Github Desktop Smudge / Clean Filter on Windows

Overview

• a filter has two scripts: clean and smudge
• filter scripts are defined in a user's .gitconfig file
• file(s) that are filtered are defined in the repo's .gitattributes file
• Github Desktop can execute sh scripts on windows

Let's say we want to replace {{BRANCH}} with the name of the current branch:

"branch": "{{BRANCH}}"

"branch": "master"

Here's a simple smudge script. I keep these in C:\workspace\git-filter-demo. Save them with unix line endings!

#!/bin/sh
export BRANCH=`git rev-parse --symbolic --abbrev-ref HEAD`
sed -e "s/{{BRANCH}}/$BRANCH/"

and the clean script (use unix line endings):

#!/bin/sh
sed -e 's/^\s*"branch"\s*:.*/\t"branch": "{{BRANCH}}"/'

Let's call the filter myFilter in [home]/.gitconfig

[filter "myFilter"]
    smudge = \"C:/workspace/git-filter-demo/smudge\"
    clean = \"C:/workspace/git-filter-demo/clean\"

To test it, checkout https://github.com/vinnyjames/git-filter-demo.git

The above repo will run myFilter on myFile via [repo]/.gitattributes

myFile.json filter=myFilter

One problem: switching branches does not trigger a clean and smudge. I.e., your {{BRANCH}} text will not change. This can be worked around with the following post-checkout script that will perform a clean and smudge on the file. This will not destory any changes in your file (assuming the clean and smudge scripts are written correctly).

#!/bin/sh

# see https://www.tygertec.com/git-hooks-practical-uses-windows/
# .../bin/sh; C:/Users/first.last/AppData/Local/GitHubDesktop/app-2.7.2/resources/app/git/usr/bin/sh.exe

# export LOGFILE=post-checkout.log
# date > $LOGFILE
# echo --- 1:$1 2:$2 3:$3 >> $LOGFILE

# FAIRLY DANGEROUS, ONLY RUN WHEN PARAM 3 = "1", I.E. A BRANCH CHANGE
# OTHEWISE THIS SCRIPT CAN BE CALLED INFINITELY
# see https://git-scm.com/docs/githooks#_post_checkout
if [ "$3" = "1" ]; then
	export SCRIPT_DIR=/c/workspace/git-filter-demo
	export CLEAN_SCRIPT=$SCRIPT_DIR/clean
	export SMUDGE_SCRIPT=$SCRIPT_DIR/smudge
	export FILE=myFile.json
	export TMPFILE=_tmp_post-checkout

	# export BRANCH=`git rev-parse --symbolic --abbrev-ref HEAD`
	# echo BRANCH:$BRANCH TMPFILE:${TMPFILE} >> $LOGFILE

	# clean; this script must use #!/bin/sh on windows
	$CLEAN_SCRIPT < ${FILE} > ${TMPFILE}
	mv ${TMPFILE} ${FILE}
	# echo cleaned >> $LOGFILE

	# smudge; this script must use #!/bin/sh on windows
	$SMUDGE_SCRIPT < ${FILE} > ${TMPFILE}
	mv ${TMPFILE} ${FILE}
	# echo smudged >> $LOGFILE

	# 2021-04-23 'git status' was saying the file was modified even though it was binary the same
	# adding it seems to clear this invalid state without losing any local changes
	git add ${FILE}
# else
# 	echo NOT_BRANCH >> $LOGFILE
fi

#echo done >> $LOGFILE