Conditional Commands

These are the familiar if...then and case structures which will execute commands if conditions are met.

Relational Operators

==

equal to

!=

not equal to

<

less than

>

greater than

<=

less than or equal to

>=

greater than or equal to

Logical Operators

&&

AND

||

OR

!

NOT

The if Command

The if command take the general form:

if ( condition ) then
   commands to do if condition returns true
else
   commands to do if condition returns false
endif

For example:

set user = $1
if ( $user == 'mary' ) then
  echo Hello $user, how are you\?
else
  echo I don\'t think we\'ve met. How do you do\?
endif

(set user=$1 sets user to be the argument given at runtime. \ means "switch off any special meaning of the next character".)

if (`who | wc -l` > 1) then
  echo Users still logged in. Do not shut down.
else
  echo All clear to shut down.
endif

File Operations

Using the if command, filenames can be tested for the following:

-d filename

true if filename is a directory

-e filename

true if filename exists

-f filename

true if filename is a text file

-o filename

true if you own filename

-r filename

true if filename is readable

-w filename

true if filename is writable

-x filename

true if filename is executable

-z filename

true if filename is empty

For example, the following shell script, listdir, will list a directory, but first will test to see if the given file is a directory.

#!/bin/csh
# csh Script listdir - lists a directory only
set file=$1
if !(-e $file) then
  echo "$file does not exist."
else if (-d $file) then
  ls -l $file | more
else
  echo "$file is not a directory."
endif

The command listdir progs will list all the files in progs, print progs does not exist or print progs is not a directory depending on the status of progs.

The switch Command

The switch command takes the general form:

switch ($variable)
      case pattern1 :
           action
           breaksw
       case pattern2 :
           action
           breaksw
          ......
      default :
           action
endsw

For example:

#!/bin/csh
# Script to find where a user is longed in from
set user=$1
set connection=`who | awk '$1=="'$user'" {print $2}'`
switch ($connection)
    case "(OXVAX)" :
        echo You are connected to linux.ox.ac.uk via VAX
        echo Remember to log out from VAX after
        echo logging out from linux.ox.ac.uk
        breaksw
    case "(LAT_08002B1DF794)" :
        echo You are connected to linux.ox.ac.uk via a
        echo Decserver. Logging out from linux.ox.ac.uk will
        disconnect you.
    breaksw
   default : echo Your connection is from
             echo $connection
endsw