Bash Shell: Take Control of PS1, PS2, PS3, PS4 and PROMPT_COMMAND shell

http://www.thegeekstuff.com/2008/09/bash-shell-take-control-of-ps1-ps2-ps3-ps4-and-prompt_command/
http://www.cyberciti.biz/tips/howto-linux-unix-bash-shell-setup-prompt.html

Your interaction with Linux Bash shell will become very pleasant, if you use PS1, PS2, PS3, PS4, and PROMPT_COMMAND effectively. PS stands for prompt statement. This article will give you a jumpstart on the Linux command prompt environment variables using simple examples.

1. PS1 – Default interaction prompt
The default interactive prompt on your Linux can be modified as shown below to something useful and informative. In the following example, the default PS1 was “\s-\v\$”, which displays the shell name and the version number. Let us change this default behavior to display the username, hostname and current working directory name as shown below.

Following PS1 codes are used in this example:

\u - Username
\h – Hostname
\w – Full pathname of current directory. Please note that when you are in the home directory, this will display only ~ as shown above
Note that there is a space at the end in the value of PS1. Personally, I prefer a space at the end of the prompt for better readability.

Make this setting permanent by adding export PS1=”\u@\h \w> ” to either .bash_profile (or) .bashrc as shown below.

ramesh@dev-db ~> vi ~/.bash_profile (or) ramesh@dev-db ~> vi ~/.bashrc [Note: Add export PS1="\u@\h \w> " to one of the above files]

In the next post, I’ll write about several practical examples of PS1 usage in detail.
2. PS2 – Continuation interactive prompt
A very long unix command can be broken down to multiple line by giving \ at the end of the line. The default interactive prompt for a multi-line command is “> “.Let us change this default behavior to display “continue->” by using PS2 environment variable as shown below.

I found it very helpful and easy to read, when I break my long commands into multiple lines using \. I have also seen others who don’t like to break-up long commands. What is your preference? Do you like breaking up long commands into multiple lines?
3. PS3 – Prompt used by “select” inside shell script
You can define a custom prompt for the select loop inside a shell script, using the PS3 environment variable, as explained below.

Shell script and output WITHOUT PS3:

【Bash Shell: Take Control of PS1, PS2, PS3, PS4 and PROMPT_COMMAND】
Shell script and output WITH PS3:

4. PS4 – Used by “set -x” to prefix tracing output
The PS4 shell variable defines the prompt that gets displayed, when you execute a shell script in debug mode as shown below.

Shell script and output WITHOUT PS4:

Shell script and output WITH PS4:
The PS4 defined below in the ps4.sh has the following two codes:

$0 – indicates the name of script
$LINENO – displays the current line number within the script

5. PROMPT_COMMAND
Bash shell executes the content of the PROMPT_COMMAND just before displaying the PS1 variable.

ramesh@dev-db ~> export PROMPT_COMMAND="date +%k:%m:%S" 22:08:42 ramesh@dev-db ~> [Note: This displays the PROMPT_COMMAND and PS1 output on different lines]

If you want to display the value of PROMPT_COMMAND in the same line as the PS1, use the echo -n as shown below.