Git: Rebase a (sub)branch from one base to another, leaving the other base's commits.

git-moveBranch.sh

#!/bin/bash

## Places a branch to a new base.
## Useful when splitting a long branch to multiple pull requests.
##
##   ---+--------master
##       \
##         --- A ---- B
##
## git-moveBranch.sh B from A to master
##
##   ---+-------- master ---- B
##       \
##         --- A

## Also, this can be used to split long branches into smaller pull requests.
## Imagine you have a long branch X
##
##      main ---------------X
##
## which you split to smaller MRs:
##
## MR #1 with branch X1.
## MR #2 with branch X2.
##
##      main --------X1-----X2
##
## It is very important to mention in MR #2 that it depends on MR #1. 
## Gitlab allows to even prevent MR #2 from being merged before MR #1.
##
## Then, when MR 1 / X1 gets merged,  you may run:
##
##     $ git co main && git pull
##
##     $ git-moveBranch.sh X2 from X1 to main
##
## That will move all commits of X2, and none from X1, onto main, which already includes X1.
##
##     mainOld---------main-----X2
##

function printUsageAndExit() {
    echo "Usage:";
    echo  "    `basename $0` <branch> from <previous-base> to <new-base>";
    echo
    git branch;
    exit 1;
}

if [ 5 != $# ] ; then printUsageAndExit; fi
if [ "$2" != "from" ] ; then printUsageAndExit; fi
if [ "$4" != "to" ] ; then printUsageAndExit; fi

WHAT="$1"
FROM="$3"
ONTO="$5"

echo "Running:   git rebase —-onto=\"$ONTO\" \"$FROM\" \"$WHAT\""
# git rebase —-onto <place-to-put-it> <last-change-that-should-NOT-move> <branch to move>
git rebase --onto "$ONTO" "$FROM" "$WHAT"