How can I so a "cascading" merge from an upstream branch and pass the merge to all downstream branches with one command?

Question

I got a "Parent" branch. Branch A is a branch of Parent, Branch B is a branch of Branch A and Branch C is a branch of Branch B:

------ Parent
  \----Branch A
     \------ Branch B
        \------- Branch C

I want to merge some changes into Branch A and then pass those same changes onto Branches B and C. Is there a way this can be done easily? Perhaps I can use the --contains mechanism which lists the contained branches and do this with bash? I googled for a solution like that but didn't find one.

Answer 1

OK, I've done it with a crude post-commit hook in git hub written in perl. My branch naming convention allows me to do this based on branch names. There are a couple of little tricks needed to get output from the branch --list, namely turning a paging setting off and back on in git.

#!/usr/bin/env perl

use strict;
use warnings;

# get the current branch name
my $curr_branch = `git branch --show`;
chomp $curr_branch;

# get the names of all the branches that have this branch as parent and merge commit into each of them
if ($curr_branch eq 'configs') {
    `git config --global pager.branch false`;
    my $out = `git branch --list --no-color --no-column`;
    `git config --global pager.branch true`;
    my @list = grep { /\scfg_/ } split /\n/, $out;
    foreach my $l ( @list ) {
        print "Merging to $l\n";
        `git checkout $l`;
        `git merge --no-edit configs`;
    }
}

`git checkout configs`;

I've symlinked the my unsued post-merge script to this post-commit script so it's also triggered on merges with another branch.