02 - Program Baics

Command Line Script

Writing shell script is a easy at

for f in 1 2 3; do echo $f done

that is a basic shell script. This script is a interactive script, or one that isn't in a file. I use A LOT of these script because the can get one thing done pretty quickly, and it isn't a task that I will need to do again, so no need to keep it around, or spend to much time on it.

Script File

You can also put all commands into a file and create a script.

The first line of the script needs to be

#!/usr/bin/bash

This tells the sytem that htis is a shell script, and that it should execute the command "/usr/bin/bash" to execute the script. (you can also use this to create scripts in other languages)

In a shell script a # signals the start of a comment, it can start anwhere in the line and the rest of the line will be ignored.

# A comment is just like this

To get options in to script you can use the variables

$#

to get the number of options passed and

$1 $2 $3 ... $x

When you exit a shell script you should use "exit" with a number to pass back the value to the calling program.

exit 0

Once you have your script file you need to make the script excutable by seting the execute bit with the chmod commands

chmod +x the_script_file.sh

Here is a simple example.

#!/bin/bash

# Name: test_script.sh 
# Description: An example script that does nothing.
#
# Copyright Brian Dolan-Goecke 2011
# Contact Brian Dolan-Goecke @ http://www.goecke-dolan.com/Brian/sendmeail.php

VERSION="0.0.3-bdg-2011"

# Global Variables
EXIT_STATUS=0

# Not needed for this script.
## Make sure we are root
#if test ${EUID} -ne 0 ; then
#  echo "Not running as root, unable to make changes."
#  echo "no point then! dieing..."
#  # Exit now with error
#  exit 2
#fi

function usage() {
  NAME=`basename ${0}`
  echo "${NAME} options for script"
  echo "explain the options here"
}

if test $# -le 0
then
  echo "No options, exiting..."
  exit 2
fi

echo "The number of options you passed was $#"
echo "They were"
while test $# -ge 1
do
  echo $1
  shift
done

# Alls well that ends well.
exit ${EXIT_STATUS}

Tips

You should put a copy right and license on your scripts. (see http://www.gnu.org/licenses/gpl-howto.html )

I try and number each "exit" uniquely, I usually increment them as a add them in the program so I can find out where it exited from the script.