4.1. PROMPT_COMMAND

Bash provides an environment variable called PROMPT_COMMAND. The contents of this variable are executed as a regular Bash command just before Bash displays a prompt.

[21:55:01][giles at nikola:~] PS1="[\u at \h:\w]\$ "
[giles at nikola:~] PROMPT_COMMAND="date +%H%M"
2155
[giles at nikola:~] d
bin   mail
2156
[giles at nikola:~] 

What happened above was that I changed PS1 to no longer include the \t escape sequence (added in a previous section), so the time was no longer a part of the prompt. Then I used date +%H%M to display the time in a format I like better. But it appears on a different line than the prompt. Tidying this up using echo -n ... as shown below works with Bash 2.0+, but appears not to work with Bash 1.14.7: apparently the prompt is drawn in a different way, and the following method results in overlapping text.

2156
[giles at nikola:~] PROMPT_COMMAND="echo -n [$(date +%H%M)]"
[2156][giles at nikola:~]$
[2156][giles at nikola:~]$ d
bin   mail
[2157][giles at nikola:~]$ unset PROMPT_COMMAND
[giles at nikola:~]

echo -n ... controls the output of the date command and suppresses the trailing newline, allowing the prompt to appear all on one line. At the end, I used the unset command to remove the PROMPT_COMMAND environment variable.