Linux Know-How provides a collection of introductory texts on often needed Linux skills.

Example Bash script

The script below executes some well-known commands (date, w, uname, uptime) to display information about you and your machine.

tom:~> cat -n
     1  #!/bin/bash
     2  clear
     3  echo "This is information provided by  Program starts now."
     5  echo "Hello, $USER"
     6  echo
     8  echo "Today's date is `date`, this is week `date +"%V"`."
     9  echo
    11  echo "These users are currently connected:"
    12  w | cut -d " " -f 1 - | grep -v USER | sort -u
    13  echo
    15  echo "This is `uname -s` running on a `uname -m` processor."
    16  echo
    18  echo "This is the uptime information:"
    19  uptime
    20  echo
    22  echo "That's all folks!"

A script always starts with the same two characters, "#!". After that, the shell that will execute the commands following the first line is defined. This script starts with clearing the screen on line 2. Line 3 makes it print a message, informing the user about what is going to happen. Line 5 greets the user. Lines 6, 9, 13, 16 and 20 are only there for orderly output display purposes. Line 8 prints the current date and the number of the week. Line 11 is again an informative message, like lines 3, 18 and 22. Line 12 formats the output of the w; line 15 shows operating system and CPU information. Line 19 gives the uptime and load information.

Both echo and printf are Bash built-in commands. The first always exits with a 0 status, and simply prints arguments followed by an end of line character on the standard output, while the latter allows for definition of a formatting string and gives a non-zero exit status code upon failure.

This is the same script using the printf built-in:

tom:~> cat
printf "This is information provided by  Program starts now."

printf "Hello, $USER.\n\n"

printf "Today's date is `date`, this is week `date +"%V"`.\n\n"

printf "These users are currently connected:\n"
w | cut -d " " -f 1 - | grep -v USER | sort -u
printf "\n"

printf "This is `uname -s` running on a `uname -m` processor.\n\n"

printf "This is the uptime information:\n"
printf "\n"

printf "That's all folks!\n"

Creating user friendly scripts by means of inserting messages is treated in Chapter 8.

NoteStandard location of the Bourne Again shell

This implies that the bash program is installed in /bin.

WarningIf stdout is not available

If you execute a script from cron, supply full path names and redirect output and errors. Since the shell runs in non-interactive mode, any errors will cause the script to exit prematurely if you don't think about this.

The following chapters will discuss the details of the above scripts.

Last Update: 2010-12-16