How does __git_ps1 update the bash prompt? - bash

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

Answer 3

What you are looking for is PROMPT_COMMAND. Bash will execute whatever is in there before displaying a prompt. If your PS1 is being updated on-the-fly, you probably already have a PROMPT_COMMAND.