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Linux Command Line

Note: As you go through this topic, do not try to memorize, learn by usage.

Goal of this unit

The goal of this unit is learning a few fundamental bash commands to get started using the terminal.

Command Line? Like in 1983?

We are all familiar with programs that have a Graphical User Interface (GUI), but in reality this is just an interface for showing us information and interact with the actual programs in an user friendly way. The actual programs are running underneath, in the background.

Programs can also be run without a GUI. Actually many programs come without a GUI. So the way to interact with these programs is through the Command Line Interface (CLI).

The environment we use is the Shell or command-line interpreter. There are many CLI interpreters. The one that comes with most distributions is Bash (Bourne Again Shell).

bash screenshot

The Unix philosophy

TODO Unix Linux and gnu-Linux

It is important to understand the Unix philosophy before you move forward. Programs in Unix were written following this philosophy:

  • Unix programs do one thing and do it well.
  • Unix programs are designed to work together.
  • Unix programs are designed to handle text streams, because that is a universal interface.

So Unix (and Linux programs) usually perform a single task only. They are not multipulpose tools programs. To achieve complex behaviours the output of a program can be piped (more on this later) to another program. Keep this is mind.

Basic syntax

The command line syntax is a command with optional options and optional arguments. For example, copy the following and paste it in the command line:

ls -a /usr/local/bin

Something's wrong! Copy and paste keys are not working The usual copy and paste key combinations have different functions in Bash. To copy text in Bash, select it and press CONTROL+SHITFT+C. To paste text in Bash use the arrow keys to go to the desired location and use CONTROL+SHITFT+V.

ls is a command which lists files and folders. -a is an option to display hidden files and /usr/local/bin is the argument telling where to operate the command. If the argument was not present it would list the current directory.

Options come in two flavors, one is called short option, a single letter like -a. In this case you don't have a clue of what that option is performing. On the other hand you can concatenate short options. For example -a -l can be reduced to -al.

Remember: Linux is a case sensitive operating system. -n and -N are different things!

Some options also have a human readable flavor called long option. Human readable options are prefixed with two dashes -- and they give you a clue of what that option is doing. For example the long option of -a is --all, giving you the clue that it will list all the files and folders, including the hidden ones. Long options cannot be concatenated.

Learning by doing: List the files of your home folder. Display them as a list. Display the hidden files also. What other options can be useful?

The manual

Wait, wait, wait... Do I need to memorize all of this? No. Just use the man command (for Manual) everytime you need help on how to use the commands. Yes, I know. We all hate manuals.

Learning by doing: Have a quick look at the manual for ls command. Get familiarised, you will need it a lot.

Navigating the filesystem

Where am I?

The information displayed in Bash varies from distribution to distribution. In Ubuntu you can see at every moment what is the current directory and the full path. Otherwise pwd command (Print Working Directory) will output the full path of your location in the filesystem.

Learning by doing: Open a new terminal window. Where are you now? What is ~?

Moving around

cd command stands for change directory and it is used to go to other directories.

Learning by doing: Navigate to your root folder /

If you need to navigate to a folder with spaces like My Documents use the escape character \ to tell bash that the space is part of the name and not another argument:

cd My\ Documents

If you need to go back to the last directory you were you can use the command cd -. This is very useful to go back and forward between 2 directories.

Tab completion

A very handy keyboard shorcut is the TAB key. It will autocomplete the name of the file or the folder name.

Learning by doing: From the root folder, navigate to your Documents folder. Use TAB to keep autocompleting.

When two or more options are available the first TAB will not autocomplete, a second TAB will show you the available options for you to type a few extra letters to diferenciate. After that another TAB will autocomplete.

Listing contents

We have seen the ls command already. Most of the time you will use ls -lh (with optional -a) which gives you a long list with human readable file sizes.

The first column tells you if the item is a file -, a directory d, or a link l. Then you can see the permissions, owner, group, date, file size, and name.

Learning by doing: List and analyse the contents of your /usr/bin. What do you see?

Creating, moving and deleting files and folders

mkdir is used to create a directory

Learning by doing: Create a personal folder using this scheme surname.name in your ~/Documents folder.

If you need to create a folder structure you can use the option -p which will create all the required parent folders.

Learning by doing: Create ~/Documents/surname.name/week1/img

touch command is used to create an empty file.

Learning by doing: create a file named readme.md inside your personal folder. Check that the file is there.

rmdir removes empty directories.

Learning by doing: Try to remove your personal folder. Ouch. What's the problem?

cp is a command used to copy files and folder. It has many options but the basic usage is cp origin destination.

Learning by doing: Copy your readme.md to day1.md

mv is used for moving files but can also me used as a rename tool.

Learning by doing: Rename day1.md to week1day1.md. Remember to use man mv if you don't know how to use the command.

When moving files from other directories to the current directory you can use the shortcut .

Example: mv ~/Downloads/somefile.zip .

At all time we can use wildcards like * that represents any number of any characters and ? that represent one of any character.

rm can be used to remove files and folders

Warning: There is no trash can in the command line. You delete a file and the file is gone forever. Be be careful with this command.

You can use rm recursively to delete files and folders

Learning by doing: rm -r ~/Documents/surname.name/ will delete the folder and everything inside it. Extreme caution!

Finding files and folders

find will recursively search for files and folders inside a specified starting location. For example, to find files and folders anywhere inside your home folder which contains "1":

find ~ -name "*1*"

This command has many other options, explore the manual if you want to learn more.

Looking inside files

cat will display your file on the screen. You can also use option -n to number the lines.

cat -n somefile.txt

You can also count the number of lines of a file by piping the output to wc (word count) command

cat file.txt | wc -l

wc can count characters, words, and lines. Check the man page.

head and tail commands will show the first and lastest 10 lines of a file. You can change this number of lines displayed by using the option -n

tail -n 5 somefile.txt

less is used for browsing long files, it will display the file you specify using the same viewer as man command.

grep is a very useful command. It looks for text inside files and display the line of text that contains the search pattern. Some useful options are:

  • -r recursive search
  • -i case insensitive search pattern
  • -n show the text line number than contains the search pattern

So for if you want to seach for "openscad" anywhere inside the current folder you would do:

grep -rin "openscad" .

The echo command

Display text on the screen

echo displays its parameters in the stdout (screen). It is usefull when creating our scripts echo hello world will display hello world on screen.

Writing to files

If you want to write text to a file you can use echo hello world > file.txt but warning!, it will overwrite the contents of the file.

echo hello me > file.txt
echo hello you > file.txt
cat file.txt

hello you

If you want to append text to an existing file (add a new line of text) use the >> operator instead.

echo hello me >> file.txt
echo hello you >> file.txt
cat file.txt

hello me
hello you

Understanding users

Linux is a multi-user operating system with users and groups. Different users have different file access levels or privileges (read, write, execute). By default bash tells you what user is logged in. Otherwise you can ask bash Who am I?

whoami command will tell you who you are.

There are 2 kinds of users in Linux. Normal users and superusers. Supersers can perfom administrative tasks and will be able to look into other users folders. Even if you are a normal user you can perform superuser tasks using the sudo command. sudo stands for Super User Do and it will perform tasks as if you were the root user. For instance, if you want to move one file out of your home folder you will be denied permission. To do that you will have to execute the command as if you were a superuser like root user.

sudo mv somefile.txt /

As soon as you have performed you administrative tasks you should revert back to your normal user privileges. You can withdraw yourself from admin privileges with sudo -k.

You can also switch user to root with su, but you need superuser privileges for that you must use sudo su. Exit to your normal user as soon as you finish with exit command.

TODO misuse of sudo.

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