Chapter 5: PS1, PS2, PS3, PS4 andPROMPT_COMMAND
__Linux 101 Hacks
Hack 29. PS1 - Default Interaction Prompt
-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]
\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.
Make this setting permanent by adding export PS1=”\u@\h \w> ” to either .bash_profile (or) .bashrc.
Hack 30. PS2 - Continuation Interactive Prompt
A very long command can be broken down to multiple lines 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 ~> export PS2="continue-> "
Hack 31. PS3 - Prompt used by “select” inside shell script
Shell script and output WITHOUT PS3:
1 select i in mon tue wed exit
2 do
3 case $i in
4 mon) echo "Monday";;
5 tue) echo "Tuesday";;
6 wed) echo "Wednesday";;
7 exit) exit;;
8 esac
9 done
10
Shell script and output WITH PS3:
代码
1 PS3="Select a day (1-4): "
2 select i in mon tue wed exit
3 do
4 case $i in
5 mon) echo "Monday";;
6 tue) echo "Tuesday";;
7 wed) echo "Wednesday";;
8 exit) exit;;
9 esac
10 done
11
Hack 32. 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:
代码
1 ramesh@dev-db ~> cat ps4.sh
2 set -x
3 echo "PS4 demo script"
4 ls -l /etc/ | wc -l
5 du -sh ~
6
7 ramesh@dev-db ~> ./ps4.sh
8 ++ echo 'PS4 demo script'
9 PS4 demo script
10 ++ ls -l /etc/
11 ++ wc -l
12 243
13 ++ du -sh /home/ramesh
14 48K /home/ramesh
15
[Note: This displays the default "++" while tracing the output using set -x]
Shell script and output WITH PS4:
代码
1 ramesh@dev-db ~> cat ps4.sh
2 export PS4='$0.$LINENO+ '
3 set -x
4 echo "PS4 demo script"
5 ls -l /etc/ | wc -l
6 du -sh ~
7
8 ramesh@dev-db ~> ./ps4.sh
9 ../ps4.sh.3+ echo 'PS4 demo script'
10 PS4 demo script
11 ../ps4.sh.4+ ls -l /etc/
12 ../ps4.sh.4+ wc -l
13 243
14 ../ps4.sh.5+ du -sh /home/ramesh
15 48K /home/ramesh
16
[Note: This displays the modified "{script-name}.{line-number}+" while tracing the output using set -x]
Hack 33. PROMPT_COMMAND
Bash shell executes the content of the PROMPT_COMMAND just before displaying the PS1 variable.
ramesh@dev-db ~> export PROMPT_COMMAND="date +%H:%M:%S"
22:08:42
ramesh@dev-db ~> export PROMPT_COMMAND="echo -n [$(date +%H:%M:%S)]"
[22:08:51]
