git-generate

#!/bin/bash

help="usage: git-generate

git-generate regenerates a commit from a script kept in the commit message.

Specifically, the topmost commit in the current git repo should have a script
preceded by [git-generate] on a line by itself. The script continues until the
end of the message or a Change-Id: line. The script starts execution in the
root of the git repo.

For example, a commit message might say:

	We are moving from Old to New.

	[git-generate]
	cd some/dir
	sed -i '' 's/Old/New/g' *

To regenerate the commit, git-generate resets the working file state to before
the commit and then runs the script. The script runs using 'bash -e', so any
single command failing will abort the generation.
"

if [ "$1" = "-help" -o "$1" = "-h" ]; then
	echo "$help"
	exit 0
fi

if [ $# != 0 ]; then
	echo >&2 'usage: git-generate'
	echo >&2 '       git-generate -help'
	exit 2
fi
if ! git log -n1 >/dev/null; then
	echo >&2 'git-generate: cannot find git repo'
	exit 2
fi

set -e
gitdir=$(git rev-parse --show-toplevel)
what=HEAD
if [ -e $gitdir/.git/rebase-merge/stopped-sha ]; then
	what=$(cat $gitdir/.git/rebase-merge/stopped-sha)
	short=$(echo $what | cut -c 1-8)
	echo >&2 "git-generate: using $short to resolve merge conflict"
fi
msg=$(mktemp /tmp/replayXXXXXX)
git log -n1 $what | sed -n 's/^    //p' | sed -n '/^ *\[git-generate\]$/,$p' | sed '1d; /Change-Id:/d' | grep -v Change-Id: >$msg
ls -l $msg
if ! [ -s $msg ]; then
	echo >&2 'git-generate: no script found in commit'
	exit 2
fi

now=$(git rev-parse HEAD)

# Reset files to HEAD^ but keep branch itself at HEAD.
# For a brief moment HEAD is pointing at HEAD^, which is unfortunate.
# It would be very nice if there were some single command to do this pair.
if [ what = HEAD ]; then
	git reset -q --hard 'HEAD^'
	git reset -q $now
else
	# Resolving merge conflict, HEAD is parent before changes,
	# which is exactly what we want.
	git reset -q --hard HEAD
fi

cd $(git rev-parse --show-toplevel)
bash -e $msg
rm -f $msg