How to customize the bash prompt in a way that doesn't squash venv

Question

I was a long time tcsh user and I have used a custom prompt, which includes the date/time, host, and a largish path. In the past, when I've used venv, I would switch to bash first, just because everything's geared toward bash, and I liked that the current environment would be added to the prompt, and I had left the bash prompt uncustomized. However, since I've been using bash more and more, I decided to migrate a version of my custom tcsh prompt. To do that, I added this to my bash profile:

changeprompt() { DIR=`dirs | perl -e '$p=scalar(<STDIN>);$p=~s/.*(?=\S{40}\Z)/.../;print($p)'`;export PS1="\h[\D{%F %T}]:$DIR\$ "; }
PROMPT_COMMAND="changeprompt"

It leaves me with a prompt like this:

gen-rl-macbookair[2021-02-12 14:01:24]:~/PROJECT-local/TRACEBASE$

I wanted a longer path in the prompt that is manually truncated with an ellipsis at a given length based on characters instead of directory depth, which is why I set PROMPT_COMMAND.

However, I realized yesterday when I started working on a venv for the first time in a long time, that activating a venv would not change the prompt to prepend (environment_name) to the prompt, since I had set PROMPT_COMMAND.

I fixed the issue by editing the activate script to unset COMMAND_PROMPT upon activate and reset COMMAND_PROMPT upon deactivate...

But I'm wondering if there exists a better way to maintain a custom command prompt like this that allows for a variably prepended string that a venv activate script adds - without needing custom edits to any activate script.

Answer 1

As a (bad) workaround, you can define bash functions that set and unset prefix for PS1, and call these when you activate/deactivate your venv instead of source activate and deactivate.

In your bashrc:

export PROMPT_PREFIX=
set_prompt_prefix(){
    PS1=$PROMPT_PREFIX$PS1
}

PROMPT_COMMAND='changeprompt && set_prompt_prefix'

activate_venv(){
    source activate && PROMPT_PREFIX="(venv) " && alias 'deactivate'='deactivate_venv'
}

deactivate_venv(){
    deactivate && PROMPT_PREFIX=
}

Then you have to call activate_venv instead of source activate. deactivate venv base function is aliased with my deactivate_venv so that prefix is always removed.

Beware that the prefix will always be (venv) . An even smarter solution could retrieve the venv name or directory and set it as prefix.