Blazing fast simple git branch name for ps1

__fastgit_ps1.sh

# Copyright (c) 2019 Will Bender. All rights reserved.

# This work is licensed under the terms of the MIT license.  
# For a copy, see <https://opensource.org/licenses/MIT>.

# Very fast __git_ps1 implementation 
# Inspired by https://gist.github.com/wolever/6525437
# Mainly this is useful for Windows users stuck on msys, cygwin, or slower wsl 1.0 because git/fs operations are just slower
# Caching can be added by using export but PROMPT_COMMAND is necessary since $() is a subshell and cannot modify parent state.
# Linux: time __ps1_ps1 (~7ms)
# Windows msys2: time __git_ps1 (~100ms)
# Windows msys2: time git rev-parse --abbrev-ref HEAD 2> /dev/null (~86ms)
# Windows msys2: time __fastgit_ps1 (~1-3ms)

# Simple PS1 without colors using format arg. Feel free to use PROMPT_COMMAND
export PS1="\u@\h \w \$(__fastgit_ps1 '[%s] ')$ "

# 100% pure Bash (no forking) function to determine the name of the current git branch
function __fastgit_ps1 () {
    local headfile head branch
    local dir="$PWD"

    while [ -n "$dir" ]; do
        if [ -e "$dir/.git/HEAD" ]; then
            headfile="$dir/.git/HEAD"
            break
        fi
        dir="${dir%/*}"
    done

    if [ -e "$headfile" ]; then
        read -r head < "$headfile" || return
        case "$head" in
            ref:*) branch="${head##*/}" ;;
            "") branch="" ;;
            *) branch="${head:0:7}" ;;  #Detached head. You can change the format for this too. 
        esac
    fi

    if [ -z "$branch" ]; then
        return 0
    fi

    if [ -z "$1" ]; then
        # Default format
        printf "(%s) " "$branch"
    else
        # Use passed format string
        printf "$1" "$branch"
    fi
}