Some very useful bash scripting tips for testing the output of a program.

This simple shell script will test your Internet connection and then tell you if it is up or not.

wgetvar=$(wget -q --tries=3 --timeout=20 --spider http://google.com)
 
if [ $? -eq '0' ]
         then
            echo "Internet is up."
         else
            #some logging
            echo "Internet is down.."
fi

This section will test if the program run at the start of the script returns 0 or not. If it has a return value of 0, then it ran successfully. Then the Internet connection is up.

if [ $? -eq '0' ]

Another way is to do this.

if [ $? -ne '1' ]

This tests if the program did not return 1. This also could return other numbers. It is better to check for the 0 return value if the scripter desires to know if a program was successful in execution.

The Linux Documentation Project has a list of reserved codes that also offers advice on what code to use for specific scenarios. These are the standard error codes in Linux or UNIX.

• 1 – Catchall for general errors
• 2 – Misuse of shell builtins (according to Bash documentation)
• 126 – Command invoked cannot execute
• 127 – “command not found”
• 128 – Invalid argument to exit
• 128+n – Fatal error signal “n”
• 130 – Script terminated by Control-C
• 255\* – Exit status out of range

This example is a case statement, this will be a good base for a script to start or stop a service, or to accept other parameters.

case "$1" in
    start)
        echo "Start"
        ;;
    stop)
        echo "Stop"
        ;;
esac

This would be useful for a script that has to do one thing or another depending upon the parameters issued to it.

This is from the /etc/init.d/gdm3 script, this is an example of the else if syntax in bash scripting.

        elif [ -e "$DEFAULT_DISPLAY_MANAGER_FILE" ] && \
           [ "$HEED_DEFAULT_DISPLAY_MANAGER" = "true" ] && \
           [ "$CONFIGURED_DAEMON" != gdm3 ] ; then
                log_action_msg "Not starting GNOME Display Manager; it is not the default display manager"
        else

This can also be used in a script to do something else when running the script.

With the case statement, it is also possible to use multiple options for each parameter.

case "$1" in
    start|force-reload|restart|reload)
        echo "Starting the server"
        ;;
    stop|shutdown)
        echo "Stopping the server deamon"
        ;;
esac

So issuing a command to force-reload, is the same as starting or reloading the service.