While Loop

$ a=2
$ i=1
$ while [ $i -le 20 ]
> do
>   echo "$a * $i = `expr $i \* $a`"
>   i=`expr $i + 1`
> done

2 * 1 = 2
2 * 2 = 4
2 * 3 = 6
2 * 4 = 8
2 * 5 = 10
2 * 6 = 12
2 * 7 = 14
2 * 8 = 16
2 * 9 = 18
2 * 10 = 20
2 * 11 = 22
2 * 12 = 24
2 * 13 = 26
2 * 14 = 28
2 * 15 = 30
2 * 16 = 32
2 * 17 = 34
2 * 18 = 36
2 * 19 = 38
2 * 20 = 40

Different loops are useful for different situations.  A poorly written loop can wind up executing indefinitely and may fill your hard drive, use all your processor or cause you to run out of free memory.  Beware of the infinite loop!

The case Statement

Instead of nesting many if-then-else or if-then-elif-then statements, another alternative is to use the case statement.  The case statement allows the user to perform a different set of commands depending upon the value associated with a string variable. 

The following example shows use of the case statement as you might see it in a shell script:

echo -n " What type of pet do you have (dog, cat, etc.)? "
read pet
echo "  "
case $pet in
  dog)    echo " So you have a dog ..."
          echo " That's nice, so do I."
          echo " My dog's name is Fido.";;
  cat)    echo " So you have a cat ..."
          echo " I'm allergic to cats!";;
  fish)   echo " So you have fish ..."
          echo " Can't take them for a walk .. ha, ha.";;
  ferret) echo " Ferrets are cool!";;
  *)      echo " A $pet ... that's a funny pet";;
esac
echo "  "

The case example script above asks the user what kind of pet they have and then provides different responses depending upon the type of pet answered.  It looks for answers of ?dog?, ?cat?, ?fish?, ?ferret?, and ?*?. 

The asterisk (*) matches any other answer supplied by the user.  When a match is found by the case statement, all commands are executed until a double semi-colon (;;) is encountered.

When the example above is entered and saved to a file called whatpet.sh, it can be run as shown below:

$ /bin/bash ./whatpet.sh

What type of pet do you have (dog, cat, etc.)? moose 
 A moose ... that's a funny pet 

$ ./whatpet.ksh

What type of pet do you have (dog, cat, etc.)? dog 

 So you have a dog ...
 That's nice, so do I.
 My dog's name is Fido.

Now that information has been presented on most of the scripting command constructs, it is possible to begin writing scripts.  The following section lists commands that may be useful in creating scripts.

Linux Command Summary

The following is a fairly comprehensive list of commands available in the Red Hat distribution of Linux.  A brief description of each command is presented in Table 8.7 below.  Some of the commands have been presented in greater detail in previous chapters. 

Rather than spend a great deal of time discussing each command in detail and giving examples of each, the commands are presented here only to give you an idea of what is available.  If you find a command here that might be useful in the development of a script, you can learn more about it with the man or info tools.  Many of the following descriptions come directly from the Linux man pages.