Beginning Shell Scripting

OK, let’s continue on with our lessons about the Linux Shell and the command line.

Today, we’re going to learn how to write a script that you can run from the command line that will actually do stuff. It’s a very simple script, so don’t expect anything spectacular. I was wondering what I could use as an example script for this lesson when I remembered a question someone at LinuxQuestions.org asked the other day about how to tell which users are currently logged into a Linux system. Let’s make a little script to give us that information.

Let’s say that Mary wants to see who is logged in on the Linux system that she maintains for her company. She could easily do this with one simple command from the command line:

mary@workplace:~$ who

bill           pts/4       April 10 09:12

james       pts/5       April 10 09:30

marjory    pts/11     April 10 10:02

ken           pts/16     April 10 10:31

That was pretty easy, right? What if Mary wanted to check this periodically during the day? Well, she could write a small shell script with the proper permissions that she could execute any time she wanted from the command line or the terminal within her graphical user interface. Let’s find out how she might do that.

Mary would first open the vim editor with the file name that she plans to use for this script:

mary@workplace:~$ vim onsys

Vim would faithfully carry out the command and open up ready to edit file onsys. At which point, Mary would enter these lines to create her script:

# custom who script – mary – 10 Apr 11

date

echo “Users currently logged into the system:”

who

# end of script

Here’s what Mary actually did… she made a comment in the first line that begins with the character #, so that the shell knows to ignore that line. In the next line, she sets the command “date” so the script will output the date along with whatever else she requests it to do. In line 4, she uses the built-in echo command to tell the shell to display whatever is being asked for following the echo command. In this case, Mary wants the script to display Users currently logged into the system: when she runs it. The next command that Mary enters into this little script is the built-in who command. And lastly, is her notation that the script has ended.

Now, to make this script work, Mary needs to make it executable. In other words, she has to change the file’s permissions to allow herself (and other, if she wants) to execute (make it run) the script onsys. She will do that this way:

mary@workplace:~$ chmod +x onsys

If she now listed the permissions for this file, they would look like this:

mary@workplace:~$ ls -l onsys

-rwxr-xr-x 1 mary users 94 Apr 10 15:21 onsys

What this means is that everyone on the system can read and execute this script, but only mary can write to (change) it.

OK, so Mary wants to test her script now, so she just types in the script name at the command line (assuming the script is in her working directory):

mary@workplace:~$ onsys

Sun Apr 10 15:48:09 EDT 2011

Users currently logged into the system:

bill           pts/4       April 10 09:12

james       pts/5       April 10 09:30

marjory    pts/11     April 10 10:02

ken           pts/16     April 10 10:31

And so, there you have it. Mary wrote a simple script using shell built-in commands to perform a function that she does repetitively every day. That’s what scripts do. They perform tasks for us at our bidding. Of course, scripts can get much more complicated; so complicated in fact as to be considered applications in their own right. A script is just a series of encoded commands and variables designed to do something, which is basically what an application is also.

Start yourself a directory in your /home directory where you can create and play around with scripting and scripts. It can really be fun. You can “automate” a lot of every day stuff that you do on your computer using custom-made scripts. Yes, you can do it. It ain’t rocket science. It is similar to the technology used to program rockets, though.

Have FUN!

~Eric

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