How does __git_ps1 update the bash prompt?

Question

I'm a Windows user with the Git Bash shell as my daily-driver. I'm curious how the __git_ps1 function updates the prompt every time you change a directory. It's really the only example of updating the bash prompt on the fly that I've seen. I want to leverage this behavior in my own function to add a display on my prompt if I have an RDP session open.

tldr: Any ideas on how the __git_ps1 function evaluates the bash prompt on the fly????

So here is my simple function to see if the RDP client is running

function __rdp_ps1() {
  local MATCH=
  if tasklist | grep --quiet mstsc; then
    MATCH="\e[41mRDP\e[0m"
  fi
  echo "$MATCH"
}

So the idea is I want to display RDP with a red background, and I want my shell to evaluate this on the fly the same way __git__ps1 is seemingly able to.

What I've investigated (without real success) so far

/etc/profile.d/git-prompt.sh

This block seems to create the PS1 my shell is using

    PS1='\[\033]0;$TITLEPREFIX:$PWD\007\]' # set window title
    PS1="$PS1"'\n'                 # new line
    PS1="$PS1"'\[\033[32m\]'       # change to green
    PS1="$PS1"'\u@\h '             # user@host<space>
    PS1="$PS1"'\[\033[35m\]'       # change to purple
    PS1="$PS1"'$MSYSTEM '          # show MSYSTEM
    PS1="$PS1"'\[\033[33m\]'       # change to brownish yellow
    PS1="$PS1"'\w'                 # current working directory
    if test -z "$WINELOADERNOEXEC"
    then
        GIT_EXEC_PATH="$(git --exec-path 2>/dev/null)"
        COMPLETION_PATH="${GIT_EXEC_PATH%/libexec/git-core}"
        COMPLETION_PATH="${COMPLETION_PATH%/lib/git-core}"
        COMPLETION_PATH="$COMPLETION_PATH/share/git/completion"
        if test -f "$COMPLETION_PATH/git-prompt.sh"
        then
            . "$COMPLETION_PATH/git-completion.bash"
            . "$COMPLETION_PATH/git-prompt.sh"
            PS1="$PS1"'\[\033[36m\]'  # change color to cyan

# tried hamjamming PS1="$PS1 `__rdp_ps1`" here, it only works on login

            PS1="$PS1"'`__git_ps1`'   # bash function
        fi
    fi
    PS1="$PS1"'\[\033[0m\]'        # change color
    PS1="$PS1"'\n'                 # new line
    PS1="$PS1"'$ '                 # prompt: always $

So I went to see where this file was being sourced to see if that could lead to the answer

/etc/bash.bashrc

Last line held the gold

# Fixup git-bash in non login env
shopt -q login_shell || . /etc/profile.d/git-prompt.sh`

So I evaluated shopt login_shell and it's always on, but I don't really know what that means because the comment leads me to believe that when login env is off, the prompt script will be evaluated

Any ideas???

Answer 1

Your problem might be that you define your $PS1 with double quotes, which bash interprets when executing. Which means that __rdp_ps1 is ran when $PS1 is defined.

In your .bashrc, try replacing the definition with:

PS1='$PS1 `__rdp_ps1`' # Note the single quote. 

I have a similar feature on my PS1 (but to display the number of jobs in the background), here is the full version (available here: https://github.com/padawin/dotfiles/blob/master/.bashrc#L70):

function j(){
    jobs | wc -l | egrep -v ^0 | sed -r 's/^([0-9]+)/ (\1)/'
}

PROMPT_COMMAND=__prompt_command # Func to gen PS1 after CMDs

__prompt_command() {
    local EXIT="$?" # This needs to be first
    PS1="$(virtual_env_name)"

    local RCol='\[\e[0m\]'

    local Red='\e[0;31m'
    local Gre='\e[0;32m'
    local Blu='\e[1;34m'

    PS1+="${Gre}\u@\h$(j)${RCol}: ${Red}\w${Blu}$(__git_ps1)"
    if [ $EXIT != 0 ]; then
        PS1+="$Red \342\234\226 (${EXIT})"
    else
        PS1+="$Gre \342\234\224"
    fi
    PS1+="$RCol\n> "
}

Which can be simplified as the following in .bashrc:

function j(){
    jobs | wc -l | egrep -v ^0 | sed -r 's/^([0-9]+)/ (\1)/'
}

PS1='\u$(j) > ' # Note the single quote here

Which behaves as follow:

padawin > vim

[1]+  Stopped                 vim
padawin (1) > fg
vim
padawin > 

Answer 2

Not directly a solution but this might help: if you want to know where the __git_ps1 function lives locally on your filesystem so you can experiment with making edits, you can do

grep -r __git_ps1 /

which searches for the string '__git_ps1' across the file contents of git bash's entire filesystem (where / is actually C:\Program Files\Git or wherever you have it installed).

For me, it was at /mingw64/share/git/completion/git-prompt.sh

My use case was removing the parenthesis from the branch name, which I did by changing this line:

local printf_format=' (%s)'

in the __git_ps1 () function