Problem

Git provides an option git cherry-pick -x to record the commit message from which the cherry picked commit was formed when backporting changes. This commit message “(cherry picked from commit …)” is very generic and does not convey why cherry pick was done. Or we may want to make additional changes to the original commit message.

For this reason most projects will have a custom message e.g. “Backport-Of: {SHA1}” to be appended for backported changes to indicate from which commit they were cherry picked from. To add this custom message we need to open a text editor each time cherry-pick was performed with git cherry-pick -xe option and manually change the SHA1 hash line.

This is a repeated and exhaustive task if there are large number of commits to backport. For this we create a custom command to search and replace the SHA1 hash line with the custom message.

Solution

Search and replace with sed

Searching and replacing cherry picked from commit line can be done by sed stream editor and regular expression as below.

Lang: bash 
sed 's/(cherry picked.*\([0-9a-f]\{40\}\))/Backport-Of: \1/'

Explanation of above regular expression is simple first we match any line that starts with “(cherry picked” and then capture the string that is made of numericals and lowercase alphabets “([0-9a-f]{40})” and it is of 40 characters length this is SHA1 hash pattern. This is then available as parameter 1 in sed substitute command.

sed substitute command performs the search and replace of cherry picked from commit line with “Backport-Of: " commit message and replaces parameter 1 with SHA1 hash.

Git custom command to automate search and replace

Add above sed command to search and replace cherry picked from commit line to a bash script as below. Give it a sensible name such as gcpx_ed and make it executable and available in $PATH e.g. copy it into ~/.local/bin/.

Lang: bash 
$ bat -p `which gcpx_ed`
#!/bin/bash
#
# Replace SHA1 line in git cherry pick commit message

sed -i 's/(cherry picked.*\([0-9a-f]\{40\}\))/Backport-Of: \1/' $1

This acts as a simple text editor which opens a file given to it and performs text substitution in-place and closes it with SUCCESS or ERROR.

Use this as a git editor before executing git cherry-pick -xe {SHA1} so that custom message is automatically added.

Lang: bash 
GIT_EDITOR=gcpx_ed git cherry-pick -xe {SHA1}

To simplify this command create an alias as below and save it in ~/.bashrc or ~/.zshrc.

Lang: bash 
gcpx='GIT_EDITOR=gcpx_ed git cherry-pick -xe'

Now we have the git custom command gcpx to backport commits with custom message cherry picked from commit line. It can be invoked as gcpx {SHA1}.