C Shell

CIS 218

C Shell overview

• Csh (new version tchs) is a command language interpreter developed under BSD UNIX. It incorporates features of other shells and a history mechanism. Tcsh development is currently maintained by tcsh.org.

• Developed for C programmers who don’t want to learn another language syntax.

• Most of the rules for command alias, job control, redirection, regular expressions, quoting and filename expansion apply to the C Shell same as the other shells.

• The C Shell parses words/tokens following the same rules as bash

• In addition to the standard file and directory manipulation commands, C Shell also supports the pushd and popd commands for directory navigation. Commands also now supported under bash.

Shell operation

• During startup the C shell begins by executing commands in sequence from:

/etc/csh.cshrc, /etc/csh.login,

~/.tcshrc or ~/.cshrc, ~/.history (or the value of the histfile shell variable)

~/.login

~/.cshdirs (or the value of the dirsfile shell variable)

.login may be read before the other files depending on C Shell copile options.

Non-login shells read only /etc/csh.cshrc and ~/.tcshrc or ~/.cshrc on startup.

• At logout, it executes:

/etc/csh.logout

~/.logout in your home directory

Shell variables

• Each shell variable has as value an array of zero or more strings. Shell variables are assigned values by the set command of the form:

set name=value

• As with bash, value can be null. $?name tests if a variable has been defined. $#name determines size of the value.

• The setenv command can be used to set variables in the environment defined during C shell startup. Thus

setenv TERM adm3a

will set the value of the environment variable TERM to ‘adm3a’. Notice the absence of “=”. A user program printenv exists which will

print out the environment variables.

• The unalias and unset commands can be used to remove aliases and variable definitions from the shell, and unsetenv removes variables from the environment.

Shell Environment Variables

• $filec turns on file completion

• $ignoreeof disables ctrl-D as logout

• $noclobber stops file overwriting with

• $notify immediate notification about background job changes

• $HOME user’s home directory path

• $PATH array of pathnames to commands

• $status exit status of last command

Variable substitution

• After each input line is broken into words and history substitutions are done on it, the input line is parsed into distinct words separate by the IFS or whitespace character.

• Variable substitution is done on these words. Variables are referenced in the same fashion as bash, by preceding the variable name with a $, i.e. echo $TERM. As with bash, this is direct string substitution.

• A number of notations are provided for accessing components and attributes of variables.

• The notation $?name expands to ‘1’ if name is set or to ‘0’ if name is not set. This is a mechanism used for checking whether particular variables have been assigned values.

• C Shell follows the same variable quoting rules as bash.

Variable arrays

• C Shell can store variables in a list called arrays. Values are assigned with a list delimited by parentheses and IFS/whitespace separating the individual values corresponding with the usual variable quoting rules. Each array element is referenced by the $variable name and an index # in square brackets[]. For example:

$ set colours = (red green blue orange)

$ echo $colours

red green blue orange

$ echo $colours[3]

blue $ echo $colours[2-4]

green blue orange

• In addition to $?name, above $#name specifies the number of elements in the array. $*name and $name both reference the entire array as one variable.

• Arrays can be defined and initialized separately:

$ set shapes = (’’ ’’ ’’ ’’ ’’)

$ echo $shapes

$ set shapes[4] = square

$ echo $shapes[4]

square

Numeric Variables

• Use the @ command for variables holding numbers.

$ @ count = 0 (or set count = 0)

$ echo $count

0

$ @ count = ( 5 + 2 )

$ echo $count

7

$ @ result = ( $count < 5 )

$ echo $result

0

• C Shell uses the same arithmetic syntax as BASH 4.0 (other than use of @):

@ count = $count + 5

$ echo $count 12

$ @ count += 5

$ @ count++ $ echo $count 13

Numeric / logical operators

• Operator What it Means

() change precedence

~ complement

! negation

*/ % multiply, divide, modulo

+ - add, subtract

<< > > left shift, right shift

<= >= < > relational operators

== != =~ !~ string comparison/pattern matching

& bitwise "and"

^ bitwise "exclusive or"

| bitwise "inclusive or"

&& logical "and"

|| logical "or"

Numeric Arrays

$ set ages = (0 0 0 0 0)

$ @ ages[2] = 15

$ @ ages[3] = ( $ages[2] + 4 )

$ echo $ages[3]

19

$ echo $ages

0 15 19 0 0

C Shell Command Line Arguments

• Referenced in C Shell as an array - $argv

• $argv[0] or $0 command name

• $argv[1] or $1 first argument

of command

• Also $argv[2] or $2, $argv[3][ or$3,... no upper limit; no need for shift

• $argv[*] or $* array of all arguments

• $#argv number of arguments

System Variables • $name or ${name} Substitutes 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.

• $name[selector] or ${name[selector]} Substitutes only the selected words from the value of name by #.

• $0 Substitutes the name of the file from which command input is being read.

• $number or ${number} Equivalent to `$argv[number]`.

• $* Equivalent to `$argv`, which is equivalent to `$argv[*]`

• $argv[N] same as above

• $argv same as above

• $?name or ${?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 not. `0` in

interactive shells.

• $#name or ${#name} Substitutes the number of words in name.

• $# Equivalent to `$#argv`.

• $%name or ${%name} Substitutes the number of characters in name.

• $%number or ${%number} Substitutes the number of characters in $argv[number].

• $? Equivalent to `$status`.

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

• $! Substitutes the (decimal) process number of the last background process started

• $_ Substitutes the command line of the last command executed.

• $( Substitutes a line from the standard input, with no further Interpretation.

Used to read from the keyboard in a shell script.

C Shell Scripts

• 1) Make executable

• 2) Invoke C

- #!/bin/csh as the first line normal execution

- #!/bin/csh -f as first line to switch off the default initial execution of the

.cshrc scripts

- $ csh script

- $ chsh csh

• Debugging

#!/bin/csh –vx

C Shell Control Structures

• File Test Conditions

• These occur in the expression part of if-then and other condition tests -d file file is a directory -f file file is a regular file -e file file exists -z file file is 0 bytes long -r file file is readable -w file writable -x file executable -o file ownership

- s file Non-zero size

- l file Symbolic link

- b file Block special file

- c file Character special file

- p file Named pipe (fifo)

C Shell Control Structures • if-then

if (expression) then

commands endif

if (expression) then

commands else commands endif

if (expression) then

commands

else if (expression) then

commands

else

commands

endif

C Shell Control Structures

• If Example: #!/bin/csh -f

# Set class depending on argument value set number = $argv[1] if ($number < 0) then @ class = 0 else if ($number >= 0 && $number < 100) then @ class = 1 else if ($number >= 100 && $number < 200)then @ class = 2 else @ class = 3 endif echo The number $number is in class $class

C Shell Control Structures

• switch (string variable)

case pattern: commands breaksw case pattern: commands breaksw : default: commands breaksw endsw

• break # causes case processing to terminate (same as bash)

• breaksw # causes case processing to terminate for this selection (switch)

C Shell Control Structures

• Switch Example:

#!/bin/csh -f # See if first argument is yes or no. # Deal with a mix of upper and lower case. # Does argv[1] exist? if ($#argv == 0) then echo “Usage: switch_1 [yes | no]” exit 1 endif

switch ($argv[1]) case [yY][eE][sS]: echo Argument 1 is yes breaksw case [nN][oO]: echo Argument 1 is no breaksw default: echo Argument 1 is neither yes or no breaksw endsw

C Shell Control Structures

• foreach foreach loop-index (argument-list)

commands end

• Example: #!/bin/csh -f

# Usage: rename arg1 arg2 # # Changes the string arg1 to arg2 # in the filenames of every file in # the current working directory. if ($#argv != 2) then echo Usage: refname arg1 arg2 exit 1 endif

foreach i (‘ls‘) set newname = ‘echo $i | sed s/$1/$2/‘ mv $i $newname end

C Shell Control Structures

• Foreach Example

• #!/bin/csh -f

# Assign up to 6 command line args # to the buffer array. Execute foo # with buffer as its arguments set buffer = (0 0 0 0 0 0) @ count = 1 if ($#argv > 6) then echo “Usage: foo-six [up to 6 args]” exit 1 endif

foreach arg ($argv[*]) set buffer[$count] = $arg @ count++ end echo There were $count arguments exec foo $buffer[*] exit 0

C Shell Control Structures

• while while (expression)

commands end

Example: Sum the numbers between 1 and

# the value in argv[1] @ limit = $argv[1] @ index = 1 @ sum = 0 while ($index <= $limit) @ sum += $index @ index++ end echo The sum is $sum

C Shell Control Structures

C Shell Control Structures

• goto Command

• goto label

...

other shell commands

...

label:

shell commands

C Shell Control Structures

• Recursion (script calling itself) is allowed in C Shell (and C):

• Example:

foreach file ($1/*) # files in directory if (-f $file) then # do something to the file else if (-d $file) then $0 $file # recursive call endif end

C Shell Control Structures • Interrupt Handling: onintr label The script must have a line: label:

Transfers control to label after intercepting Ctrl C • Example: #!/bin/csh -f

onintr close while (1) echo Program is running. sleep 2 end close: echo End of Program

• Keyboard input:

set x = $<

- or –

set x = `head -1`

• reads input from the keyboard up until RETURN <LF> and places results in x.

C Shell Futures

• Csh is now tcsh under newer systems.

• Development now separated from UNIX BSD flavors; supported by tcsh.org.

• Programatically not as sophisicated as Korn shell. But Korn contains most C shell arithmetic and logical functions.

• More C shell features in Korn shell and later versions of bash.

• UNIX programming now usually done in environments other than UNIX shell, limiting audience for C shell.

• LINUX/UNIX system scripts in Bourne (again) Shell.