In UNIX, writing script is the main task. We use Scripts in many places to increase productivity. So my post gives you top ideas on writing scripts.
Tip 1 – Comments in Script
Lines not preceded by a hash ‘#’ character are taken to be Unix commands and are executed. Any Unix command can be used.
For example, the script below displays the current directory name using the pwd command, and then lists the directory contents using the ls command.
#!/bin/sh # purpose: print out current directory name and contents pwd ls
Tip 2 – Writing Variables
The script below shows how to assign and use a variable. In general, shell variables are all treated as strings (i.e. bits of text).
Shells are extremely fussy about getting the syntax exactly right; in the assignment there must be no space between the variable name and the equals sign, or the equals sign and the value.
To use the variable, it is prefixed by a dollar ‘$’ character.
#!/bin/sh # name is a variable name="fred" echo "The name is $name"
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Tip 3 – Writing Conditions IF THEN ELSE FI
Conditionals are used where an action is appropriate only under certain circumstances. The most frequently used conditional operator is the if-statement. For example, the shell below displays the contents of a file on the screen using cat, but lists the contents of a directory using ls.
#!/bin/sh # show script if [ -d $1 ] then ls $1 else cat $1 fi
Tip 4 – Looping commands DO WHILE
The simplest looping command is the while command. An example is given below:
#!/bin/sh # Start at month 1 month=1 while [ $month -le 12 ] do # Print out the month number echo "Month no. $month" # Add one to the month number month=`expr $month + 1` done
Tip 5 – Unix Script an Example
One of the best example of sample script is as follows.
#!/bin/sh # DOS interpreter. Impersonates DOS as follows: # DOS command UNIX equivalent Action # cd cd Change directory # dir ls List directory contents # type cat List file contents # del rm Delete a file # ren mv Rename a file # copy cp Copy a file echo "Welcome to the DOS interpreter" echo "Type Ctrl-C to exit" # Infinite loop while [ "forever" ] do # Show DOS prompt; c stops a new line from being issued echo "DOS> c" # Read in user's command read command arg1 arg2 # Do a UNIX command corresponding to the DOS command case $command in cd) cd $arg1 ;; dir) ls ;; type) cat $arg1 ;; del) rm $arg1 ;; ren) mv $arg1 $arg2 ;; copy) cp $arg1 $arg2 ;; *) echo "DOS does not recognise the command $command" ;; esac done
Here’re some additional examples.