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

by Ramesh Natarajan on September 4, 2008

PS[1-4] and PROMPT_COMMANDYour 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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{ 3 comments… read them below or add one }

1 Ajith Edassery September 4, 2008 at 1:23 am

Ramesh,
An unrelated question but specific to Ubuntu… I heard that there’s a directory copy installation available for Ubuntu. The last time I tried the normal install a couple of years back, it was scary. Any thoughts on this simple installation procedure.

May be you could post an article about that as well.

Cheers,
Ajith
PS: As such I am not a Linux user yet

2 Sitaram September 13, 2008 at 4:13 am

I like having this in my .bashrc:

PS1=’\[\e[32m\]\t \h:\W \$ \[\e[m\]‘
[[ $EUID == 0 ]] && PS1=’\[\e[31m\]\t \h:\W \$ \[\e[m\]‘

(1) it colors the prompt green when I’m logged in as me, but when I have su’d into root, it goes red — a very useful distinction!

(2) it shows the current time in 24hr HH:MM:SS format due to the \t. There are lots of times when I run a command and walk away, and I would like an approximate idea of how long it actually took. (If I want “exact”, I’ll use the time command, but that’s not always needed)

3 Gurumurthy January 17, 2014 at 9:06 am

I use this

C=$(tput setaf 6)
export PS1=’\[$C\]\u@\[$C\]\h:\[$C\]\w $ \[$RESET\]‘

to know different colors supported by the terminal, use this

for i in {0..255}; do tput setab $i; echo -n ” “; done; tput setab 0; echo

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