Shell Programming

CONTROL STRUCTURES

The control flow commands alter the order of execution of commands within a shell script. They include the if...then, for... in, while, until, and case statements. In addition, the break and continue statements work in conjunction with the control flow structures to alter the order of execution of commands within a script.

if.. .then

The syntax of the if… then control structure is

       If  test-command
         then
           commands
       fi

The test builtin returns a status of true if the two words are the same and false if they are not. Double quotation marks around $word1 and $word2 make sure that test works properly if you enter a string that contains a SPACE or other special character.  

bash$ cat ifi

echo -n "word 1: "
read word1
echo —n "word 2: "
read word2

if test "$word1" = "$word2"
    then
     echo "Match"
fi
    echo "End of program."

bash$ ifi

word1: peach
word2: peach
Match
End of program.

BUILTINS

 exec

§       Runs a command without creating a new process, therefore it makes program to run faster.

§       Redirects standard input, output, or error of a shell script from within the script

          When you run a command using the exec, exec executes the new command in place of (overlays) the current process. Exec does not return control to the original program, the exec bultin can only be used with the last command that you want to run the script.

$ cat exec_demo

who
exec date
echo This echo building is never executed.

$ exec_demo

[wmorales@rc33uxas01 wmorales]$ bash ~/exec_demo

nwijerat pts/0    Mar  1 02:00 (ABDECEB6.ipt.aol.com)

rhussein pts/2    Mar  1 04:36 (ip1.portland12.or.pub-ip.psi.net)

wmorales pts/3    Mar  1 05:15 (net22-cust43.pdx.wantweb.net)

Wed Mar  1 05:28:38 PST 2000

[wmorales@rc33uxas01 wmorales]$            

trap

          The trap builtin catches or traps a signal. A signal is a report to a process about a condition. The Linux system uses signals to report interrupts generated by the user (ie. Pressing the interrupt key). Using trap you can direct the action a script takes when it receives a signal.

          trap ['commands'] [singal-numbers]

$cat inter

#!/bin/bash

trap 'echo PROGRAM INTERRUPTED; exit 1' 2

while true

do

        echo "Program running."

done                           

[wmorales@rc33uxas01 wmorales]$ bash ~/inter

Program running.  
Program running.
Program running.
Program running.
Program running.
Program running.
PROGRAM INTERRUPTED

FUNCTIONS

A shell function is similar to shell script. Shell functions are a way to group commands for later execution using a single name for the group. They are executed just like a "regular" command. Shell functions are executed in the current shell context; no new process is created to interpret them.

You can:

§       Declare a funcition in your .bash_profile  

§       In a script

§       Or in the command line

§       Use unset builtin to remove functions

function-name ()

          {  
                    commands
          }

$cat whofunction

echo This is an example of a function

whoson()

{
        date
        echo "Users logged on"
        who
}

whoson

[wmorales@rc33uxas01 wmorales]$ ./whofunction

This is an exemple of a function

Wed Mar  1 06:37:05 PST 2000

Users logged on

wmorales pts/0    Mar  1 06:32 (net22-cust43.pdx.wantweb.net)

rhussein pts/2    Mar  1 04:36 (ip1.portland12.or.pub-ip.psi.net)

wmorales pts/3    Mar  1 05:15 (net22-cust43.pdx.wantweb.net)

rhussein pts/4    Mar  1 06:08 (ip184.portland11.or.pub-ip.psi.net)

[wmorales@rc33uxas01 wmorales]$   

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This page was last modified September 26, 2004
wmorales@pcc.edu