NAME

csh − invoke a shell command interpreter that uses C-like syntax

SYNOPSIS

csh [ −cefinstvVxX ] [ arg ... ]

DESCRIPTION

csh is a command language interpreter.  It 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 user’s .login file.  In the normal case, the shell will then begin reading (see read(2)) 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 &, |, ;, <, >, (, and ) form separate words.  If doubled (for example, &&, ||, <<, or >>) these character pairs form single words.  These parser metacharacters may be made part of other words, or you can take away their special meaning by preceding them with a backslash character (\).  A new-line character preceded by a \ is equivalent to a blank. 

Strings enclosed in matched pairs of quotations, ´, `, or ", form parts of a word; metacharacters in these strings, including blanks and tabs, do not form separate words.  The semantics of these quotations are described below.  Within pairs of ´ or " characters, a new-line character preceded by a \ gives a true new-line character. 

When the shell’s input is not from a terminal, the character # introduces a comment which continues to the end of the input line.  It does not have this special meaning when preceded by \ or when placed inside the quotation marks `, ´, and ". 

Commands

A simple command is a sequence of words, the first of which specifies the command to be executed.  A simple command or a sequence of simple commands separated by | characters forms a pipeline.  The output of each command in a pipeline provides the input for the next command.  Sequences of pipelines may be executed sequentially by separating them with the ; character.  A sequence of pipelines may be executed in the background by following it with the & character. 

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

Jobs

The shell associates a job with each pipeline.  It keeps a table of current jobs, printed by the jobs command, and assigns them small integer numbers.  When a job is started asynchronously with &, the shell prints a line that looks like:

[1] 1234

indicating that the job that was started asynchronously was job number 1 and it had one (top-level) process, whose process ID was 1234. 

If you are running a job and wish to do something else, you may use ^Z (CTRL-Z) which sends a STOP signal to the current job.  (By default, the switch key­stroke used to suspend a job is undefined (job control is disabled).  To define the job control switch character (enable job control), use the stty command, for example, stty swtch ^z.) The shell will then normally indicate that the job has been “Stopped” and print another prompt. You can then manipulate the state of this job, putting it in the background with the bg command, or run some other commands and then eventually bring the job back into the foreground with the foreground command fg. A ^Z takes effect immediately and is like an interrupt in that pending output and unread input are discarded when it is typed. 

A job being run in the background will stop if it tries to read from the terminal.  Background jobs are normally allowed to produce output, but this can be disabled by giving the command “stty tostop.” If you set this terminal option, then background jobs will stop when they try to produce output as they do when they try to read input. 

There are several ways to refer to jobs in the shell.  The character % introduces a job name.  If you wish to refer to job number 1, you can name it as %1.  Just naming a job brings it to the foreground; thus %1 is a synonym for fg %1, bringing job 1 back into the foreground.  Similarly, saying %1& resumes job 1 in the background.  Jobs can also be named by prefixes of the string typed in to start them, if these prefixes are unambiguous; thus %ex would normally restart a suspended ex(1) job, if there were only one suspended job whose name began with the string ex.  It is also possible to say %? string which specifies a job whose text contains string, if there is only one such job.

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

Status Reporting

This shell learns immediately whenever a process changes state.  It normally informs you whenever a job becomes blocked so that no further progress is possible, but only just before it prints a prompt.  This is done so that it does not otherwise disturb your work.  If, however, you set the shell variable notify, the shell will notify you immediately of changes of status in background jobs.  There is also a shell command notify which marks a single process so that its status changes will be immediately reported.  By default notify marks the current process; simply say notify after starting a background job to mark it. 

When you try to leave the shell while jobs are stopped, you will be warned that “You have suspended jobs.”  You may use the jobs command to see what they are.  If you do this or immediately try to exit again, the shell will not warn you a second time, and the suspended jobs will be terminated. 

Substitutions

The following sections describe the various transformations the shell performs on the input in the order in which they occur. 

History Substitutions

History substitutions can be used to reintroduce sequences of words from previous commands, possibly performing modifications on these words.  Thus, history substitutions provide a generalization of a redo function. 

History substitutions begin with the !  character and may begin anywhere in the input stream if a history substitution is not already in progress.  This !  may be preceded by a \ to prevent its special meaning; a !  is passed unchanged when it is followed by a blank, tab, new-line character, =, or (.  History substitutions also occur when an input line begins with ^.  This special abbreviation will be described later. 

Any input line that contains a history substitution is echoed on the terminal before it is executed as it could have been typed without a history substitution. 

Commands input from the terminal that consist of one or more words are saved on the history list, the size of which is controlled by the history variable.  The previous command is always retained.  Commands are numbered sequentially from 1. 

For example, 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 !w 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 re-introduce 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. The form !# references the current command (the one being typed in).  It allows a word to be selected from further left in the line, to avoid retyping a long name, as in !#:1. 

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

Designator Description

0 First (command) word

n nth argument

^ First argument, i.e., 1

$ Last argument

% Word matched by (immediately preceding) ?s? search

x−y Range of words

−y Abbreviates 0−y

∗ Abbreviates ^−$, or nothing if only one word in event

x∗ Abbreviates x−$

x− Like x∗ but omitting word $

The colon 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:

Modifier Description

h Removes a trailing path name component. 

r Removes a trailing .xxx component. 

s/l/r/ Substitutes l for r. 

t Removes all leading path name components. 

& Repeats the previous substitution. 

g Applies the change globally, prefixing the above. 

p Prints the new command but does not execute it. 

q Quotes the substituted words, preventing substitutions. 

x Like q, but breaks into words at blanks, tabs, and new-line characters. 

Unless preceded by a g, the modification is applied only to the first modifiable word.  In any case, it is an error for no word to be ­applicable. 

The left 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 slash (/); a backslash (\) quotes the delimiter into the l and r strings.  The character & in the right-hand side is replaced by the text from the left-hand side.  A \ quotes & also.  A null l uses the previous string either from a l or from a contextual scan string s in !?s?.  The trailing delimiter in the substitution may be omitted if a new-line character follows immediately, as may the trailing ?  in a contextual scan. 

A history reference may be given without an event specification, e.g., !$.  In this case, the reference is to the previous command unless a previous history reference occurred on the same line, in which case this form repeats the previous reference.  Thus, !?foo?^!$ gives the first and last arguments from the command matching ?foo?. 

A special abbreviation of a history reference occurs when the first nonblank character of an input line is a ^.  This is equivalent to !:s^, providing a convenient shorthand for substitutions on the text of the previous line.  Thus, ^lb^lib fixes the spelling of lib in the previous command.  Finally, a history substitution may be surrounded with { and } if necessary to insulate it from the characters that follow.  Thus, after ls −ld ~paul we might do !{l}a to do ls −ld ~paula, while !la would look for a command starting with la.

Quotations With Single Quotes (’) and Double Quotes (")

The quotation of strings by ´ and " can be used to prevent all or some of the remaining substitutions.  Strings enclosed in ´ are prevented from any further interpretation.  Strings enclosed in " are variable, and command expansion may occur. 

In both cases, the resulting text becomes (all or part of) a single word; only in one special case (see Command Substitution below) does a " quoted string yield parts of more than one word; ´ quoted strings never do. 

Alias Substitution

The shell maintains a list of aliases that can be established, displayed, and modified by the alias and unalias commands.  After a command line is scanned, it is parsed into distinct commands, and the first word of each command, left-to-right, is checked to see if it has an alias.  If it does, then the text 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 −l /usr.  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, you can alias print ´pr \!∗ | lpr´ to make a command that paginates its arguments to the line printer. 

Variable Substitution

The shell maintains a set of variables, each of which has as value zero or more words.  Some of these variables are set by the shell or referred to by it.  For instance, the argv variable is an image of the shell’s argument list, and words of this variable’s value are referred to in special ways. 

The values of variables may be displayed and changed by using the set and unset commands.  Of the variables referred to by the shell, a number are toggles; the shell does not care what their value is, only whether they are set or not.  For instance, the verbose variable is a toggle that causes command input to be echoed.  The setting of this variable results from the −v command line option. 

Other operations treat variables numerically.  The at-sign (@) command permits numeric calculations to be performed and the result assigned to a variable.  However, variable values 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, keyed by dollar sign ($) characters.  This expansion can be prevented by preceding the dollar sign with a backslash (\) except within double quotation marks ("), in which case it always occurs, and within single quotation marks (´) where it never occurs.  Strings quoted by back quotation marks (`) are interpreted later (see Command Substitution below) so dollar sign substitution does not occur there until later, if at all.  A dollar sign is passed unchanged if followed by a blank, tab, or end-of-line. 

Input and output redirections are recognized before variable expansion and are expanded separately.  Otherwise, the command name and entire argument list are expanded together.  It is thus possible for the first (command) word 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 quotation marks or given the :q modifier, the results of variable substitution may eventually be command and file name substituted.  Within double quotation marks (") a variable whose value consists of multiple words expands to a portion of a single word, with the words of the variable’s value separated by blanks.  When the :q modifier is applied to a substitution, the variable expands to multiple words with each word separated by a blank and quoted to prevent later command or file name substitution. 

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

$name

${name}
Are replaced by the words of the value of variable name, each separated by a blank.  Braces insulate name from following characters which would otherwise be part of it.  Shell variables have names consisting of up to 120 letters, digits, and underscores.  If name is not a shell variable, but is set in the environment, then that value is returned.  However, modifiers and the other forms shown in the following list 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 variable 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.  Only one : modifier is allowed on each $ expansion. 

The following substitutions may not be modified with : modifiers:

$?name

${?name}
Substitutes the string 1 if name is set and 0 if it is not.

$?0 Substitutes 1 if the current input file name is known and 0 if it is not. 

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

Command and File Name Substitution

Command and file name substitution are applied selectively to the arguments of built-in commands.  This means that portions of expressions that are not evaluated are not subjected to these expansions.  For commands that are not internal to the shell, the command name is substituted separately from the argument list.  This occurs very late, after input-output redirection is performed, and in a child of the main shell. 

Command Substitution

Command substitution is indicated by a command enclosed in back quotation marks.  The output from such a command is normally broken into separate words at blanks, tabs, and new-lines, with null words being discarded.  This text then replaces the original string.  Within double quotation marks, only new-line characters force new words; blanks and tabs are preserved. 

In any case, the single final new-line character 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 displays a complete line. 

File Name Substitution

If a word contains any of the characters ∗, ?, [, or {, or begins with the character ~, then that word is a candidate for file name substitution, also known as “globbing.” This word is then regarded as a pattern and is replaced with an alphabetically sorted list of file names that match the pattern.  In a list of words specifying file name 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 file names, the character .  at the beginning of a file name 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 file name is used to refer to home directories.  Standing alone it expands to the invoker’s home directory as reflected in the value of the variable home.  When followed by a name consisting of letters, digits, and − characters, the shell searches for a user with that name and substitutes the user’s home directory; thus, ~ken might expand to /usr/ken and ~ken/chmach to /usr/ken/chmach.  If the ~ character is followed by a character other than a letter or /, or does not appear at the beginning of a word, it is left unchanged. 

The metanotation a{b,c,d}e is a shorthand for abe ace ade.  Left to right order is preserved, with results of matches being sorted separately at a low level to preserve this order.  This construct may be nested.  Thus, ~source/s1/{oldls,ls}.c expands to /usr/source/s1/oldls.c /usr/source/s1/ls.c, whether or not these files exist, without any chance of error if the home directory for source is /usr/source.  Similarly ../{memo,∗box} might expand to ../memo ../box ../mbox.  (Note that memo was not sorted with the results of matching ∗box.)  As a special case {, }, and {} are passed unchanged. 

Input/Output

The standard input and standard output of a command may be redirected with the following syntax:

< name
Opens file name (which is first variable, command, and file name expanded) as the standard input. 

<< word
Reads the shell input up to a line which is identical to word. word is not subjected to variable, file name, or command substitution, and each input line is compared to word before any substitutions are made on this input line.  Unless a quoting backslash, double or single quotation mark, or a back quotation mark appears in word, variable and command substitution is performed on the intervening lines, allowing \ to quote $, \, and `.  Commands that are substituted have all blanks, tabs, and new-line characters preserved except for the final new-line character, which is dropped.  The resulting 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, and its previous contents are lost. 

If the variable noclobber is set, then the file must not already exist or it must 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 to 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 file names 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, it is similar to >. 

If a command is run detached (followed by &), then the default standard input for the command is the empty file /dev/null.  Otherwise, the 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 |. 

Expressions

A number of the built-in commands (to be described later) 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, with the following operators forming groups at the same level:

== and !=
<=, >=, <, and >
<< and >>
+ and −
∗ / and %

The == and != operators compare their arguments as strings; all others operate on numbers.  Strings that begin with 0 are considered octal numbers.  Null or missing arguments are considered 0.  The result of all expressions are strings, which represent decimal numbers.  It is important to note that no two components of an expression can appear in the same word; they should be surrounded by spaces except when adjacent to components of expressions which are syntactically significant to the parser [& | < > ( )]. 

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 the following characters:

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

The specified name is command- and file name-expanded, 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 that can be used to control command files (shell scripts) and, in limited but useful ways, 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 command line. 

If the shell’s input is not seekable, the shell buffers up input whenever a loop is being read and performs seeks in this internal buffer to accomplish the rereading implied by the loop.  (To the extent that this allows, backward goto commands will succeed on nonseekable inputs.) 

Built-In Commands

Built-in commands are executed within the shell.  If a built-in command occurs as any component of a pipeline except the last, then it is executed in a subshell.  The following list describes the syntax and function of the built-in commands:

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 file name substituted.  name is not allowed to be alias or unalias. 

bg

bg% job... Puts the current or specified jobs into the background, continuing them if they were stopped.

break
Causes execution to resume after the end of the nearest enclosing foreach or while statement.  The remaining commands on the current line are executed.  Multilevel 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
Changes the shell’s working directory to directory name. If no argument is given, then it changes 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
Continues execution of the nearest enclosing while or foreach. The rest of the commands on the current line are executed.

default:
Labels the default case in a switch statement.  The default should come after all case labels. 

dirs Prints the directory stack; the top of the stack is at the left, the first directory in the stack being the current directory.

echo wordlist
The specified words are written to the shell’s standard output. A \c causes the echo to complete without printing a new-line character.  A \n in wordlist causes a new-line character to be printed.  Otherwise, the words are echoed, separated by spaces. 

else

end

endif

endsw
See the following descriptions of the foreach, if, switch, and while statements. 

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 built-in command continue may be used to continue the loop prematurely, and the built-in command break may be used 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. 

glob wordlist
Like echo, but no \ escapes are recognized, and words are delimited by null characters in the output. Useful for programs that wish to use the shell to file name-expand a list of words.

goto word
The specified word is file name-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
Displays the history event list. Using the −r flag gives the history in reverse order (most recent events first). 

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. 

if (expr) then

    ... 

else if (expr2) then

    ... 

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.)

jobs

jobs −l
Lists the active jobs; given the −l options lists process IDs in addition to the normal information. 

kill %job

kill −sig %job

kill pid

kill −sig pid ...

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

limit

limit resource

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

Resources controllable currently include filesize (the largest single file which can be created) and datasize (the maximum growth of the data+stack region via sbrk(2) beyond the end of the program text).

The maximum-size may be given as a (floating point or integer) number followed by a scale factor.  For all limits the default scale is “k” or “kilobytes” (1024 bytes); a scale factor of “m” or “megabytes” may also be used. 

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

logout
Terminates a login shell. This is the only way to log out 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 superuser may specify negative niceness by using “nice −number ....” The command is always executed in a subshell, and the restrictions placed on commands in simple if statements apply. 

nohup

nohup command
The first form can be used in shell scripts to cause hangups to be ignored for the remainder of the script. The second form causes the specified command to be run with hangups ignored.  Unless the shell is running detached, nohup has no effect. 

notify

notify%job ...
Causes the shell to notify the user asynchronously when the status of the current or specified jobs changes; normally notification is presented before a prompt. This is automatic if the shell variable notify is set. 

onintr

onintr  −

onintr  label
Controls the action of the shell on interrupts. The first form restores the default action of the shell on interrupts: 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. 

rehash
Causes the internal hash table of the directories’ contents 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 be necessary only 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 that 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 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 file name-expanded. These arguments may be repeated to set multiple values in a single set command. Note, however, that variable expansion happens for all arguments before any setting occurs.

setenv

setenv name value
The first form of the command shows all environment variables. The second form sets the value of the environment variable name to be value, a single string. Useful environment variables are TERM, the type of your terminal, and SHELL, the shell you are using. 

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
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.  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 stop in its tracks, much as if it had been sent a stop signal with ^Z.  This is most often used to stop shells started by su(1).

switch (string)

case str1:

    ... 

  breaksw

    ... 

default:

    ... 

  breaksw

endsw
Each case label is successively matched against the specified string that is first command- and file name-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 match the unalias pattern. 

unhash
Use of the internal hash table to speed location of executed programs is disabled.

unlimit

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

unset pattern
All variables whose names match the specified pattern are removed.  Thus, all variables are removed by unset ∗; this has noticeably undesirable side-effects.  It is not an error for nothing to be unset.

wait
All child processes are waited for. It the shell is interactive, then an interrupt can disrupt the wait, at which time the shell prints names and process numbers of all children known to be ­outstanding.

while(expr)

    ... 

end
While the specified expression evaluates nonzero, the commands between the while and the matching end are evaluated.  Break and continue may be used to terminate or continue the loop prematurely.  (The while and end must appear alone on their input lines.)  Prompting occurs here the first time through the loop as for the foreach statement if the input is a terminal. 

%job Brings the specified job into the foreground. 

%job &
Continues the specified job in the background.

@

@ name = expr

@ name [ index ] = expr
The first form prints the values of all the shell variables. The second form sets the specified name to the value of expr. If the expression contains <, >, &, or |, then at least this part of the expression must be placed within (). The third form assigns the value of expr to the index argument of name. Both name and its index component must already exist. 

Assignment operators, such as ∗= and +=, are available as in C.  The space separating the name from the assignment operator is optional.  Spaces 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++. 

Predefined Variables

The following variables have special meaning to the shell.  Of these, argv, child, home, path, prompt, shell, and status are always set by the shell.  Except for child and status, this setting occurs only at initialization; these variables will not then be modified unless done explicitly by the user.

Variable Description

argv Set to the arguments of the shell; from this variable, positional parameters are substituted, i.e., $1 is replaced by $argv[1]. 

cdpath Gives a list of alternate directories searched to find subdirectories in cd commands. 

child The process number printed when the last command was forked with &.  This variable is unset when this process terminates. 

echo Set when the −x command line option is given.  Causes each command and its arguments to be echoed just before it is executed.  For nonbuilt-in commands, all expansions occur before echoing.  Built-in commands are echoed before command and file name substitution since these substitutions are then done selectively. 

histchars Can be assigned a two-character string.  The first character is used as a history character in place of !; the second character is used in place of the ^ substitution mechanism.  For example, set histchars = ,; will cause the history characters to be comma and semicolon. 

history Can be given a numeric value to control the size of the history list. Any command that has been referenced in this many events will not be discarded. A history that is too large may run the shell out of memory.  The last executed command is always saved on the history list. 

home The home directory of the user, initialized from the environment. The file name expansion of ~ refers to this variable. 

ignoreeof If set, the shell ignores the end-of-file from input ­devices that are terminals. This prevents a shell from accidentally being terminated by typing a CTRL-D. 

mail The files where the shell checks for mail. This is done after each command completion results in a prompt, if a specified interval has elapsed. The shell sends 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, in seconds, than the default, which is 10 minutes.  If multiple mail files are specified, then the shell sends the message “New mail in name” when there is mail in the file name.

noclobber Restrictions are placed on output redirection to insure that files are not accidentally destroyed and that >> redirections refer to existing files. 

noglob If set, file name expansion is inhibited. This is most useful in shell scripts that are not dealing with file names or after a list of file names has been obtained and further expansions are not desirable.

nonomatch If set, it is not an error for a file name 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. 

notify If set, the shell notifies asynchronously of job completions. The default is to rather present job completions just before printing a prompt.

path Each word of the path variable specifies a directory in which commands are to be sought for execution. A null word specifies the current directory. If there is no path variable, then only full path names will execute.  The usual search path is ., /bin, and /usr/bin, but this may vary from system to system.  A shell that is given neither the −c nor the −t option will normally hash the contents of the directories in the path variable after reading .cshrc and each time the path variable is reset.  If new commands are added to these directories while the shell is active, it may be necessary to give the rehash, or the commands may not be found.

prompt The string that is printed before each command is read from an interactive terminal input. If a !  appears in the string, it will be replaced by the current event number unless a preceding \ is given.  The default is % or # for the superuser. 

shell The file in which the shell resides. This is used in forking shells to interpret files that have execute bits set but are not executable by the system (see the section Nonbuilt-In Command Execution below.)  shell is 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. Abnormal termination results in a core dump. Built-in commands that fail return exit status 1; all other built-in commands set status 0.

time Controls automatic timing of commands. If set, then any command that takes more than this many CPU seconds will cause a line giving user, system, and real times and a utilization percentage (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. 

The shell copies the environment variable PATH into the variable path and copies the value back into the environment whenever path is set.  Thus, 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. 

Nonbuilt-In Command Execution

When a command to be executed is found to not be a built-in command, the shell attempts to execute the command via 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 cd from affecting the current shell. 

If the file has execute permissions but is not an executable binary to the system, then it is assumed to be a file containing shell commands and a new shell is spawned to read it. 

If there is an alias for shell, then the words of the alias will be prepended to the argument list to form the shell command. The first word of the alias should be the full path name of the shell (e.g., $shell).  Note that this is a special, late occurring case of alias substitution and only allows words to be prepended to the argument list without modification. 

Argument List Processing

If argument 0 to the shell is −, then this is a login shell.  The flag arguments are interpreted as follows:

Flag Description

−c Reads commands from the (single) following argument which must be present.  Any remaining arguments are placed in argv.

−e Causes the shell to exit if any invoked command terminates abnormally or yields a nonzero exit status. 

−f Lets the shell start faster because it will neither search for nor execute commands from the file .cshrc in the user’s home ­directory. 

−i Makes the shell interactive.  The shell prompts for its top-level input even if it appears not to be a terminal.  Shells are interactive without this option if their inputs and outputs are terminals. 

−n Causes commands to be parsed but not executed.  This may aid in syntactic checking of shell scripts. 

−s Causes command input to be taken from the standard input. 

−t Reads and executes a single line of input.  A backslash (\) can be used to escape the new-line character 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 Causes the echo variable to be set even before .cshrc is ­executed. 

After processing of flag arguments, if arguments remain but none of the −c, −i, −s, or −t options were given, the first argument is taken as the name of a file of commands to be executed.  The shell opens this file and saves its name for possible resubstitution by $0.  Since on a typical system most shell scripts are written for the standard shell (see sh(1)), the C shell will execute such a standard shell if the first character of a script is not a #, i.e., if the script does not start with a comment.  Remaining arguments initialize the variable argv.

Signal Handling

The shell normally ignores quit signals.  The signals have the values that the shell inherited from its parent.  The shell’s handling of interrupts 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. 

NEW ENVIRONMENT VARIABLES

The new environment variable described in this section has been added to the C shell.  The C shell will behave normally for those users who do not set DOSPATH.  Users who wish to be able to execute MS-DOS (DOS) programs directly from the C shell, that is, bypassing the normal DOS bootup that occurs when running vpix, should set DOSPATH to include those directories in PATH that contain DOS executables. 

DOSPATH is a string with the same format as PATH; it contains a subset of the list of directories from PATH.  When searching a directory in PATH for a program, the C shell determines whether that directory is also in DOSPATH.  If it is not, the C shell acts as usual.  If it is, the C shell looks first for the command with the suffix .com, then .exe, then .bat, and finally, for the command without any suffix.  Whenever the result of a path search gives a file with one of these DOS suffixes, the shell runs the vpix program via a standard search path and adds arguments −c and the full path name of the DOS program (including the suffix). 

The internal shell variable DOSPATH also does this in a way analogous to PATH and path. 

For example, if PATH is set to :/bin:/usr/bin, DOSPATH is set to ., the current directory is /usr/john/dosbin, and there is a DOS program named abc.com in the current directory, then typing abc to the C shell will cause the command vpix −c /usr/john/dosbin/abc.com to be executed, which will run the DOS program abc.com without the normal vpix DOS bootup. 

FILES

~/.cshrc Read by each shell at the beginning of execution

~/.login Read by login shell after .cshrc at login

~/.logout Read by login shell at logout

/bin/sh Shell for scripts not starting with a #!  /bin/csh

/tmp/sh∗ Temporary file for <<

/dev/null Source of empty file

/etc/passwd Source of home directories for ~name

/etc/default/.cshrc
Default file of automatically invoked commands

SEE ALSO

umask(1), wait(1). 
access(2), exec(2), fork(2), pipe(2), signal(2), a.out(4), environ(5) in the INTERACTIVE SDS Guide and Programmer’s Reference Manual.

CREDIT

This utility was developed at the University of California at Berkeley and is used with permission. 

NOTES

Words can be no longer than 512 characters.  The number of arguments to a command which involves file name expansion is limited to 1/6 of the number of characters allowed in an argument list, which is 5120 less the characters in the environment.  Also, 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. 

When a command is restarted from a stop, the shell prints the directory it started in if this is different from the current directory; this can be misleading (i.e., wrong) as the job may have changed directories internally. 

Built-in control structure commands like foreach and while cannot be used with the pipe symbol (|), ampersand (&), or semicolon (;). 

Shell built-in functions are not stoppable/restartable.  Command sequences of the form “a;b;c” are also not handled gracefully when stopping is attempted.  If you suspend “b,” the shell will then immediately execute “c.” This is especially noticeable if this expansion results from an alias. It suffices to place the sequence of commands in parentheses to force it to a subshell; i.e., “(a;b;c).”

Commands within loops prompted for by ?  are not placed in the ­history list. 

It is not possible to use the colon (:) modifiers on the output of command substitutions. 

csh attempts to import and export the PATH variable for use with regular shell scripts.  This only works for simple cases, where the PATH contains no command characters. 

You can modify the list of commands that csh automatically invokes by editing the /etc/default/.cshrc file.  For example, if you want to automatically assign the alias h to the history command, add the following line to the /etc/default/.cshrc file using the computer editor of your choice:

alias history h

\*U  —  Version 1.0