Making __git_ps1 and virtualenvwrapper play nicely

I had to reinstall my ubuntu installation today after I completely broke it upgrading from 13.04 to 14.04. This meant that one of my first tasks was to reconfigure my shell how I liked it. Upon searching around, I found that the way I had been including the git branch status in my bash prompt had been superceded by the following new syntax:

export GIT_PS1_SHOWUNTRACKEDFILES=1
export GIT_PS1_SHOWUPSTREAM="auto"
export GIT_PS1_SHOWCOLORHINTS=1
export GIT_PS1_SHOWDIRTYSTATE=1

export PROMPT_COMMAND='__git_ps1'

which has a few nice updates to it! It will not only tell you which branch you are on, but it will also tell you the status of the branch AND colour it accordingly. Notice the use of PROMPT_COMMAND in order to allow the __git_ps1 command to properly set colours in bash. PROMPT_COMMAND is a special bash command that is executed before every prompt is updated. Hence the syntax used (excluding the $(...)).

Unfortunately, this messed up how virtualenvwrapper was displaying my active virtualenv, which was no use to me at all. After some tinkering, I found two pieces of important information:

1. __git_ps1 can take two parameters. The first a string to place before the branch information, and the second a string to place after.
2. PROMPT_COMMAND can take a string of commands to call, seperated by semicolons.

Therefore, we can make a nice prompt that is friendly to both our git status and our virtualenv status as follows:

export GIT_PS1_SHOWUNTRACKEDFILES=1
export GIT_PS1_SHOWUPSTREAM="auto"
export GIT_PS1_SHOWCOLORHINTS=1
export GIT_PS1_SHOWDIRTYSTATE=1
export VIRTUAL_ENV_DISABLE_PROMPT=1

set_active_venv() {
    export ACTIVE_VENV=""
    if [ "$VIRTUAL_ENV" != "" ]; then
        export ACTIVE_VENV="(`basename \"$VIRTUAL_ENV\"`)"
    fi
}

export PROMPT_COMMAND='set_active_venv; __git_ps1 "${ACTIVE_VENV}\w" "\\\$ "'

We have to set VIRTUAL_ENV_DISABLE_PROMPT=1 in order to prevent virtualenvwrapper from providing its normal behaviour. We then just set our own environment variable to provide the same functionality!