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Previous: 7.12 External Commands Send Signals to Set Variables Chapter 7
Setting Your Shell Prompt
Next: II. Let the Computer Do the Dirty Work

7.13 Pre-Prompt Commands in bash

bash can run a UNIX command, or multiple commands, before it prints every prompt. This command does not have to set the prompt; it just happens to be run before each prompt is printed. The command could do some system checking, reset shell variables, or almost anything that you could type at a shell prompt. Store the command(s) in the PROMPT_COMMAND shell variable. If the commands run slowly, though, they'll delay your prompt.

Here's a silly example that I used to have in my bash setup file ( 2.2 ) :



shift $#

# Save old $IFS; set IFS to tab:
OIFS="$IFS"; IFS="   "
# Put x in $1, face in $2, explanation[s] in $3[, $4, ...]:
set x `smiley`
# Put face into $face and explanation(s) into $explan:
face="$2"; shift 2; explan="$*"
# Restore shell environment:
shift $#; IFS="$OIFS"'

# Prompt I use (includes the latest $face):
PS1='\u@\h $face '

The first part is a series of shell commands that are stored in the PROMPT_COMMAND variable; they're surrounded by a pair of single quotes ( ' ' ), one on the first line (after the = ) and the other after IFS is reset. That series of commands is executed before every prompt. It sets two shell variables, $face and $explan , with new values before each prompt is set. The prompt is set on the last line; it includes the value of $face .

Here's what my screen looked like with this ridiculous setup. Notice that the prompt keeps changing as the PROMPT_COMMAND resets $face and $explan . If I wanted the explanation of a face I saw as I went along, I could type echo <">$explan<"> :

jerry@ruby :-{) 

echo "$explan"

normal smiling face with a moustache
jerry@ruby +<||-) 

vi proj.cc

jerry@ruby :-O 

echo "$explan"

Mr. Bill
        ohh, big mouth, Mick Jagger
        uh oh
jerry@ruby :-)   < 

g++ -Wall proj.cc


(It was even more useless than psychoanalyze-pinhead ( 32.13 ) , but it was fun while it lasted.) Seriously now, I'll say again: PROMPT_COMMAND does not have to be used to set a prompt. You can use it to run any commands. If the commands in PROMPT_COMMAND write to standard output or standard error, you'll see that text before the prompt.

- JP

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