git-unpull: undoes the merge commit created by an accidental 'git pull', plus some added helpful information!

git-unpull

#!/bin/bash
# Reverts HEAD back to ORIG_HEAD, for example after a 'git pull' accidentally
# creates a merge. This is identical to running 'git reset --hard ORIG_HEAD',
# except that unpull prints some helpful information along the way.
# Useage:
#     $ git unpull
#          HEAD: a0ac0fd Merge branch 'master' of /tmp/foo
#                2284c9d some remote commit
#     ORIG_HEAD: 35431fd my local commit
#     Really reset HEAD to ORIG_HEAD? (y/n) y
#     + git reset --hard ORIG_HEAD
#     HEAD is now at 35431fd bar
# Installation:
#     copy git-unpull to ~/bin/ or /usr/local/bin/, or wherever you prefer to
#     keep such things.

IFS="`printf "\n\t"`"
set -eu

if [[ "$*" ]]; then
    echo "usage: git unpull"
    echo
    echo "Reverts HEAD back to ORIG_HEAD, for example after a 'git pull'"
    echo "accidentally creates a merge. This is identical to running"
    echo "'git reset --hard ORIG_HEAD', except that unpull prints some"
    echo "helpful information along the way."
    echo
    echo "Note that unpull can be run repeatedly; subsequent unpulls will"
    echo "undo the previous unpull."
    echo
    echo "Hint: after an unpull, you can use 'git rebase --onto origin/master'"
    echo "to rebase (as if you had run 'git pull --rebase')."
    exit 1
fi

git log --format="%h %s" ORIG_HEAD..HEAD | {
    num_changes=0
    while read line; do
        let "num_changes += 1"
        if [[ "$num_changes" -eq 1 ]]; then
            echo "     HEAD: $line"
        else
            echo "           $line"
        fi
    done
    head="$(git log --format="%h %s" -n 1 HEAD)"
    orig_head="$(git log --format="%h %s" -n 1 ORIG_HEAD)"
    if [[ "$num_changes" -eq 0 ]]; then
        echo "     HEAD: $head"
    fi
    if [[ "$head" == "$orig_head" ]]; then
        echo "ORIG_HEAD: $orig_head"
        echo "HEAD and ORIG_HEAD are the same (nothing to do)"
        exit 42
    fi
    if [[ "$num_changes" -eq 0 ]]; then
        echo "           (no common commits between HEAD and ORIG_HEAD)"
    fi
    echo "ORIG_HEAD: $orig_head"

    if [[ "$num_changes" -gt 2 ]]; then
        echo "warning: $num_changes commits between HEAD and ORIG_HEAD!"
    fi
}
subst="$?"
if [[ "$subst" -eq 42 ]]; then
    exit 0
fi
if [[ "$subst" -ne 0 ]]; then
    echo "$subst"
fi

read -p "Really reset HEAD to ORIG_HEAD? (y/n) " confirm
if [[ "$confirm" == "y" || "$confirm" == "yes" ]]; then
    set -x
    git reset --hard ORIG_HEAD
fi

or... git reset --hard HEAD@{1}