where expansion never occurs.  Strings quoted by ‘`’ are interpreted later (see Command substitution below); therefore, ‘$’ substitution does not occur at this point until later, if at all.  If followed by a blank, tab, or end-of-line a ‘$’ is passed unchanged. 

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 with a value that 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 expands to multiple words, with each word separated by a blank and quoted to prevent later command or filename substitution. 

The following metasequences are provided for introducing variable values into the shell input.  Except as noted, it is an error to reference a variable which is not set. 

$name

${name}
Replaced by the words of the value of variable name, with each separated by a blank.  Braces insulate name from following characters which would otherwise be part of name.  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; : however, 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 unknown.

$number

${number}
Equivalent to ‘$argv[number]’.

$∗
Equivalent to ‘$argv[∗]’.

The modifiers ‘:h’, ‘:t’, ‘:r’, ‘:q’ and ‘:x’ may be applied to the above substitutions, as may ‘:gh’, ‘:gt’ and ‘:gr’.  If braces ‘{’ ’}’ appear in the command form, 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 unknown.

$$
Substitute the (decimal) process number of the (parent) shell.

$<
Substitutes a line from the standard input, with no further interpretation thereafter.  $< can be used to read from the keyboard in a shell script.

Command and filename substitution

The remaining, command and filename substitution are applied selectively to the arguments of built-in commands.  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 discarded.  This text replaces the original string.  Within ‘"’s, only newlines force new words; blanks and tabs are preserved. 

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 ‘~’, that word qualifies for filename substitution, also known as ‘globbing’.  This word is 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, as the characters ‘~’ and ‘{’ are more compatable to abbreviations. 

In matching filenames, the character ‘.’ at the beginning of a filename or immediately following a ‘/’, as well as the character ‘/’, must be explicitly matched.  The character ‘∗’ matches any string of characters, including the null string.  The character ‘?’ matches any single character.  The sequence ‘[...]’ matches any one of the enclosed characters.  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.  If standing alone, (i.e. ‘~’ ) this character 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 his 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 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.  For example, ‘~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 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 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 performed 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, it is created; if the file exists, its is truncated, as its previous contents are lost. 

If the variable noclobber is set, the file must not exist or be a character special file (e.g. a terminal or ‘/dev/null’) or an error results.  This condition helps prevent accidental destruction of files.  In this case, the ‘!’ forms can be used to suppress this check. 

The forms involving ‘&’ route the diagnostic output into the specified file as well as the standard output.  Name is expanded identically as ‘<’ input filenames. 

>> name

>>& name

>>! name

>>&! name
Uses file name as standard output, as with ‘>’, but places output at the end of the file.  If the variable noclobber is set, 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.  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 use of ‘<<’ 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 detached command run is not modified to be the empty file ‘/dev/null’; rather, the standard input remains as the original standard input of the shell.  If using terminal and the process attempts to read from the terminal, then the process blocks and the user is notified (see Jobs above). 

Diagnostic output may be directed through a pipe with the standard output.  Simply use the form ‘|&’ rather than only ‘|’.

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:

||  &&  |  ↑  &  ==  !=  =~  !~  <=  >=  <  >  <<  >>  +  −  ∗  /  %  !  ~  (  )

In this case, the precedence increases to the right, ‘==’ ‘!=’ ‘=~’ with ‘!~’, ‘<=’ ‘>=’ ‘<’ 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 similar to ‘!=’ and ‘==’ except the right hand side is a pattern (e.g., containing ‘∗’s, ‘?’s and instances of ‘[...]’) which the left hand operand is matched against.  This reduces the use of the switch statement in shell scripts, when all that is required 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 (‘&’ ‘|’ ‘<’ ‘>’ ‘(’ ‘)’), the components should be surrounded by spaces. 

Command executions  enclosed in ‘{’ and ‘}’ and file inquiries of the form ‘−l  name’ are also available in expressions as primitive operands, where l is one of the following:

rread access
wwrite access
xexecute access
eexistence
oownership
zzero size
fplain file
ddirectory

The specified name is command and filename expanded and tested to ensure it has the specified relationship to the real user.  If the file does not exist or is inaccessible then all inquiries return false, (i.e. ‘0’).  If the command exits with status 0; command executions succeed, returning true, (i.e., ‘1’), otherwise, they fail, returning false, (i.e. ‘0’).  If more detailed status information is required, 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 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 cannot be obtained, the shell buffers up input whenever a loop is read and performs seeks in this internal buffer to accomplish the rereading implied by the loop.  (To the extent that this procedure allows, backward goto’s succeed on non-seekable inputs.) 

Built-in commands

Built-in commands are executed within the shell.  If a built-in command occurs as any component of a pipeline, except the last, it is executed in a subshell. 

alias

alias name

alias name wordlist
The first form prints all aliases and the second form prints the alias for name. The final form assigns the specified wordlist as the alias of name; wordlist is command and filename substituted.  Name is not allowed to be alias or unalias. 

alloc
Shows the amount of dynamic core in use, broken down into used and free core, and the address of the last location in the heap. With an argument alloc shows each used and free block on the internal dynamic memory chain, indicating its address, size, and whether it is used or free. This is a debugging command and may not work in production versions of the shell; it requires a modified version of the system memory allocator.

bg

bg %job ... 
Places the current or specified jobs into the background and continues the jobs if they were stopped.

break
Causes execution to resume after the end of the nearest enclosing foreach or while.  The remaining commands on the current line are executed.  Multi-level breaks are thus possible by writing them all on one line. 

breaksw
Causes a break from a switch, resuming after the endsw. 

case label:
A label in a switch statement, as discussed below. 

cd

cd name

chdir

chdir name
Change the shells working directory to directory name.  If no argument is given, 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 ‘../’), each component of the variable cdpath is checked to ensure it has a subdirectory name.  Finally, if all attempts fail but name is a shell variable with a value that begins with ‘/’, then this command is tried to see if it is a directory. 

continue
Continue execution of the nearest enclosing while or foreach.  The remaining 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 is the current directory.

echo wordlist

echo −n wordlist
The specified words are written to the shells 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.  This command 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 with either the value of the status variable (first form) or the specified expr (second form). 

fg

fg %job ... 
Brings the current or specified jobs into the foreground, and continues the jobs if they were stopped.

foreach name (wordlist)

    ... 

end
The variable name is successively set to each member of wordlist and the sequence of commands between this command and the matching end are executed.  (Both, foreach, and end, must appear alone on separate lines.) 

The built-in command, continue, may be used to continue the loop prematurely and the built-in command, break, to terminate the loop 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 a typing error occurs in a loop at the terminal, it can be erased. 

glob wordlist
Similar to echo, but no ‘\’ escapes are recognized and words are delimited by null characters in the output.  Useful for programs which use the shell to filename expand a list of words. 

goto word
The specified word is filename and command expanded to yield a string of the form ‘label’.  The shell rewinds its input as much as possible and searches for a line of the form ‘label:’, possibly preceded by blanks or tabs.  Execution continues after the specified line. 

hashstat
Print a statistics line indicating how effective the internal hash table has been at locating commands (and avoiding exec’s).  An exec is attempted for each component of the path where the hash function indicates a possible hit, and for each component which does not begin with a ‘/’. 

history

history n

history −r n
Displays the history event list; if n is given, only the most recent n events are printed.  The −r option reverses the order of printout. 

if (expr) command
If the specified expression evaluates true, the single command with arguments is executed.  Variable substitution on command occurs early; specifically, at the same time for the rest of the if command.  Command must be a simple command, rather than a pipeline, command list, or 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; if else if expr2 is true, the commands to the second else are executed, etc.  Any number of else-if pairs are possible; only one endif is required.  The else part is also 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; if given the −l options, lists process id’s in addition to the normal information. 

kill %job

kill −sig %job ... 

kill pid

kill −sig pid ... 

kill −l
Sends either the TERM (terminate) signal or the specified signal to the specified jobs or processes. Signals are either given by number or names (as given in /usr/include/signal.h, stripped of the prefix “SIG”).  The signal names are listed by “kill −l”.  There is no default; entering only ‘kill’ does not send a signal to the current job.  If the signal being sent is TERM (terminate) or HUP (hangup), the job or process is sent a CONT (continue) signal as well. 

limit

limit resource

limit resource maximum-use
Limits the consumption by the current process and each process it creates to not individually exceed maximum-use on the specified resource.  If no maximum-use is given, the current limit is printed; if no resource is given, all limitations are given. 

Resources controllable currently include cputime (the maximum number of cpu-seconds to be used by each process), filesize (the largest single file which 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), and coredumpsize (the size of the largest core dump that will be created). 

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 scaling is ‘seconds’, ‘m’ for minutes, or ‘h’ for hours, or a time of the form ‘mm:ss’ giving minutes and seconds also may be used. 

For both resource names and scale factors, unambiguous prefixes of the names suffice. 

login
Terminate a login shell, replacing it with an instance of /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. 

newgrp
Changes the group identification of the caller; (for details see newgrp(1)). A new shell is executed by newgrp so that the shell state is lost. 

nice

nice +number

nice command

nice +number command
The first form sets the nice for this shell to 4.  The second form sets the nice to the given number.  The final two forms run command at priority 4 and number, respectively.  The super-user may specify negative niceness by using ‘nice −number ...’.  Command is always executed in a sub-shell, and the restrictions place on commands in simple if statements apply. 

nohup

nohup command
The first form can be used in shell scripts to cause hangups to be ignored for the remainder of the script. The second form causes the specified command to be run with hangups ignored. All processes detached with ‘&’ are effectively nohup’ed. 

notify

notify %job ... 
Causes the shell to asynchronously notify the user when the status of the current or specified jobs changes; normally, notification is presented before a prompt.  This command 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 are to terminate shell scripts or 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.

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, pushd rotates 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 recomputing is required 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 the above statement 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 index’th component of name to word; this component must already exist.  The final form sets name to the list of words in wordlist.  In all cases, the value is command and filename expanded. 

These arguments may be repeated to set multiple values in a single set command.  Note, however, that variable expansion happens for all arguments before any setting occurs. 

setenv name value
Sets the value of environment variable name to be value; a single string.  The most commonly used environment 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 variables. 

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 this same function on the specified variable. 

source name
The shell reads commands from name.  Source commands may be nested; however, if the commands 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.  Input during source commands is never placed on the history list. 

stop

stop %job ... 
Stops the current or specified job which is executing in the background.

suspend
Causes the shell to immediately stop, as if it had been sent a stop signal with ^Z.  This command 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 a ‘default’ label is found, 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 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 with names that 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 resource

unlimit
Removes the limitation on resource.  If no resource is specified, all resource limitations are removed. 

unset pattern
All variables with names that match the specified pattern are removed. As a result, all variables are removed by ‘unset ∗’; this command has noticeable side-effects. It is not an error for nothing to be unset. 

unsetenv pattern
Removes all variables with names that match the specified pattern from the environment.  (See also the setenv command above and printenv(1).)

wait
All background jobs are waited for. If the shell is interactive, 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 and 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 ‘|’, 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 however, are 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 variables, 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 are not modified unless this is performed explicitly by the user. 

This shell copies the environment variable USER into the variable user, TERM into term, and HOME into home, and then copies these variables back into the environment whenever the normal shell variables are reset.  The environment variable PATH is likewise handled; note that path will not set other than in the file .cshrc, as inferior csh processes import the definition of path from the environment, and re-export it if it is changed.  (Path could be set once in the .login, except commands through net(1) would not see the definition.)

argv Set to the arguments to the shell, this variable enables positional parameters to be 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 execution.  For non-built-in, commands all expansions occur before echoing.  Built-in commands are echoed before command and filename substitution, as these substitutions are then selectively performed. 

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-D’s.

mail The files where the shell checks for mail. This check is performed after each command completion, which results in a prompt if a specified interval has elapsed. The shell displays the message ‘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; the time is specified in seconds, rather than the default, which is 10 minutes. 

If multiple mail files are specified, the shell displays the message ‘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 files are not accidentally destroyed, and ‘>>’ redirections refer to existing files. 

noglob If set, filename expansion is inhibited. This command 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. However, it is still an error for the primitive pattern to be malformed, (i.e., ‘echo [’ still gives an error).

notify If set, the shell asynchronously notifies about 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, only full path names execute.  The usual search path is ‘.’, ‘/bin’ and ‘/usr/bin’, but this may vary in various systems.  For the super-user, the default search path is ‘/etc’, ‘/bin’ and ‘/usr/bin’.  A shell which is not given the −c or −t option normally hashes the contents of the directories in the path variable after reading .cshrc, resetting the path variable each time.  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 is replaced by the current event number unless a preceding ‘\’ is given. Default is ‘% ’, or ‘# ’ for the super-user.

shell The file in which the shell resides. This file is used in forking shells to interpret files which have execute bits set, but are not executable by the system. (See the description of Non-built-in Command Execution below.)  Initialized to the (system-dependent) home of the shell. 

status The status returned by the last command. If the command terminated abnormally, 0200 is added to the status. Built-in commands which fail return exit status ‘1’; all other built-in commands set status ‘0’.

time Controls automatic timing of commands. If set, then any command which exceeds more than this many cpu seconds causes a line giving user, system, and real times; this command also provides 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, verbose causes the words of each command to be printed after history substitution. 

Non-built-in command execution

When a command to be executed is found to not be a built-in command, the shell attempts to execute the command via exec(2). Each word in the variable path names a directory from which the shell attempts to execute the command.  If not given a −c or a −t option, the shell hashes the names in these directories into an internal table; as a result, the shell only attempts 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.  The function of the shell differs if this mechanism has been turned off (via unhash), if the shell was given a −c or −t argument, and for any case for each directory component of path which does not begin with a ‘/’; in these cases, 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.  Therefore, ‘(cd ; pwd) ; pwd’ prints the home directory, leaving you at the point you reached (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, it is assumed to be a file containing shell commands.  A new shell is then spawned to read this file. 

If there is an alias for shell, the words of the alias are 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 abnormally terminates or yields a non-zero exit status.

−f The shell starts faster, because it neither searches for or executes commands from the file ‘.cshrc’ in the invokers home directory.

−i The shell is interactive and prompts for its top-level input, even if the input 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 command 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 This command has the same relation to −x as −V has to −v. 

If after processing of flag arguments, 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’.  Many systems use either the standard version 6 or version 7 shells that have shell scripts incompatible with this shell.  Therefore, the shell executes 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. 

Signal handling

The shell normally ignores quit signals.  Jobs running detached (by either ‘&’, or the bg, or %... & commands) are immune to signals generated from the keyboard, including hangups.  Other signals have the values which the shell inherited from its parent.  The shells handling of interrupts and terminate signals in shell scripts can be controlled by onintr.  Login shells catch the terminate signal; otherwise, this signal is passed on to children from the state in the shell’s parent.  In no case are interrupts allowed when a login shell is reading the file ‘.logout’. 

AUTHOR

William Joy.  Job control and directory stack features first implemented by J.E. Kulp of I.I.A.S.A, Laxenburg, Austria, with different syntax than that used now. 

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 not exceed 1024 characters.  The system limits argument lists to 10240 characters.  The number of arguments to a command which involves filename expansion is limited to 1/6’th which is the number of characters allowed in an argument list.  Command substitutions may not exceed the amount of characters allowed in an argument list.  To detect looping, the shell restricts the number of alias substititutions on a single line to 20. 

SEE ALSO

sh(1), newcsh(1), access(2), exec(2), fork(2), killpg(2), pipe(2), sigsys(2), umask(2), vlimit(2), wait(2), jobs(3), sigset(3), tty(4), a.out(5), environ(5), ‘An introduction to the C shell’

BUGS

When a command is restarted from a stop, the shell prints the directory it started in if this differs from the current directory; this print can be misleading or incorrect as the job may have internally changed directories. 

Shell built-in functions are not stoppable/restartable.  Command sequences of the form ‘a ; b ; c’ are also not handled inadequately when stopping is attempted.  If you suspend ‘b’, the shell immediately executes ‘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 )’). 

 After processes are started, control over tty output is primitive. 

Alias substitution is most often used to clumsily simulate shell procedures, shell procedures should be provided rather than aliases. 

Commands within loops, prompted by ‘?’, are not placed in the history list.  Control structure should be parsed rather than recognized as built-in commands.  Parsing allows control commands to be placed anywhere, to be combined with ‘|’, and 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. 

4th Berkeley Distribution  —  9/28/80