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

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]

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