Create a git repo whose last commit has four parents

four-parents.sh

#!/bin/sh

GIT=$(which git)
TARGET_DIR="four-parents"

if [[ -d $TARGET_DIR ]]; then
    echo $TARGET_DIR already exists.
    exit 1
fi

$GIT init $TARGET_DIR && cd $TARGET_DIR

MSG_MASTER_1="1 on master"
echo "$MSG_MASTER_1" > master.txt
$GIT add -A && $GIT commit -m "$MSG_MASTER_1"

for BRANCH in alpha beta gamma
do
    MSG="1 on $BRANCH"
    $GIT checkout -b $BRANCH
    echo "$MSG" > $BRANCH.txt
    $GIT add -A && $GIT commit -m "$MSG"
    $GIT checkout master
done

MSG_MASTER_2="2 on master"
echo "$MSG_MASTER_2" >> master.txt
$GIT commit -a -m "$MSG_MASTER_2"

$GIT merge -m "Merge branches 'alpha', 'beta', and 'gamma'" \
alpha beta gamma

$GIT log --oneline --graph --decorate

$GIT cat-file -p HEAD

cd ..