Extending the Bash Prompt

Descended from the Bourne shell, Bash (Bourne Again Shell) is a GNU product that is 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 may not realize how useful it can be.

Most Linux systems have a default prompt in one color (gray) that includes your user name, the name of the machine you are working on and your current working directory. In addition, you can display even more information, use ANSI colors and manipulate the title bar of an xterm to provide useful information.

Beyond looking cool, prompts are also useful for keeping track of system information. One idea with appeal to many is the use of different color prompts on different machines. If you have several xterms open on different machines or if you tend to forget which machine you are working on, you'll find this a great reminder.

To change your prompt, you need a basic understanding of shell programming and UNIX utilities. The more you know, the more complex the prompts you will be able to create.

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 way. Since controlling it is exactly the same, I'll mostly be modifying the PS1 string. (PS3 and PS4 strings are also available, but 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 appears, change the PS1 variable. For experimentation purposes, the PS1 string can be entered at the prompt to show the results immediately. Doing so affects only your current session. If you want to make a permanent change, modify the ~/.bashrc file by adding the new definition of PS1. If you have root permission, you can modify the PS1= line in the /etc/profile file. On some distributions (Red Hat 5.1 at least), the /etc/bashrc file overrides the /etc/profile setting of PS1 and PS2.

My default prompt includes my user name “giles”, the name of my work machine “nikola” and my home directory /home/giles. The simplest prompt is a single character. I can change my default prompt to a simple $ by typing:

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

I use the quotes to force a space after the prompt, making it more readable.

Bash 2.02 Man Page

Many escape sequences are offered by the Bash shell for insertion in the prompt. See the sidebar which shows the Bash 2.02 man page.

$ PS1="\u@\h \W> "<\n>
giles@nikola giles>

This example creates a prompt that is close to the default on most Linux distributions. I wanted a slightly different appearance, so I changed it to include the time by typing:

giles@nikola giles> PS1="[ ][\u@\h:\w]$ "<\n>
[21:52:01][giles@nikola:~]$

[21:55:01][giles@nikola:~] PS1="[\u@\h:\w]$ "<\n>
[giles@nikola:~] PROMPT_COMMAND="date +%H%M"
2155
[giles@nikola:~] ls
bin   mail
2156
[giles@nikola:~]$ unset PROMPT_COMMAND
[giles@nikola:~]

As I discuss the use of external commands in prompts, I'll use the $(command) convention for command substitution; that is,

$(date +%H%M)

means “substitute the output from the date +%H%M command here.” This works in Bash 1.14.7 and 2.0+. In some older versions of Bash, you may need to use backquotes (`date +%H%M`). Backquotes can be used in Bash 2.0+ but are being phased out in favor of $(), which nests better. If you are using an earlier version of Bash, substitute backquotes wherever you see $(). If the command substitution is escaped (i.e., \$(command) ), then use backslashes to escape BOTH your backquotes (i.e., \`command\`).

You don't want to insert much material from an external command into the prompt, as a prompt of great length may be created. You also want to use a fast command, because it will be executed each time your prompt appears on the screen. Delays in the appearance of the prompt while you are working can be annoying.

[giles@nikola:~]$ PS1="[\$(date +%H%M)][\u@\h:\w]$ "[2159][giles@nikola:~]$

Note the backslash before the dollar sign of the command substitution. Without it, the external command is executed exactly once: when the PS1 string is read into the environment. For this prompt, it would display the same time no matter how long the prompt was used. The backslash prevents immediate shell interpretation of the command, so date is called each time a prompt is generated.