Module #02 - Serious CLI Usage

This module will pick up where the previous one left as we will dig a bit deeper into the Linux CLI. The Bash shell is a complete programming language. Once you get used to interacting with an operating system via the command line, you generally discover a wealth of possibilities in terms of automating complex and, at times, boring tasks.

We will eventually talk about scripting in this course but, before we get there, we will start by getting familiar with the Bash shell variables, options, and various configurations possibilities. First, our modifications will be transient; i.e. only applying to the current Bash session with which we are experimenting. By the end of the module, we will learn how to make such modifications "stick around" from shell session to shell session and even make them apply to all users on our system.

Topics Studied in this Module

T1 - Globbing

Bash allows you to use so-called meta-characters to build expressions allowing you to designate sets of filenames or folder names on which you may apply all sorts of CLI tools.

T2 - Shell Quoting & Escaping

One of the most interesting topics when learning Bash; the syntax allowing you to control the interpretation of the above-mentioned meta-characters or even substitute the result of executing code in an expression.

T3 - Bash Environment, Variables, & Option

We then examine Bash options & variables.

T4 - Bash Initialization Files

Last but not least, we are going to take a look at how we may configure the Bash shell for your user accounts. We will consider individual configuration files first, then system-wide ones.

Learning Activities

This module features the following learning activities; DF, PA, GQ

Linux+ Exams Objectives

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

• Exam LX0-101 - Objective 103.1 - Work on the command line
• Exam LX0-101 - Objective 103.3 - Perform basic file management
• Exam LX0-102 - Objective 105.1 - Customize & use the shell environment

T1 - Globbing

The first topic we are going to study is related to one of the expansions the Bash shell applies to the commands you type in order to interpret special characters, also known as meta-characters, that you might have placed in them. More specifically, we are going to look into globbing, the mechanism that allows you to specify multiple file or folder names in a concise expression such as "*.txt" to mean "all files with suffic ".txt".

T2 - Shell Quoting & Escaping

Next topic is related to a very interesting aspect of interacting with the Bash shell; understanding how to neutralize the special meaning of meta-characters when we want to make sure a '*' is just used by the shell as the symbol star without further expansions.

Now that we have covered the basics of quoting in Bash, we are going to look at a topic that is related, at least from the syntactical perspective, yet different. We want to specify parts of our command line that need to be executed and replaced by their result.

T3 - Bash Environment, Variables, & Option

We are now ready to jump into how variables are handled in Bash.

T4 - Bash Initialization Files

Our next topic will focus on the initialization files used by the Bash command line shell when it is started. The video will give you an overview of the various configuration files that are involved as well as their contents.

To supplement the above-video, make sure to also study the relevant sections of the Bash manpage.

This Discussion Forum assignment will allow us to further our previous initial exploration of what open source means. The format we will adopt will be that of a pro vs. cons discussion.

Topic - Deciding whether to go Open Source or not

The idea of this DF is to have you discuss with one another the pro and cons of adopting open source solutions. To this end, you will have to put yourself in the shoes of an IT professional working in one of the following sectors;

• Private Corporations (non IT)
• Private Corporations (IT related)
• Non-profit Organizations
• Military Organizations
• Scientific & Research Institutions
• Educational Organizations
• Governmental Organizations

Make sure that the title of your post indicates which of the above sectors you selected.

Requirements for your Posts

You will have to make two posts on this module's DF forum.

Your first post will state arguments in favor, or against, the adoption of open source software from the perspective of one of the above-listed sectors.

After you make this first post on the DF forum, you will be able to see the posts made by other students. Pick one and respond to the arguments that were developed in it. You may either agree and provide new arguments or references to new resources that further establish the points being made, or disagree and provide similar material contesting the points being made.

How to get a good DF grade

If everyone waits late in the week, e.g. thursday, to make their first post, this assignment will end up rushed and mostly frustrating for those who wanted a bit of time to formulate their responses to their peers. Post late and you will lose points, potentially all.

Opinions are easy to come by, try to Google anything subject to controversy and you will find thousands of them. As such, their value is nil, except if they are backed by facts and logically articulated arguments. Make sure that both in your original post, and your response, you reference verifiable information that you found in order to support the points you are making. Students who simply post arbitrary opinions will not receive any points in this kind of DF.

This module's PA will use short and simple exercises to help you practice hands-on the various syntaxes we discussed in the study material. For each exercise, we will provide you with details on what you need to submit in order to receive credit. Make sure that you collect all required information in a single document that you will then upload as a PDF file.

Exercise #1 - Setting Variables

Let us start by practicing our skills in creating different types of variables in the Bash shell;

• Create a local shell variable mylocalvar and assign to it the string value "This is local"
• Create a global shell variable, also known as environment variable, MYENVVAR and assign to it the value "This is in the environment"

Now use echo to display the contents of both variables, just to make sure they were appropriately assigned.

For this exercise, submit a screenshot of your command line terminal that shows your full name the contents of both variables, as well as the commands you used to set them.

Exercise #2 - Verifying the Nature of these Variables

While the above exercise allowed us to verify that the proper value was assigned to each variable, we did not do anything to ensure they really were local and global, respectively.

To do so we are going to cheat a bit and introduce syntax that we will fully study in a later module. The study material mentioned both printenv (or env) and set as ways to list local shell variables, global environment variables, and also a whole lot more of information we do not need right now.

We are going to use these commands but follow them with a bit of syntax that we will study only in a later module. The result will be to filter out every line from the output of these commands that does not refer to the variables we are interested in looking up.

This is what we get when filtering for both variables while using the previoulsy mentioned commands;

tux@tuxbox$ printenv | grep mylocalvar
tux@tuxbox$ printenv | grep MYGLOBALVAR
MYGLOBALVAR=This is global
tux@tuxbox$ set | grep mylocalvar
mylocalvar='This is local'
tux@tuxbox$ set | grep MYGLOBALVAR
MYGLOBALVAR='This is global'

This tells us that printenv shows no output related to mylocalvar but only MYGLOBALVAR. Makes sense. But then, set shows output for both variables even though one is local and the other global. Why is that so?

For this exercise, submit a screenshot of your command line terminal that shows your full name, the output when you run the commands shown in the above sample shell output. In addition, provide a brief explanation of why set shows output for both variables even though one is local and the other is global.

Exercise #3 - Common Bash Environment Variables

How would you go about finding out which Bash environment variables contain the following information and then display their contents using echo?

• The full path of the executable program being used as shell in this terminal
• The type of terminal being used in the console you are typing
• Your host name
• The type of architecture of your host; e.g. x86_64, amd64...
• The name of the user as which you are logged

For this exercise, submit a screenshot which displays the environment variables that contain the following information;

• The full path of the executable being used as the shell in your terminal
• The type of terminal being used in the console you are typing.
• Your host name
• The type of architecture of your host (x86_64, amd64, etc).
• The name of the user which you are logged in as.

Also display the contents of those variables in your screenshot. You must find this information using the appropriate environment variable, if you display it using a command (like hostname) you will not receive credit. Also explain how you found this information, namely what commands did you use. If the commands you typed are in your shell output, that is sufficient.

Exercise #4 - The Path is Closed

The PATH variable was mentioned in the study material. For this exercise, do a little bit of research on the CDPATH variable and explain, in your own words, how it is similar, as well as how it differs, from the PATH variable. In addition, you will also describe a scenario for which you would use CDPATH.

Exercise #5 - Bash Quoting

Provide the echo command you would type in your shell to display each of the following outputs;

The dog & the cat; a tale of getting along just fine
I remember now... "Bash is fun", they said. Liars!

To do so, you will provide solutions that only use the backslash character, and then solutions that only use the single quote character.

Try to do the same with the following outputs. Why isn't it working?

  It's rather annoying (and even at times infuriating) to see quoting fail

Exercise #6 - Command Substitution

The commands whoami and date are quite handy. We want to display with a single echo command the following message;

Hello, tux today is Tue May 8 17:20:40 EDT 2018 what do you want to do?

Obviously, we want "tux" to be replaced by the name of the user you are logged as, and the date by the current date of the system.

There are two syntaxes that will allow to specify what part of an echo statement you want the shell to actually execute and substitue by its output. Use both to achieve the above goal.

Exercise #7 - While we are at it...

The above message would be a nice personalized prompt for a shell. How would we go about telling our current shell to use it as such for the rest of the session?

Do you notice something strange about the so-called current date that is now being displayed in your prompt for each line?

How would you fix this? This question is tricky. Do not simply try things at random and make sure you understand why things are working, or not.

Exercise #8 - More Bash Quoting

The double quotes are used by Bash to allow the interpretation of some special characters while blocking others. There are actually only a few meta-characters that are still interpreted as such when encased in double quotes. Which ones are these?

Now that you remembered them, or looked them up, come up with a single output that you would want a candidate at a job interview to be able to echo. Your challenge must require them to use all three meta characters. Document in your PA answer what is the echo statement the candidate would have to come up with in order to display the output you crafted.

Exercise #9 - Hacker Wannabe

It is a good day for a prank. A friend left their workstation unlocked with a terminal running bash opened. We want to make it so that, when they use ls, the following message is displayed instead of running the command;

tux@tuxbox$ ls
ERROR 0xFF42 (bad sector, data corrupted). Contact NSA for a backup copy of your data.
tux@tuxbox$ ls

How would you go about doing this?

You should definitively take a snapshot of your machine BEFORE you make any modifications to your system for this exercise. This way, you will be able to easily revert back to a working state if you mess up something.

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

Resources on the Bash shell

Wooledge.org tutorials;

• Bash FAQ
• Bash Pitfalls
• Bash Programming
• Bash Guide

The following website was recommended by one of our students, Issam Halabi, during summer 2019. It takes a gamified approach to guiding your first steps on the Linux Command Line; OverTheWire: Bandit.

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.

We will have an opportunity to dig into other aspects of the material presented in that guide when we study Bash scripting in a later module.

By the time we are done with this course, you should be able to read that guide without problems. The following sections are the ones directly relevant to this module;

Harley Hahn's Guide to UNIX and Linux

If you are interested in learning more about the topics covered in this module, I recommend you look for Harley Hahn's Guide to UNIX and Linux. It is unfortunately out of print but is a great (and entertaining) read about the topic.

The following sections of the above are particularly relevant to this module;

• Section 11: The Shell
• Section 12: Using the Shell: Variables & Options
• Section 13: Using the Shell: Commands & Customization