Module #09 - Bash Scripting II

This module is the 2nd half of our introduction to Bash Scripting. This time we are going to focus more specifically on control flow with conditional statements and loops.

This will allow you to recombine all the tools we studied so far, and learned to insert in a script in the last module, with the control you are used to in any programming language.

Topics Studied in this Module

T1 - Testing Conditions

Most scripts will need you to test things. Does the filename you were given as parameter actually exist? Did that grep pipeline we just executed succeeded or failed? This topic will show you how to implement such tests in Bash scripts.

T2 - Repetitive Tasks

We are going to add to our scripts the ability to repeat groups of instructions multiple times. In programming, this is refered to as iteration. We are going to learn to iterate a fixed number of times, until a condition is met, or simply over a list of words resulting from the expansion of a Bash variable; e.g. $*

T3 - Interactive Scripts

While you may write many scripts in order to just automate some tasks based on a few parameters, there are times where your scripts will need to either get information interactively from the user while they run, or read it from other files. Guess what we are going to study in this topic?

Learning Activities

This module features the following learning activities; (see announcements for details)

Linux+ Exams Objectives

This module deals with material related to the following objectives from the CompTIA Linux+ and LPI certifications exams.

• Exam LX0-102 - Objective 105.2 - Customize or Write simple Scripts

We are going to, as with the previous module, leverage one of Ryan Chadwick Linux tutorials.

T1 - Testing Conditions

T2 - Repetitive Tasks

Quick note about the videos below; Do not be alarmed if the audio seems absent, it is indeed absent :) I am trying my hand at a different type of live coding videos where the narration is provided in written as comments to the code. I have seen it done with courseware related videos and figured it would give a break to some of you from my accent :)

Let me know if you like it or not on the forums! I know preference on such matters is highly subjective so I would not be surprised by a 50% like 50% dislike result but it always help to get feedback from students to realize what works and what does not.

T3 - Interactive Scripts

We continue our hands-on practice with more exercises focusing on more advanced features

Exercise #1 - howmany.sh - version #1

Let us start with a script which will define a variable containing a small integer of your choice. This value will represent the number of positional parameters the script expects to be passed. We want our script to display a message stating whether the expected number of parameters was passed or not.

Here is an example of how it would be used;

  tux@LinuxBox > ./howmany.sh one two three
  Sorry, you passed 3 parameters but I expected 5 parameters 
  tux@LinuxBox > ./howmany one two three four five
  Master gave Dobby the right number of parameters!

Exercise #2 - howmany.sh - version #2

Let us improve our howmany.sh script by enabling it to tell us whether we provided too many or too few parameters.

Here is an example of how it would be used;

  tux@LinuxBox > ./howmany.sh one two three
  Too few parameters! You gave me 3 parameters but I expected 5 parameters 
  tux@LinuxBox > ./howmany one two three four five six seven eight nine
  Too many parameters! You gave me 9 parameters but I expected 5 parameters
  tux@LinuxBox > ./howmany one two three four five
  Master gave Dobby the right number of parameters!

It would be silly to have the same message in two different echo statements, right? Make sure that the part of the two messages telling you how many parameters were given vs. how many were expected is not typed twice in your script. There are different ways to get this done, explore a bit

Exercise #3 - paramsloop-v1.sh

Write and test a Bash Script which will displays all of its parameters by iterating over them with a for loop based on expanding the list of parameters first.

Here is an example of how it would be used;

  tux@LinuxBox > ./paramsloop.sh one two three
  one
  two
  three

You will use different ways to expand the list of parameters;

• using the $* syntax
• using the $@ syntax
• using the "$*" syntax
• using the "$@" syntax

What are the differences you observe? Which of the above syntaxes are equivalent and which are necessary in some scenarios?

Exercise #4 - paramsloop-v2.sh

Write and test a Bash Script which will displays all of its parameters by iterating over them by using a while loop with shift.

Here is an example of how it would be used;

  tux@LinuxBox > ./paramsloop.sh one two three
  one
  two
  three

Exercise #5 - paramsloop-v3.sh

Write and test a Bash Script which will displays all of its parameters by iterating over them by using a until loop with shift.

Here is an example of how it would be used;

  tux@LinuxBox > ./paramsloop.sh one two three
  one
  two
  three

Exercise #6 - whatisthis.sh - version #1

Write and test a Bash Script which, given a single parameter, a file name, will display one of the following messages

• "Java Source File" if the file suffix is .java
• "C Source File" for .c
• "C++ Source File" for .cpp
• "Ada Source File" for .ada
• "Python Source File" for .py
• If the file doesn't have one of these suffix, your script will simply display "Unknown Source File"

Here is an example of how it would be used;

  tux@LinuxBox > ./whatisthis.sh something.diff
  Unknown File Suffix
  tux@LinuxBox > ./whatisthis.sh Something.java
  Java Source File

Please note:

• Use the CASE statement
• Do not verify that the parameter given to your script is the name of a file that actually exists. For now, just take the name given to us and look at its suffix.
• In order to get the file suffix out of a string containing the whole filename, you might want to read section 10.1 Manipulating Strings of the Advanced Bash Scripting Guide.

Exercise #7 - whatisthis.sh - version #2

The next version of our whatisthis.sh script will also verify whether the file actually exists and display a message accordingly. In addition, if the file exists, it will display the number of lines it contains

Here is an example of how it would be used;

  tux@LinuxBox > ./whatisthis.sh something.diff
  File not found, Unknown File Suffix
  tux@LinuxBox > ./whatisthis.sh Something.java
  File not found, Java Source File
  tux@LinuxBox > ./whatisthis.sh Something.java
  File exists! Java Source File with 23 lines

Exercise #8 - repeat.sh

We want a script to repeat a given command an arbitrary number of times.

Here is an example of how it would be used;

  tux@LinuxBox > ./repeat.sh 5 "echo something"
  something
  something
  something
  something
  something

To write this script, you will have to do a bit of research on the eval bash syntax as it is not covered in the assigned readings.

This tab provides you with optional resources meant for those who want to learn a bit more about the topics covered in this module.

Machtelt Garrels' Bash Beginners Guide

If you are interested in getting a bit deeper into Bash scripting, we recommend you study Machtelt Garrels' Bash Beginners Guide. The following links will provide you with a local PDF copy and the URL of the website where you might find the latest version. Please note that this document is freely available as part of the TLDP - The Linux Documentation Project.