Bash Shell: Take Control of PS1, PS2, PS3, PS4 and PROMPT_COMMAND
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.
-bash-3.2$ export PS1="\u@\h \w> " ramesh@dev-db ~> cd /etc/mail ramesh@dev-db /etc/mail> [Note: Prompt changed to "username@hostname current-dir>" format] 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.
ramesh@dev-db ~> myisamchk --silent --force --fast --update-state \ > --key_buffer_size=512M --sort_buffer_size=512M \ > --read_buffer_size=4M --write_buffer_size=4M \ > /var/lib/mysql/bugs/*.MYI [Note: This uses the default ">" for continuation prompt] ramesh@dev-db ~> export PS2="continue-> " ramesh@dev-db ~> myisamchk --silent --force --fast --update-state \ continue-> --key_buffer_size=512M --sort_buffer_size=512M \ continue-> --read_buffer_size=4M --write_buffer_size=4M \ continue-> /var/lib/mysql/bugs/*.MYI [Note: This uses the modified "continue-> " for continuation prompt]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:
ramesh@dev-db ~> cat ps3.sh select i in mon tue wed exit do case $i in mon) echo "Monday";; tue) echo "Tuesday";; wed) echo "Wednesday";; exit) exit;; esac done ramesh@dev-db ~> ./ps3.sh 1) mon 2) tue 3) wed 4) exit #? 1 Monday #? 4 [Note: This displays the default "#?" for select command prompt]
Shell script and output WITH PS3:
ramesh@dev-db ~> cat ps3.sh PS3="Select a day (1-4): " select i in mon tue wed exit do case $i in mon) echo "Monday";; tue) echo "Tuesday";; wed) echo "Wednesday";; exit) exit;; esac done ramesh@dev-db ~> ./ps3.sh 1) mon 2) tue 3) wed 4) exit Select a day (1-4): 1 Monday Select a day (1-4): 4 [Note: This displays the modified "Select a day (1-4): " for select command prompt]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:
ramesh@dev-db ~> cat ps4.sh set -x echo "PS4 demo script" ls -l /etc/ | wc -l du -sh ~ ramesh@dev-db ~> ./ps4.sh ++ echo 'PS4 demo script' PS4 demo script ++ ls -l /etc/ ++ wc -l 243 ++ du -sh /home/ramesh 48K /home/ramesh [Note: This displays the default "++" while tracing the output using set -x]
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
ramesh@dev-db ~> cat ps4.sh export PS4='$0.$LINENO+ ' set -x echo "PS4 demo script" ls -l /etc/ | wc -l du -sh ~ ramesh@dev-db ~> ./ps4.sh ../ps4.sh.3+ echo 'PS4 demo script' PS4 demo script ../ps4.sh.4+ ls -l /etc/ ../ps4.sh.4+ wc -l 243 ../ps4.sh.5+ du -sh /home/ramesh 48K /home/ramesh [Note: This displays the modified "{script-name}.{line-number}+" while tracing the output using set -x]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.
ramesh@dev-db ~> export PROMPT_COMMAND="echo -n [$(date +%k:%m:%S)]" [22:08:51]ramesh@dev-db ~> [Note: This displays the PROMPT_COMMAND and PS1 output on the same line]