Chapter 5: PS1, PS2, PS3, PS4 and PROMPT_COMMAND __Linux 101 Hacks
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
:0
8
:
42
ramesh@dev
-
db
~>
export PROMPT_COMMAND
=
"
echo -n [$(date +%H:%M:%S)]
"
[
22
:0
8
:
51
]