The first thing that you should notice is something like:

dud@shadowplay:~ $
or
[dud@shadowplay ~]$

What you see here is called the prompt. It signifies that the computer is ready and awaiting user input. In my case, dud is the user that I'm logged in as. shadowplay is the computer's hostname, and ~ is the current directory (the user's home directory).

Concepts:

• A terminal is a "physical" (direct) interface to your Linux Operating System.
• A terminal emulator is what we'll be using here. This is a CLI wrapped within your running GUI. Any applications running in a terminal emulator will be killed if you close the terminal emulator.
• A shell is an interpreter for commands entered into the terminal.
• A command is usually a small utility that the shell will execute for you.
• Output is what a command returns; most often this is returned on the terminal.
• Input consists of the arguments or data that any given command will take. Input will change the way a given command acts.
• A process is a running application on your computer. It can be active, sleeping, or in a number of other states.

Command Syntax

This section will try to give you a good rundown of the basic usage for the bash shell, which is the default user shell in Ubuntu.

Single Command

The command syntax will vary with each command. Here are some of the basics.

The simplest way to use some commands is to type just the command.

• command

Example:

dud@shadowplay:~ $ ls
file1.txt
file2.pdf
file3.mp3
file1.pdf
another_file.txt
Yet-Another_file.txt
file-with_other-NAME.TXT

The above example displays the contents of the current working directory, while other commands may require one or more arguments.

• command <argument>

Example:

dud@shadowplay:~ $ cat file1.txt
Roses are red.
Violets are blue,
and you have the bird-flu!

Sometimes the desired task may require the use of more than one command to be completed. Here is the syntax for the use of multiple commands.

If you want to execute two commands consecutively, then you would use the following syntax:

• command1 ; command2

Example:

dud@shadowplay:~ $grep red file1.txt ; grep blue file1.txt
Roses are red,
Violets are blue,

In the example above, command1 and command2 are executed. However, if you need command1 to complete successfully before executing command2, then you would use the following syntax:

• command1 && command2

Example:

dud@shadowplay:~ $ grep red file1.txt && grep blue file1.txt
Roses are red,
Violets are blue,
dud@shadowplay:~ $ grep purple file1.txt && grep blue file1.txt
dud@shadowplay:~ $

In the example above, you will notice nothing happened when the first command did not complete successfully. If you want command2 to execute only if command1 fails, then you would use the following syntax:

• command1 || command2

Example:

dud@shadowplay:~ $ grep red file1.txt || grep blue file1.txt
Roses are red,
dud@shadowplay:~ $ grep purple file1.txt || grep blue file1.txt
Violets are blue,
dud@shadowplay:~ $

In the example above, you will notice command2 was only executed when command1 failed.

Wildcards

Wildcards are a useful feature that allows an unknown value or values to be used with another command. This becomes very useful with commands such as "ls" allowing only a range of filenames to be displayed.

There are three operators used with wildcards - "*", "?" and "[x-y]".

Specifing a single character

The "?" is used to represent a single unknown character, consider we have a folder containing four files: file1.pdf, file2.pdf, file2.mp3 and file23.pdf. If wanted to know which PDF filenames contained numbers, then we could use:

dud@shadowplay:~ $ ls file?.pdf
file1.pdf
file2.pdf

Specifying multiple characters

Using the same files as the previous example, if we wanted to search for all files called "file2" of any type we could:

Example:

dud@shadowplay:~ $ ls file2.*
file2.pdf
file2.mp3

Specifying a range

If we wanted to know all PDF filenames beginning with "file" and a number between 2 and 23 then we use:

dud@shadowplay:~ $ ls file[2-23].pdf
file2.pdf
file23.pdf

Control Flow

Commands read input from the keyboard (standard input, or stdin) and write to output (standard out, or stdout). There is also a special output category for error messages called standard error (or stderr). These three locations are created automatically for each program.

We can redirect input and output to and from a command.

Redirection

If you wanted the output of a command to go to a file instead of the terminal, then you would use the following syntax:

• command > filename

Example:

dud@shadowplay:~ $ ls > file4.txt
dud@shadowplay:~ $ cat file4.txt
file1.txt
file2.pdf
file3.mp3
file1.pdf
another_file.txt
Yet-Another_file.txt
file-with_other-NAME.TXT
file4.txt

The above example will create file4.txt if it is not found. NOTE: If file4.txt exists already, the above command will overwrite its contents. If you want to add to the end of a existing file, then you would use the following syntax:

• command >> filename

Example:

dud@shadowplay:~ $ ls >> file4.txt
dud@shadowplay:~ $ cat file4.txt
file1.txt
file2.pdf
file3.mp3
file1.pdf
another_file.txt
Yet-Another_file.txt
file-with_other-NAME.TXT
file4.txt
file1.txt
file2.pdf
file3.mp3
file1.pdf
another_file.txt
Yet-Another_file.txt
file-with_other-NAME.TXT
file4.txt

In the example, you will notice the file was appended with the new information.

But bash takes it even further. You will have noticed that ls can produce error messages if a directory ls is suppose to list does not exist or the user lacks access rights. In bash, Error messages and the "regular results" (i.e. dir listing) are 2 separate streams. To ease use of results only find a detailed explanation of bash.

Now we are going to do a different redirection: We are going to take the input from a file for the command to be executed. Here is the syntax for this redirection:

• command < filename

Example:

dud@shadowplay:~ $ sort < file4.txt
another_file.txt
another_file.txt
file1.txt
file1.txt
file2.pdf
file2.pdf
file3.mp3
file3.mp3
file4.txt
file4.txt
file-with_other-NAME.TXT
file-with_other-NAME.TXT
Yet-Another_file.txt
Yet-Another_file.txt

As you can see from this example, we used the file4.txt as input into the sort command.

Pipe

When you need the output from command 1 for the input into command 2, then you would use pipe character '|'. Here is the syntax for the pipe character:

• command | command

Example:

dud@shadowplay:~ $ ls | sort
another_file.txt
file1.pdf
file1.txt
file2.pdf
file3.mp3
file-with_other-NAME.TXT
Yet-Another_file.txt

The above example is using the output from ls as input to the sort command. You will notice the list has been sorted.

As you can see, the command line is an easy and powerful way of completing many tasks. If you want more information on using the command line, then look at the further reading section of this document.

Further reading

• UsingTheTerminal

• BasicCommands has a list of basic commands.

• AdvancedCommandlineHowto has some advanced command line features such as scripting.

CategoryCommandLine