Subject: Re: [PATCH 19/19] bash prompt: alternative git prompt without command substitution

From: Andrew Sayers <[email-masked]>
Date: Wed, 09 May 2012 20:38:22 +0100

On 09/05/12 01:44, SZEDER Gábor wrote:
> __git_ps1() prints the branch name, status indicators, etc. to stdout,
> therefore it has to be included in $PS1 through a command substitution
> to display that information in the prompt.  The configuration is
> straightforward, but it imposes the overhead of fork()ing a subshell
> for the command substitution.
> 
> However, bash has the $PROMPT_COMMAND shell variable, which "if set,
> the value is executed as a command prior to issuing each primary
> prompt" (quoted from bash man page).  Its value isn't executed in a
> subshell but in the context of the "main" shell, hence (non-local)
> variables set in invoked shell functions are available when expanding
> $PS1.  We can use this facility to avoid that command substitution for
> __git_ps1().
> 
> So split out the meat of __git_ps1() into the new
> __git_prompt_command() function, which stores the branch name & co.
> in the $__git_ps1_string variable.  This function, as its name
> suggests, should be included in $PROMPT_COMMAND, and $__git_ps1_string
> should in turn be included in $PS1 with a bit of a twist to put the
> parentheses around it:
> 
>    PROMPT_COMMAND=__git_prompt_command

Rather than overwrite any existing PROMPT_COMMAND, it would be better to
do something like:

PROMPT_COMMAND="__git_prompt_command; $PROMPT_COMMAND"

>    PS1='[\u@\h \W${__git_ps1_string:+ ($__git_ps1_string)}]\$ '
> 
> Turn __git_ps1() into a wrapper around __git_prompt_command() such
> that it's functionality remains unaltered, so already configured
> prompts won't break.
> 
> The whole series speeds up the bash prompt on Windows/MinGW
> immensely, in many cases brings it down to around 10ms on my
> machine while in powersave mode.  Here are some timing results in
> three common scenarios (repeated 10 times, because the after cases
> were too fast to measure a single execution accurately with 'time'):
> 
> In my home directory, i.e. not in a git repository, before:
> 
>     /c/Users/szeder
>     $ time for i in {0..9} ; do prompt=$(__git_ps1) ; done
> 
>     real    0m0.952s
>     user    0m0.214s
>     sys     0m0.444s
> 
>   After:
> 
>     /c/Users/szeder
>     $ time for i in {0..9} ; do __git_prompt_command ;
>            prompt=${__git_ps1_string:+ ($__git_ps1_string)} ; done
> 
>     real    0m0.718s
>     user    0m0.136s
>     sys     0m0.354s
> 
>   After, with discovery across filesystems enabled:
> 
>     /c/Users/szeder
>     $ time for i in {0..9} ; do __git_prompt_command ;
>            prompt=${__git_ps1_string:+ ($__git_ps1_string)} ; done
> 
>     real    0m0.078s
>     user    0m0.016s
>     sys     0m0.062s
> 
> At the top of a work tree, before:
> 
>     /c/Users/szeder/repo (master)
>     $ time for i in {0..9} ; do prompt=$(__git_ps1) ; done
> 
>     real    0m2.901s
>     user    0m0.391s
>     sys     0m1.468s
> 
>   After:
> 
>     /c/Users/szeder/repo (master)
>     $ time for i in {0..9} ; do __git_prompt_command ;
>            prompt=${__git_ps1_string:+ ($__git_ps1_string)} ; done
> 
>     real    0m0.094s
>     user    0m0.047s
>     sys     0m0.047s
> 
> In a subdirectory, stash indicator enabled, before:
> 
>     /c/Users/szeder/repo/subdir (master $)
>     $ time for i in {0..9} ; do prompt=$(__git_ps1) ; done
> 
>     real    0m4.118s
>     user    0m0.468s
>     sys     0m2.056s
> 
>   After:
> 
>     /c/Users/szeder/repo/subdir (master $)
>     $ time for i in {0..9} ; do __git_prompt_command ;
>            prompt=${__git_ps1_string:+ ($__git_ps1_string)} ; done
> 
>     real    0m0.858s
>     user    0m0.152s
>     sys     0m0.322s
> 
>   After, discovery across filesystems enabled:
> 
>     /c/Users/szeder/repo/subdir (master $)
>     $ time for i in {0..9} ; do __git_prompt_command ;
>            prompt=${__git_ps1_string:+ ($__git_ps1_string)} ; done
> 
>     real    0m0.109s
>     user    0m0.047s
>     sys     0m0.063s
> 
> Well, that's about 97% improvement.
> 
> The performance gain on Linux is smaller, the latter case goes down
> from 0.264s to 0.047, but since it was fast enough to begin with I
> won't lengthen this commit message with further timing results on
> Linux.
> 
> Signed-off-by: SZEDER Gábor <[email-masked]>
> ---
> 
> We had some discussions recently about putting user-facing functions into
> a separate "namespace".  This patch doesn't take that into account, but
> once a consensus is reached __git_prompt_command() should be put in that
> namespace.
> 
>  contrib/completion/git-completion.bash | 25 +++++++++++++++++++++----
>  1 file changed, 21 insertions(+), 4 deletions(-)
> 
> diff --git a/contrib/completion/git-completion.bash b/contrib/completion/git-completion.bash
> index 5ea19018..1c29f3d0 100755
> --- a/contrib/completion/git-completion.bash
> +++ b/contrib/completion/git-completion.bash
> @@ -29,6 +29,11 @@
>  #       are currently in a git repository.  The %s token will be
>  #       the name of the current branch.
>  #
> +#       Alternatively, to make the above Bash prompt a bit faster:
> +#               PROMPT_COMMAND=__git_prompt_command

As above, I'd recommend a simple documentation change:
PROMPT_COMMAND="__git_prompt_command; $PROMPT_COMMAND"
(to show people how to chain any other prompt commands they have)

> +#               PS1='[\u@\h \W${__git_ps1_string:+ ($__git_ps1_string)}]\$ '
> +#               GIT_DISCOVERY_ACROSS_FILESYSTEM=true
> +#
>  #       In addition, if you set GIT_PS1_SHOWDIRTYSTATE to a nonempty
>  #       value, unstaged (*) and staged (+) changes will be shown next
>  #       to the branch name.  You can configure this per-repository
> @@ -258,11 +263,12 @@ __git_ps1_show_upstream ()
>  }
>  
>  
> -# __git_ps1 accepts 0 or 1 arguments (i.e., format string)
> -# returns text to add to bash PS1 prompt (includes branch name)
> -__git_ps1 ()
> +# Stores the text to be added to the bash prompt (branch name, status
> +# indicators, etc.) in the $__git_ps1_string variable.
> +__git_prompt_command ()
>  {
>  	local __git_dir=""
> +	__git_ps1_string=""
>  	__gitdir >/dev/null
>  	if [ -z "$__git_dir" ]; then
>  		return
> @@ -365,7 +371,18 @@ __git_ps1 ()
>  	fi
>  
>  	local f="$w$i$s$u"
> -	printf -- "${1:- (%s)}" "$c${b##refs/heads/}${f:+ $f}$r$p"
> +	__git_ps1_string="$c${b##refs/heads/}${f:+ $f}$r$p"
> +}
> +
> +# __git_ps1 accepts 0 or 1 arguments (i.e., format string)
> +# returns text to add to bash PS1 prompt (includes branch name)
> +__git_ps1 ()
> +{
> +	local __git_ps1_string
> +	__git_prompt_command
> +	if [ -n "$__git_ps1_string" ]; then
> +		printf -- "${1:- (%s)}" "$__git_ps1_string"
> +	fi

How hard/appropriate would it be to export individual parts of the
prompt here?  Something like:

__git_ps1_string_dirtystate="$i"
__git_ps1_string_untrackedfiles="$u"

There have been requests in the past to let people individually
colourise different bits of the prompt, which this would make practical.

>  }
>  __gitcomp_1 ()