7.2.5.2. Some simple examples

A very simple script consisting of only one command, that says hello to the user executing it:

[jerry@nowhere ~] cat hello.sh
#!/bin/bash
echo "Hello $USER"

The script actually consists of only one command, echo, which uses the value of ($) the USER environment variable to print a string customized to the user issuing the command.

Another one-liner, used for displaying connected users:

#!/bin/bash
who | cut -d " " -f 1 | sort -u

Here is a script consisting of some more lines, that I use to make backup copies of all files in a directory. The script first makes a list of all the files in the current directory and puts it in the variable LIST. Then it sets the name of the copy for each file, and then it copies the file. For each file, a message is printed:

tille:~> cat bin/makebackupfiles.sh
#!/bin/bash
# make copies of all files in a directory
LIST=`ls`
for i in $LIST; do
        ORIG=$i
        DEST=$i.old
        cp $ORIG $DEST
        echo "copied $i"
done

Just entering a line like mv * *.old won't work, as you will notice when trying this on a set of test files. An echo command was added in order to display some activity. echo's are generally useful when a script won't work: insert one after each doubted step and you will find the error in no time.

The /etc/rc.d/init.d directory contains loads of examples. Let's look at this script that controls the fictive ICanSeeYou server:

#!/bin/sh
# description: ICanSeeYou allows you to see networked people

# process name: ICanSeeYou
# pidfile: /var/run/ICanSeeYou/ICanSeeYou.pid
# config: /etc/ICanSeeYou.cfg

# Source function library.
. /etc/rc.d/init.d/functions

# See how (with which arguments) we were called.
case "$1" in
        start)
                echo -n "Starting ICanSeeYou: "
                daemon ICanSeeYou
                echo
                touch /var/lock/subsys/ICanSeeYou
                ;;
        stop)
                echo -n "Shutting down ICanSeeYou: "
                killproc ICanSeeYou
                echo
                rm -f /var/lock/subsys/ICanSeeYou
                rm -f /var/run/ICanSeeYou/ICanSeeYou.pid
                ;;
        status)
                status ICanSeeYou
                ;;
        restart)
                $0 stop
                $0 start
                ;;
        *)
                echo "Usage: $0 {start|stop|restart|status}"
                exit 1
esac

exit 0

First, with the . command (dot) a set of shell functions, used by almost all shell scripts in /etc/rc.d/init.d, is loaded. Then a case command is issued, which defines 4 different ways the script can execute. An example might be ICanSeeYou start. The decision of which case to apply is made by reading the (first) argument to the script, with the expression $1.

When no compliant input is given, the default case, marked with an asterisk, is applied, upon which the script gives an error message. The case list is ended with the esac statement. In the start case the server program is started as a daemon, and a process ID and lock are assigned. In the stop case, the server process is traced down and stopped, and the lock and the PID are removed. Options, such as the daemon option, and functions like killproc, are defined in the /etc/rc.d/init.d/functions file. This setup is specific to the distribution used in this example. The initscripts on your system might use other functions, defined in other files, or none at all.

Upon success, the script returns an exit code of zero to its parent.

This script is a fine example of using functions, which make the script easier to read and the work done faster. Note that they use sh instead of bash, to make them useful on a wider range of systems. On a Linux system, calling bash as sh results in the shell running in POSIX-compliant mode.

The bash man pages contain more information about combining commands, for- and while-loops and regular expressions, as well as examples. A comprehensible Bash course for system administrators and power users, with exercises, from the same author as this Introduction to Linux guide, is at https://tille.xalasys.com/training/bash/. Detailed description of Bash features and applications is in the reference guide Advanced Bash Scripting.