The Command Prompt


title: The Command Prompt

The Command Prompt

As one of the oldest User Interfaces (UI), the command prompt (a.k.a. shell, terminal, console, tty) has been implemented in many ways.

This has led to a few words being used interchangeably in modern conversation that actually have slightly different meanings.


Table of Contents


A Very, Very Short History

In the 19th century was the telegraph. This allowed two people to exchange encoded messages over long distances. Later technological advancements led to the teletype machine (tty), where the person required to receive the message was replaced by a kind of printer.

At the same time, early computers like ENIAC, were programmed with some kind of hardware, like switches, dials, or patch cables. As the computers advanced, better Input/Output (IO) was needed, and so the commonly available teletype machines were converted for use.

Because the tty’s were large pieces of furniture on their own, and earned the name console for their similarities to other floor-standing furniture such as console televisions. As an electronic end-point for a mainframe, these devices were also called terminals.

TTY printers were eventually replaced by Cathode Ray Tube (CRT) screens, which were also used in televisions before LCD and plasma was available. Interestingly, modern Linux computers can still be controlled using a tty machine!

Click the image below to go to a short Youtube video.


Today

Today, Linux and Unix users still use the same terms, but with slight differences. Real and virtual terminals are available, and virtual terminals can be accessed using Alt + Ctrl + [F1-F12].

In Graphical User Environments (GUI), users can access the command prompt using a terminal emulator, which provides the features of a tty, but within a window. There are many terminal emulators available to Linux users, such as xterm, kterm, and rxvt.

The ones available will depend on which Linux distribution (distro, for short) you are using, and its defaults. Check your package manager to install others. Windows users can use PuTTY or other utilities to connect to a Linux system.

The Shell

Shells are programs that interpret commands.

There are a number of them, such as Bourne Again SHell (BASH), C Shell (csh/tcsh), and Z SHell (zsh).

BASH

The most common default shell in Linux is BASH, but each user can switch temporarily or permanently to any other available shell. The shell is fully scriptable, meaning that programming concepts can be combined with shell and system utilities in order to create more complicated functions.

Commands entered at a command prompt may be built in to the shell, such as cd, exit, or export.

They can also come from external programs, and in the case of most Linux distributions, is provided by the Gnu tools.

See below for the most common commands.

CommandUsage
cdchange current directory
lslist files in current directory
mvmove files and directories
manopen command documentation
mkdirmake a directory
rmdirdelete a director
touchcreate an empty file
rmremove files
lncreate links to files and directories
chownchange ownership of files and directories
chmodchange permissions
findlocate files
catwrites files to standard output
lessallows scrolling of standard input
grepsearch for matches in plain text
diffshow differences between files
passwdchange your password

Getting Help

Immediate help is available for commands in one or more places.

Add --help after the command.

This prints usage information for the command.

Its output is similar to the man command, but man is used before the command that you want the manual for.

The info command is the third help option, and is used just like man.

ls --help man ls info ls

The Prompt

The prompt, which is the bit of text in the shell to the left of the cursor, can change to show your current status, such as which directory you are currently in, which user you are logged in as, your computer’s name, and what privileges you have.

That last one is important to recognize. Usually the last character in the prompt, you will see a $, which indicates normal user privileges.

If you have root privileges, which belong to the system administrator, you will usually see a # as the last character. When browsing forums and getting help online, the commands that you must type will often be shown with this character.

You don’t have to type it!

For example:

$ ls -l

means you type ls -l at a normal prompt.

# apt-get install node

means that you type apt-get install node using administrator privileges. How you elevate your privileges depends on your Linux distribution.

Getting Root

login

Logging in as root is a very bad idea. This is why some versions of Linux disable users’ ability to log this way. Those users are encouraged to use the next method, sudo from within their own user account.

If you do have to use a root console, be aware of its power. You will not be warned, or asked to confirm for most tasks, even if a simple typo means deleting something important.

sudo

Add “sudo” before a command in order to switch to Super User and DO (SUDO). This is how Ubuntu and its derivatives are configured to allow administrator access, and is given on a per-command basis.

You are not given a root shell, and the next command you type will not have elevated privileges, unless you use sudo again.

sudo apt-get update

Except for the first created user on certain distros, users have to be added to a special list (found in /etc/sudoers) in order to use sudo.

This is done with the visudo command.

You should never edit the sudoers file with a regular text editor!

visudo will make sure that you don’t lock yourself out of your own system.

su

su, like sudo, allows you to change to another user, except that by default, you will get another prompt as the user you switched to.

On it’s own, su will switch you to a root prompt, but with the current user’s environment variables, such as $HOME for your home folder, and $PATH for the system path.

This can lead to unexpected results, and if you want to use su to switch to another user, add a hyphen after the command:

su -

This will switch you fully to a root prompt.

A username can be added to the command to switch to that user, but will require that user’s password.

sudo can be used in combination with su to allow an administrator to switch to any user.

myUser@linux $ su - otherUsername Password: (typed my password) su: Authentication failure [email protected] $ sudo su - otherUsername Password: (typed my password) [email protected] $

Relative and Absolute Paths

When using a command on a file, such as copying or deleting, you can refer to the file in one of two ways.

Relative

File location in relation to the current directory.

There are two relative path operators in the shell, . and ...

The first, . means the current directory, so cat file.txt and cat ./file.txt are the same thing if file.txt is in the current directory.

The other is .., and means one directory up in the tree.

So if you are in /home/user/projects/project-a and issue the command cd .. you will change to /home/user/projects.

If the projects directory has sub-directories named project-a, project-b, project-c, and you were in the project-a directory, you could switch to project-b using cd ../project-b.

There is also an environment variable in the shell called $HOME which points to your home directory.

You can use this in BASH using a tilde character ~.

The shell replaces the tilde for you when you hit enter, so as an example, you can change to your own home folder using cd ~.

Absolute

File locations are the full path from the root of the filesystem, and always have a leading slash.

For example, cd /home/quincy/Desktop will go to Quincy’s desktop directory, regardless of current path or logged in user.

Command Options

Most shell commands follow the same syntax, which is command options files.

ls -l *.txt

where

  • ls gives a list of files and directories,
  • -l changes the output of ls to a long listing,
  • and *.txt restricts the list to files ending with .txt.

Each command has different options, and multiple options can be listed together, as in the tar example tar -cvf in the next section.

Individual commands can be connected together in a chain, where the output of one command becomes the input to another command.

This is done with the | character, often called pipe or bar. This is not a capital i or lowercase L, nor is it the number 1. On US keyboards, it’s found on one of the keys near Enter.

In the following example, I will use 2 commands.

The first, cat, is short for concatenate, and can be use to put the contents of one file at the end of another (concatenation!). When using it with one file only, it writes the contents to the terminal.

The second command, grep is a program that outputs text found based on some input, and a search pattern. The search pattern can be simple text, or a Regular Expression (regex) for more advanced searches.

cat index.html | grep img

There are many ways to do this, but this will output every line in index.html that contains img to the terminal. This example only uses one |, but you are not limited to that.

Chaining Commands

While the single ampersand operator & is a job control operator in BASH (next section), the double ampersand has another meaning. It is logical AND, and you use it between two commands so that the second command only runs if the first exits successfully (without error).

The following example is how many Debian & Ubuntu users update their list of software, and then run a system upgrade.

sudo apt-get update && sudo apt-get dist-upgrade

Another option is the double-pipe ||, which means logical OR. You would use it when you want to run a command only when the first exits with an error.

The following will create an archive called project.tar on the user’s desktop from the files in a project directory, and if that fails, echo a message.

tar -cvf /home/user/Desktop/project.tar /home/user/project/* || echo "archive failed"

Background Jobs

When you run a command in a terminal, the terminal is busy until the command is finished, and no other commands can be run. There is a job control system in Linux that allows you to suspend running commands, resume suspended commands in the background, and resume suspended commands in the foreground.

This is useful for long-running scripts, or when you need to push something in to the background so that the terminal can be used for other things.

o suspend a program that is running in the terminal use the key combination Ctrl + Z.

You will get back to your normal prompt, and the command appears to have quit. It hasn’t, but has only been suspended. It’s still visible in the jobs list by using jobs command to list all currently tracked jobs. I did man ls to get a manual page, and then suspended it.

When I type jobs I get the following output:

$ jobs [1] + suspended man ls

From here, I can let it resume in the background by typing bg %1 where the 1 is the job number found in the square brackets.

I can bring it back to the foreground by typing fg %1.

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