Lab 13: BASH Scripting

Have completed the reading on BASH scripting. If you haven’t done this you will almost certainly have difficulty completing the lab!

Open an SSH remote terminal session to your Linux server’s IP address

Login with your standard user’s username and password

At their core BASH scripts are really just lists of commands which could have been entered directly into the command line but have been pre-entered into a file to be run later instead. We can see how this works by creating a couple of very simple BASH scripts which each just run a single command. Your ability to write BASH scripts that do something useful is really only limited by how well you know Linux commands and utilities which could be included in the script. The echo command can be used on your system to print information out to the standard output.

Lets try running the echo command on your server like echo Hello World!:

ben@2480-Z:~$ echo Hello World!
Hello World!
ben@2480-Z:~$

You should see the text "Hello World!" printed out on the screen below your command.

In order to turn that command into a script we just need to put the command into a text file and add one more line, called a sha-bang because it is made up of a hash (pound) sign and a bang (exclamation) sign. This line tells the system what program to use to interpret the commands in the script. In this case we’re going to be writing BASH scripts so we’ll set it for the BASH program.

Now try using what you have learned to create and test another script named simple-backup.sh which executes the command tar -czf /tmp/jsmith-backup.tar.gz /home/jsmith/ when the script is run (substitue your own username for jsmith).

Variables are a key part of turning a simple list of instructions you could type into a command line into something much more powerful by allowing you to reuse code and do something very similar (remember programmers are lazy!) but with a little tweak each time without re-writing the whole thing. Variables can be defined in the script itself but there are also special variables which are part of the BASH environment we can use. Variables can be identified in bash scripts as starting with a $. One type of these variables allows us to access arguments which have been supplied as part of the command line calling the script. Sometimes we only want to run certain sections of the script or we want to modify the script depending on the outcome of some test, called a conditional. This is often used in combination with variables as we’ll see in the example below.

In BASH the script filename is assigned to the $0 variable, the first argument to $1 and so on. Let’s see how we can modify our hello.sh script to make it a bit more personable by including a name.

Wait, what if someone doesn’t know they are supposed to include their name as an argument though? Try just running ./hello.sh again and see what happens. It might be the case that we want to go back to the simple Hello World! and maybe add some instructions if they don’t specify an argument.

It’s also possible to capture the standard output of a command executed by the script and put that into a variable for later use. For example, try running the date +%Y%m%d command now. We can use the output of this command in a variable to make our backup script a little bit nicer.

Sometimes you want a program to do something over and over again several times. This is typically done using a loop. Let’s say you wanted to create a bunch of sample files to do some further practice with in the current directory named sample-fileX.extension where X was a number that would keep incrementing by 1 each time and you could specify any extension you wanted. We could certainly use arguments to capture that user input but it is also possible to accept input from the user directly while the script is running. This would create what we call an "interactive" script because the user is interacting with it while it runs.

Create a file-generator.sh script on your system like the one below and see if you can figure out how it works:

#!/bin/bash
BASENAME=sample-file
echo "Welcome to the sample file generation utility!"
echo "Files will be created using the filename format "$BASENAME"X.extension"
echo "Enter the number of files you wish to create, then press Enter:"
read NUM
echo "Enter the extension (without the leading period) to put on the files"
read EXTENSION
for CURNUM in `seq 1 $NUM`; do
    touch $BASENAME$CURNUM.$EXTENSION
done
echo "Created "$NUM" file(s)."

There are a few things to note about this script.

Try running the script and see if you were correct about what it does and how it works.

See if you can modify the script so you can specify a starting and an ending number for the files rather than just a number of files to be created.

See if you can add a check to the script which will display an example of a filename that will be created and asking the user if that looks correct before actually creating all the files. If the user enters N then do not create the files.

One final programming structure we should discuss is the function. In shell scripting you can think of a function like something you could have put into a separate script so that you could call it to do something multiple times from inside of your script but instead of making it a completely separate file you decided to include it in a single script for organizational purposes.

Now that you have a grasp on the fundamentals of BASH scripting see if you can write a script which can be used to modify the name of all files in the current directory which share the same extension such as those created by file-generator.sh. The goals for your script should be:

Don’t be afraid to try using search engines for help with this. The goal of this lab is not to make you into a professional programmer but to introduce you enough to shell scripting so that you can create useful scripts with resources such as search engines available to you.

Close the SSH session

If you are using the Administrative PC in Netlab instead of your own computer as the administrative computer you should also shut down that system in the usual way each time you are done with the Netlab system and then end your Netlab Reservation. You should do these steps each time you finish using the adminsitrative PC in future labs as well.