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Unix Dweeb

What is the Linux command line? An introduction

Jan 31, 20204 mins

Here are some warm-up exercises for anyone just starting to use the Linux command line. Warning: It can be addictive.

If you’re new to Linux or have simply never bothered to explore the command line, you may not understand why so many Linux enthusiasts get excited typing commands when they’re sitting at a comfortable desktop with plenty of tools and apps available to them. In this post, we’ll take a quick dive to explore the wonders of the command line and see if maybe we can get you hooked.

First, to use the command line, you have to open up a command tool (also referred to as a “command prompt”). How to do this will depend on which version of Linux you’re running. On RedHat, for example, you might see an Activities tab at the top of your screen which will open a list of options and a small window for entering a command (like “cmd” which will open the window for you). On Ubuntu and some others, you might see a small terminal icon along the left-hand side of your screen. On many systems, you can open a command window by pressing the Ctrl+Alt+t keys at the same time.

You will also find yourself on the command line if you log into a Linux system using a tool like PuTTY.

Once you get your command line window, you’ll find yourself sitting at a prompt. It could be just a $ or something as elaborate as “user@system:~$” but it means that the system is ready to run commands for you.

Once you get this far, it will be time to start entering commands. Below are some of the commands to try first, and here is a PDF of some particularly useful commands and a two-sided command cheatsheet suitable for printing out and laminating.

Command		What it does
pwd		show me where I am in the file system (initially, this will be your
        	home directory)
ls		list my files
ls -a		list even more of my files (including those that start with a period)
ls -al		list my files with lots of details (including dates, file sizes and
who		show me who is logged in (don’t be disappointed if it’s only you)
date		remind me what day today is (shows the time too)
ps		list my running processes (might just be your shell and the “ps”

Once you’ve gotten used to your Linux home from the command line point of view, you can begin to explore. Maybe you’ll feel ready to wander around the file system with commands like these:

Command		What it does
cd /tmp		move to another directory (in this case, /tmp)
ls		list files in that location
cd		go back home (with no arguments, cd always takes you back to your home
cat .bashrc	display the contents of a file (in this case, .bashrc)
history		show your recent commands
echo hello	say “hello” to yourself
cal		show a calendar for the current month

To get a feeling for why more advanced Linux users like the command line so much, you will want to try some other features – like redirection and pipes. Redirection is when you take the output of a command and drop it into a file instead of displaying it on your screen. Pipes are when you take the output of one command and send it to another command that will manipulate it in some way. Here are commands to try:

Command				What it does
echo “echo hello” > tryme	create a new file and put the words “echo hello” into
chmod 700 tryme			make the new file executable
tryme				run the new file (it should run the command it
				contains and display “hello”)
ps aux				show all running processes
ps aux | grep $USER		show all running processes, but limit the output to
				lines containing your username
echo $USER			display your username using an environment variable
whoami				display your username with a command
who | wc -l			count how many users are currently logged in


Once you get used to the basic commands, you can explore other commands and try your hand at writing scripts. You might find that Linux is a lot more powerful and nice to use than you ever imagined.

Unix Dweeb

Sandra Henry-Stocker has been administering Unix systems for more than 30 years. She describes herself as "USL" (Unix as a second language) but remembers enough English to write books and buy groceries. She lives in the mountains in Virginia where, when not working with or writing about Unix, she's chasing the bears away from her bird feeders.

The opinions expressed in this blog are those of Sandra Henry-Stocker and do not necessarily represent those of IDG Communications, Inc., its parent, subsidiary or affiliated companies.

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