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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.

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c shell

C Shell

CIS 218

c shell overview
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
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
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
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
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
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
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 $count12

$ @ count += 5

$ @ count++$ echo $count13

numeric logical operators
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
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
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
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
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
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 structures1
C Shell Control Structures
  • if-then

if (expression) then commandsendif

if (expression) then commandselse commandsendif

if (expression) then

commands

else if (expression) then

commands

else

commands

endif

c shell control structures2
C Shell Control Structures
  • If Example:

#!/bin/csh -f# Set class depending on argument valueset number = $argv[1]if ($number < 0) then @ class = 0else if ($number >= 0 && $number < 100) then @ class = 1else if ($number >= 100 && $number < 200)then @ class = 2else @ class = 3endifecho The number $number is in class $class

c shell control structures3
C Shell Control Structures
  • switch (string variable) case pattern: commands breaksw case pattern: commands breaksw : default: commands breakswendsw
  • break # causes case processing to terminate (same as bash)
  • breaksw # causes case processing to terminate for this selection (switch)
c shell control structures4
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 1endif

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 breakswendsw

c shell control structures5
C Shell Control Structures
  • foreach

foreach loop-index (argument-list) commandsend

  • 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 1endif

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

c shell control structures6
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 argumentsset buffer = (0 0 0 0 0 0)@ count = 1if ($#argv > 6) then echo “Usage: foo-six [up to 6 args]” exit 1endif

foreach arg ($argv[*]) set buffer[$count] = $arg @ count++endecho There were $count argumentsexec foo $buffer[*]exit 0

c shell control structures7
C Shell Control Structures
  • while

while (expression) commandsend

Example:

Sum the numbers between 1 and # the value in argv[1]@ limit = $argv[1]@ index = [email protected] sum = 0while ($index <= $limit) @ sum += $index @ index++endecho The sum is $sum

C Shell Control Structures

c shell control structures8
C Shell Control Structures
  • goto Command
  • goto label

...

other shell commands

...

label:

shell commands

c shell control structures9
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 endifend

c shell control structures10
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 -fonintr closewhile (1) echo Program is running. sleep 2endclose: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
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.
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