Need explanation for $PS1, $PS2, $PS3, $PS4

Shell Programming and Scripting
1

Hi Masters,

could you plz. any of you explain what are $PS1, $PS2, $PS3, $PS4.

Thanks & Regards
Ramesh

2

PS1 - to set the prompt string, ofcourse!
PS2 - to set the subshell prompt string, which by default is ">", when we enter a command half and press enter, we get the subshell prompt string which is >, then we can complete the command and press enter, it runs. We can change the > to something by modifying this var
PS3 - mostly used in ksh, this is used to define the prompt string displayed by 'select' command, which creates a menu, so in the menu, the prompt string for each option can be defined by changing this one
PS4- we can debug a shell script by putting a set -x while running it, this enables to print each command and then its results a + sign is put before each command, this "+" prompt string can be changed by defining the PS4 var.

3

The $PS1 is used to display the user and host information
For example: thillaiselvan@webserver
\u - Username
\h - Hostname
\w - Full pathname of current directory.

$PS2 this is used to continuation of command.
Assume that you are typing a very lengthy commands that may be a sequence of some pipes.
So at this time we can broken this into multiple lines using \ character.
The default is '>'

If the command is not ended properly this will appear.

$PS3 this is the prompt for select inside a script

Example: if you are not setting this variable then your select script will appear like this

select i in mon tue wed exit
do
  case $i in
    mon) echo "Monday";;
    tue) echo "Tuesday";;
    wed) echo "Wednesday";;
    exit) exit;;
  esac
done

The output will look like this
1) mon
2) tue
3) wed
4) exit
#? 1
Monday
#? 4
[Note: This displays the default "#?" for select command prompt]
[/code]

If you are using the $PS3 then at this time this will be like this

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

At this time the output will be like this

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]

PS4 - Used by �set -x� to prefix tracing output
defines the prompt that gets displayed, when you execute a shell script in debug mode as shown below.

Shell script and output WITHOUT PS4:

cat ps4.sh

set -x
echo "PS4 demo script"
ls -l /etc/ | wc -l
du -sh ~

Output is as follows

++ echo 'PS4 demo script'
PS4 demo script
++ ls -l /etc/
++ wc -l
243
++ du -sh /home/thillaiselvan
123M     /home/thillaiselvan
[Note: This displays the default "++" while tracing the output using set -x]

If we are using the PS4:

cat ps4.sh

export PS4='$0.$LINENO+ '
set -x
echo "PS4 demo script"
ls -l /etc/ | wc -l
du -sh ~

Output of this script is as follows

../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/thillaiselvan
123M     /home/thillaiselvan
[Note: This displays the modified "{script-name}.{line-number}+"
       while tracing the output using set -x]
4

Hi Nila and Selven,

Thank you very much for your prompt repy. Nila I didn't find any url in your reply.

Regards
Ramesh

5

From man bash, similar in man ksh

6

Please check now

7

Hi Nila,

Now I have found the link in your reply.

Thank you very much.

Regards
Ramesh