Useful shell commands

Gnome

When you log in to the department Linux system using a ThinLinc client you will be using the Gnome graphical desktop environment.

The Gnome terminal

You can open a terminal in two ways:

  • From the Applicatinos menu at the top left of the desktop: Applications → Accessories → Terminal.
  • By pressing the keyboard shorcut CTRL + ALT + T .

After a few seconds a new terminal window should open.

In the upper left corner of the white area of the terminal window you see the shell prompt abcd1234@arrhenius:~$. The shell prompt you see might be different.

The shell prompt

In the above example, the prompt shows the username of the logged in user abcd1234 together with the name of the physical Linux server arrhenius used. You should see your own user name. If you are logged into a different physical Linux server you will also see a different server name in the prompt.

It is also possible to tweak the prompt to show custom information such as your username, local time etc.

Since the appearance of the shell prompt might vary, in all further instructions $ will be used to refer to the Linux shell prompt. In the instructions, interaction with the Linux shell in the terminal will be presented in a box like this.

$

Your username (whoami)

Every user on the Linux system has a unique username. The whoami command will show your username. Type whoami at the shell prompt.

$ whoami

Press enter to execute the command. Now the result will be printed on the next line in the terminal and a new shell prompt will appear on the line after that.

$ whoami
abcd1234
$

In the above example the username of the logged in user abcd234 is printed as the result of the whoami command.

Username

In all examples and instructions you should replace abcd1234 with your actual username.

The shell has a concept of a current working directory. The pwd (print working directory) commands prints the full path of the current working directory.

Type pwd at the shell prompt.

$ pwd

Press enter to execute the command.

$ pwd
/home/abcd1234
$

In the above example the current working directory /home/abcd1234 is printed as the result of the pwd command.

Home directory

On the Linux system each user has a private home directory to where she/he can save files and create sub directories.

When you first log in to the Linux system the home directory will be used as the current working directory in the shell.

For user abcd1234 the full path to the home directory is /home/abcd1234.

List files and directories (ls)

To list the files and directories in the current working directory the ls command can be used. The name ls is a short form of list (files).

Type ls at the shell prompt.

$ ls

Press enter to execute the ls command. You should see something similar to the below as result but you might see other files and folders listed.

$ ls
foo.txt  Desktop  public_html
$

In the above example the only content in the current working directory is the text file foo.txt and two sub directories Desktop and public_html. You may see many more directories and files.

Distinguish between files and folders (ls -F)

To get some more information about files and folder various options can be given to the ls command. One useful option is -F that marks directories with a trailing slash /.

$ ls -F
foo.txt  Desktop/  public_html/
$

Visualize a directory as a tree

The tree command displays the contents of the current directory and subdirectories as a tree structure.

$ tree

The output takes a graphical form which will resemble the following example:

.
├── README.md
├── one.txt
├── sub
│   └── three.txt
└── two.txt

1 directory, 4 files

In the above example, there are three files (README.md, one.txt and two.txt) and one sub directory (sub) in the current working directory. In the sub directory sub there is a single file three.txt.

You can provide three with the path to a directory to visualize its content.

$ tree sub
sub/
└── three.txt

0 directories, 1 file

Change directory (cd)

The cd command navigates to a different folder. The name cd means change directory.

First print the current working directory.

$ pwd
/home/abcd1234
$

To navigate to the Desktop folder, type cd Desktop at the shell prompt and press enter.

$ pwd
/home/abcd1234
$ cd Desktop
$

Now, execute the pwd command.

$ pwd
/home/abcd1234
$ cd Desktop
$ pwd
/home/abcd1234/Desktop
$

Note how the current working directory changed from /home/abcd1234 to /home/abcd1234/Desktop as the result of the cd Desktop command.

The directory above the current working directory can be referred to using ... To navigate to the parent directory, type cd .. and press enter.

$ pwd
/home/abcd1234
$ cd Desktop
$ pwd
/home/abcd1234/Desktop
$ cd ..
$

Now, execute the pwd command again.

$ pwd
$ /home/abcd1234
$ cd Desktop
$ pwd
/home/abcd1234/Desktop
$ cd ..
$ pwd
/home/abcd1234
$

Note how the current working directory changed back from /home/abcd1234/Desktop to /home/abcd1234 as the result of the cd .. command.

Print content of file to the terminal (cat)

The cat command can be used to print the content of a file to the terminal.

Assume you have the following file named foo.txt in the current working directory.

The first line of the file.

The third line. The second line is empty.
The last line of the file.

You can now print the content of foo.txt to the terminal using the cat command.

$ cat foo.txt
The first line of the file.

The third line. The second line is empty.
The last line of the file.
$

The name cat is a short form of concatenate which means to join together. If more than one argument is given to cat the contents of the provided files will be joined together and printed to the terminal.

In the below example cat is used to concatenate the file foo.txt with itself.

$ cat foo.txt foo.txt
The first line of the file.

The third line. The second line is empty.
The last line of the file.
The first line of the file.

The third line. The second line is empty.
The last line of the file.
$

One useful option to the cat command is -n which prefixes each line with a line number.

$ cat -n foo.txt
     1	The first line of the file.
     2
     3	The third line. The second line is empty.
     4	The last line of the file.
$

Count words, lines and bytes (wc)

The wc command counts the number of words, lines and bytes.

$ wc foo.txt
       4      20      98
$

In the above example we see that the file foo.txt contains for lines, 20 words and 98 bytes.

Filter (grep)

The grep command searches its input for a pattern and prints all lines in the input that contains that pattern.

To search for the the string X in the input type grep X at the shell prompt and press enter.

$ grep X

Note that we don’t get back the shell prompt. This is because the grep command is still running waiting for input. The grep command will now read input from the terminal and print back all lines containing the character X.

Now type Hello and press enter.

$ grep X
Hello

There is no X in the string Hello and therefore grep will not print back the string Hello to the terminal.

Type Hello mr X and press enter and watch what happens.

$ grep X
Hello
Hello mr X
Hello mr X

Once you type Hello mr X the grep command will print Hello mr X right back to the terminal since it contains a matching X.

Lets try a few more lines and observe what happens.

$ grep X
Hello
Hello mr X
Hello mr X
abc
abcXdef
abcXdef
xxx

Only lines containing a matching X will be echoed back to the terminal.

No more input

To tell grep that you are done (no more input), press Ctrl D (press and hold down the control key and while you still hold down the control key press the D key).

Press Ctrl D. Now grep terminates and you get back to the shell prompt.

$ grep X
Hello
Hello mr X
Hello mr X
abc
abcXdef
abcXdef
ABC
$

To filter the lines i a file, the name of the file can be given together with a search pattern to grep.

Assume you have the file foo.txt in your current directory.

$ cat foo.txt
The first line of the file.

The third line. The second line is empty.
The last line of the file.
$

In the below example only lines containing of in the file foo.txt will be printed to the terminal.

$ grep of foo.txt
The first line of the file.
The last line of the file.
$

Filter the output of ls using grep (ls | grep)

The usefulness of grep might not obvious at this point. To make grep useful we will combine grep with ls to filter the output of ls.

First we use ls to list all files and folders.

$ ls
foo.txt    Desktop    public_html
$

If we are only interested in files (and folders) with names ending in .txt we can combine ls and grep to using the pipe character |.

$ ls | grep .txt
foo.txt
$

In the above example, first the ls command exectutes but it does not print its result back to the terminal. Instead, the result of the ls command becomes the input to the grep command. The only file or folder name containing .txt is foo.txt.

Piping commands togehter

Using the pipe character | the output of the command to the left becomes the input to the command to the right. This is called piping the two commands together.

Compressed file arhives (tarballs)

It is often useful to compress multiple files and folders into a single file that can later be decompressed and expanded to get back the original files and folders. There exists many file formats for compressed file archives.

  • Windows users commonly use the zip file format.
  • Unix users commonly use the tar file format.

Tarball

The name tarball is often used to refer to a tar archive file.

Download the following gziped compressed tar archive (tarball) to your home folder:

Verify that you have the tarball in your current working directory

From the terminal, make sure you have the downloaded tarball in the current working directory.

$ ls | grep .tar
archive.tar.gz
$

In the above example the output of ls is piped together with grep to filter the output of ls to only print any files (or folders) containing .tar. You should see archive.tar.gz among the results.

Sneak peek inside a tarball (tar tf)

To see the contents of a tarball without extracting all the files you can use tar with options t and f.

$ tar tf archive.tar.gz
archive/
archive/large.txt
archive/small.txt
archive/sub_folder/
archive/sub_folder/info.txt
$

In the above example we see that the tarball archive.tar.gz contains the top level directory archive with sub folder sub_folder. In the top level directory archive there are two files (large.txt and small.txt) and in the sub folder sub_folder there is a single file (small.txt).

Unpack a tarball (tar xvfz)

To unpack and extract the contents of a gzipped tarball we need to use the xvfz options together with the tar command.

$ tar xvfz archive.tar.gz
x archive/
x archive/large.txt
x archive/small.txt
x archive/sub_folder/
x archive/sub_folder/info.txt
$

Now the tarball have been unpacked. Use ls to see what happened to the current working directory.

$ ls | grep archive
archive
archive.tar.gz
$

In the above example we now have a new directory named archive inside the current working directory.

Use cd to “step inside” the archive directory and then ls -F to list the content.

$ cd archive
$ ls -F
large.txt
small.txt
sub_folder/
$

Using the -R option ls will be run recursively stepping inside every sub-directory.

$ ls -R
large.txt	small.txt	sub_folder

./sub_folder:
info.txt
$

In the result printed by ls -R a single period . means the current working directory.

Print text back to the terminal (echo)

To print anything to the terminal simply type echo followed by the text you want to print.

$ echo Hello
Hello
$

Note that HELLO is echoed back to the terminal as the result of executing the echo Hello command before the shell prints the next command prompt.

Shell variables

The shell can set and read variables. Sometimes it is useful to use the value of a built-in shell variable to make a command more generic and/or portable.

Remember that the command woami can be used to print your username.

$ whoami
abcd1234
$

$USER

An alternative to woami is to use echo together with the shell variable USER. In order for echo to know if you want to print the string "USER" or the value of the shell variable USER shell variables must be prefixed with $ or enclosed within ${ }.

$ echo Hello USER
Hello USER
$ echo Hello $USER
Hello abcd1234
$ echo Hello ${USER}
Hello abcd1234
$

$HOME

Another useful shell variable is HOME with the full path to the home directory for the logged in user.

$ echo $HOME
/home/abcd1234
$

Command history

Often you type and run a command in the terminal and later you wants to run the very same command again. To prevent you from having to type the same thing again the shell keeps a history of executed command. To navigate the history, simply press the up-arrow to move backwards in history and press the down-arrow to move forward in history.

Try the following command in the terminal.

$ pwd
/home/abcd1234
$

And now this command.

$ whoami
abcd1234
$

If you want to repeat the whoami command, simply press the up-arrow key once. Instead if you wish to run the pwd command again, press the up-arrow key twice.

Reading manual pages (man)

For more information about command you can always refer to the corresponding built in manual page. For example, to read the manual page for the ls command simply type man ls and press enter at the shell prompt.

$ man ls

This will print the manual one page at a time to the terminal. To view the next page, press the space bar. To quit, press q.

To learn more about the build in manual pages read the manual page about the man command.

$ man man

A summary of usefull controll keys when reading man pages.

Key Behaviour
q Quit and get back to the terminal
Space bar or F Move forward one page
D Move forward half a page
B Move backwards one page
U Move backwards half a page

Learn more

To learn more about the Ubuntu Linux shell:

To learn more about tar file archives (tarballs):