Command line overview

Command line overview

Description of a command line interface

A command-line interface (commonly abbreviated to CLI) is a human-machine interface in which communication between the user and the computer takes place in text mode:

  • the user types a command line, i.e., text on the keyboard, to request the computer to perform an operation;

  • the computer displays text corresponding to the result of the execution of the typed commands or to questions that a software poses to the user.

A command-line interface can be used to launch the execution of various software by means of a command interpreter, as well as for dialogs with the user of this software. This is the fundamental interaction between a man and a computer (or any other computer equipment).

When an interface is ready to receive a command, it indicates it by a command prompt. This, sometimes referred to as prompt anglicism, consists of a few characters at the beginning of the line (usually the user’s account name, and / or the default logical drive, and / or the default path , and / or date, …), ending with a well-known character (often “]”, “#”, “$” or “>”), inviting the user to type an order.

The BASH shell and other interpreters

Under GNU / Linux the default command interpreter named Bash, there is much more like the GUI, however a majority of people use it.

Here is the name of another shell available:

 

Like all script-based shell interpreters, Bash performs four basic operations:

  1. It provides a list of commands to operate on the computer (launching programs, copying files, etc.);

  2. It allows to group these commands in a single file, called script;

  3. It checks the command line, during its execution or during a possible verification procedure, and returns an error message in the event of a syntax error;

  4. In case of validation, each order line is interpreted; that is to say, translated in a language understandable by the operating system, which executes it then.

Command Prompt (prompt)

If you start a terminal, you will immediately have the command prompt. Many cast presents the prompt as follows:

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UserName@Hostname:Répertoire_courant$

If you are logged in with the root user, it should look like:

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root@Hostname_de_la_machine:Répertoire_courant#

  • UserName: obviously represents the name of the user in use

  • Hostname: The name of the machine waiting for instructions at the command line.

  • Current_directory: The directory where you are. If you have the ~ character, this is equivalent to the user’s home directory. So if the user names it thomas, ~ == / home / thomas

  • $ or #: $ indicates that you are logged in as a “regular” user. # indicates that you are logged in with the administrator account.

Of course this invitation is customizable; we will see later how to perform this operation.

Using the command line (Shell / Terminal)

General syntax of commands

The elementary commands under Unix are of the form:

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command options files_or_data

The command appearing at the beginning of the line is almost always the name of a software. This software can be a command of the operating system, a software written by a user.

A command line option or an option (also sometimes called a flag) changes the operation of a command. The effect of the option depends on the command. Usually, the options immediately follow the name of the command on the command line and are separated by spaces.

It is impossible to know in advance what options a program recognizes, unless you consult its documentation. However, it is common (but not systematic) for a program to display a summary of its options when it is launched from the command line with one of the following options:? – ?; -h; / ?; / H; -help; or –help.