Welcome to my blog. I write Instruction Guides, How-Tos, and Tips & Tricks on Linux, Database, Hardware, Security and Web. Visit Best of the Blog section to get started.

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]

If you liked this article, please bookmark it on del.icio.us and Stumble using the link provided below, under ‘What Next?’ section.

What Next?   Bookmark on del.icio.us   Stumble it   Digg it   Subscribe to The Geek Stuff   Leave a comment

Related Articles Execution sequence for .bash_profile, .bashrc, .bash_login, .profile and .bash_logout Bash Shell PS1: 10 Examples to Make Your Linux Prompt like Angelina Jolie 15 Examples To Master Linux Command Line History 6 Awesome Linux cd command Hacks - Productivity Tip#3 for Geeks

Tags: bash, command line