2. Bash and Bash Prompts

2.1 What is Bash?

Descended from the Bourne Shell, Bash is a GNU product, the "Bourne Again SHell." It's the standard command line interface on most Linux machines. It excels at interactivity, supporting command line editing, completion, and recall. It also supports configurable prompts - most people realize this, but don't know how much can be done.

2.2 What Can Tweaking Your Bash Prompt Do For You?

Most Linux systems have a default prompt in one colour (usually gray) that tells you your user name, the name of the machine you're working on, and some indication of your current working directory. This is all useful information, but you can do much more with the prompt: all sorts of information can be displayed (tty number, time, date, load, number of users, uptime ...) and the prompt can use ANSI colours, either to make it look interesting, or to make certain information stand out. You can also manipulate the title bar of an Xterm to reflect some of this information.

2.3 Why Bother?

Beyond looking cool, it's often useful to keep track of system information. One idea that I know appeals to some people is that it makes it possible to put prompts on different machines in different colours. If you have several Xterms open on several different machines, or if you tend to forget what machine you're working on and delete the wrong files (or shut down the server instead of the workstation), you'll find this a great way to remember what machine you're on.

For myself, I like the utility of having information about my machine and work environment available all the time. And I like the challenge of trying to figure out how to put the maximum amount of information into the smallest possible space while maintaining readability.

2.4 The First Step

The appearance of the prompt is governed by the shell variable PS1. Command continuations are indicated by the PS2 string, which can be modified in exactly the same ways discussed here - since controlling it is exactly the same, and it isn't as "interesting," I'll mostly be modifying the PS1 string. (There are also PS3 and PS4 strings. These are never seen by the average user - see the Bash man page if you're interested in their purpose.) To change the way the prompt looks, you change the PS1 variable. For experimentation purposes, you can enter the PS1 strings directly at the prompt, and see the results immediately (this only affects your current session, and the changes go away when you log out). If you want to make a change to the prompt permanent, look at the section below Setting the PS? Strings Permanently.

Before we get started, it's important to remember that the PS1 string is stored in the environment like any other environment variable. If you modify it at the command line, your prompt will change. Before you make any changes, you can save your current prompt to another environment variable:

[giles@nikola giles]$ SAVE=$PS1
[giles@nikola giles]$

The simplest prompt would be a single character, such as:

[giles@nikola giles]$ PS1=$
$ls
bin   mail
$

This demonstrates the best way to experiment with basic prompts, entering them at the command line. Notice that the text entered by the user appears immediately after the prompt: I prefer to use

$PS1="$ "
$ ls
bin   mail
$ 

which forces a space after the prompt, making it more readable. To restore your original prompt, just call up the variable you stored:

$ PS1=$SAVE
[giles@nikola giles]$

2.5 Bash Prompt Escape Sequences

There are a lot of escape sequences offered by the Bash shell for insertion in the prompt. From the Bash 2.02 man page:

When executing interactively, bash  displays  the  primary
prompt  PS1  when  it  is ready to read a command, and the
secondary prompt PS2 when it needs more input to  complete
a  command.   Bash  allows these prompt strings to be cus-
tomized by inserting a number of backslash-escaped special
characters that are decoded as follows:
       \a     an ASCII bell character (07)
       \d     the  date  in  "Weekday  Month  Date" format
              (e.g., "Tue May 26")
       \e     an ASCII escape character (033)
       \h     the hostname up to the first `.'
       \H     the hostname
       \n     newline
       \r     carriage return
       \s     the name of the shell, the  basename  of  $0
              (the portion following the final slash)
       \t     the current time in 24-hour HH:MM:SS format
       \T     the current time in 12-hour HH:MM:SS format
       \@     the current time in 12-hour am/pm format
       \u     the username of the current user
       \v     the version of bash (e.g., 2.00)
       \V     the  release  of  bash, version + patchlevel
              (e.g., 2.00.0)
       \w     the current working directory
       \W     the basename of the current  working  direc-
              tory
       \!     the history number of this command
       \#     the command number of this command
       \$     if  the effective UID is 0, a #, otherwise a
              $
       \nnn   the character  corresponding  to  the  octal
              number nnn
       \\     a backslash
       \[     begin a sequence of non-printing characters,
              which could be used to embed a terminal con-
              trol sequence into the prompt
       \]     end a sequence of non-printing characters

Continuing where we left off:

[giles@nikola giles]$ PS1="\u@\h \W> "
giles@nikola giles> ls
bin   mail
giles@nikola giles>

This is similar to the default on most Linux distributions. I wanted a slightly different appearance, so I changed this to:

giles@nikola giles> PS1="[\t][\u@\h:\w]\$ "
[21:52:01][giles@nikola:~]$ ls
bin   mail
[21:52:15][giles@nikola:~]$ 

2.6 Setting the PS? Strings Permanently

Various people and distributions set their PS? strings in different places. The most common places are /etc/profile, /etc/bashrc, ~/.bash_profile, and ~/.bashrc . Johan Kullstam ( johan19@idt.net) writes:

the PS1 string should be set in .bashrc. this is because non-interactive bashes go out of their way to unset PS1. the bash man page tells how the presence or absence of PS1 is a good way of knowing whether one is in an interactive vs non-interactive (ie script) bash session.

the way i realized this is that startx is a bash script. what this means is, startx will wipe out your prompt. when you set PS1 in .profile (or .bash_profile), login at console, fire up X via startx, your PS1 gets nuked in the process leaving you with the default prompt.

one workaround is to launch xterms and rxvts with the -ls option to force them to read .profile. but any time a shell is called via a non-interactive shell-script middleman PS1 is lost. system(3) uses sh -c which if sh is bash will kill PS1. a better way is to place the PS1 definition in .bashrc. this is read every time bash starts and is where interactive things - eg PS1 should go.

therefore it should be stressed that PS1=..blah.. should be in .bashrc and not .profile.

I tried to duplicate the problem he explains, and encountered a different one: my PROMPT_COMMAND variable (which will be introduced later) was blown away. My knowledge in this area is somewhat shaky, so I'm going to go with what Johan says.