git-ff

#!/bin/bash

#   Simple fast-forward without working tree

#   Advances the branch ref if it is and ancestror of target.
#   No need of a working tree as it does not try to merge but uses git
#   merge-base.
#
#   use an alias in .gitconfig:
#     ff   = !git-ff "$@"
#
#   author: piotr.lesnicki@ext.bull.net

set -euo pipefail

git_is_ancestor() {
    ## git merge-base has no `--is-ancestor` option in v1.7
    local a="$1"
    local b="$2"
    if git merge-base --is-ancestor 2>/dev/null ; then
        git merge-base --is-ancestor "$a" "$b"
    else
        local common=$(git merge-base "$a" "$b")
        [[ "$common" == $(git rev-parse "$a") ]]
    fi
}

git_merge_status() {
    ## pre: ref and upstream are correct refs
    local ref="$1"
    local upstream="$2"
    if [[ $(git rev-parse "$ref") == $(git rev-parse "$upstream") ]]; then
        echo "up-to-date"
    elif git_is_ancestor "$ref" "$upstream"; then
        n=$(git rev-list "${ref}..${upstream}" | wc -l)
        echo "behind $n"
    elif git_is_ancestor "$upstream" "$ref"; then
        n=$(git rev-list "${upstream}..${ref}" | wc -l)
        echo "ahead $n"
    else
        echo "merge/rebase required"
    fi
}


gitff() {
    local branch="$1"

    if [[ "$branch" == "" ]]; then
	branch=$(git rev-parse --abbrev-ref HEAD)
    fi

    if ! git show-ref --heads -q "$branch"; then
        echo "error: branch '$branch' is not a local head reference" >&2
        return 1
    fi
    branch=$(git show-ref --heads "$branch" | cut -d' ' -f2)

    if ! branch_sha=$(git rev-parse --verify -q "$branch"); then
        echo "error: branch '$branch' invalid" >&2
        return 1
    fi

    if ! upstream=$(git rev-parse --symbolic-full-name \
            --verify -q "${branch#refs/heads/}@{upstream}" 2>/dev/null); then
        echo "error: branch '$branch' is not tracking anything" >&2
        return 1
    fi

    local status=$(git_merge_status "$branch" "$upstream")
    printf "%-20s %-25s : %-s\n" "${branch#refs/}" "[${upstream#refs/}]" "$status"
    case "$status" in
        behind*)
            printf "  fast-forwarding %s..%s\n"  \
                $(git rev-parse --short "$branch") \
                $(git rev-parse --short "$upstream")
            ;;
        ahead*) 
            local remote=$(echo "${upstream#refs/remotes/}" | cut -d'/' -f 1)
            local upref=$(echo "${upstream#refs/remotes/}" | cut -d'/' -f 2-)
            echo "  you can push: git push $remote ${branch#refs/}:${upref}"
            return 0
            ;;
        up-to-date) return 0;;
        merge*) echo "  merge/rebase required"; return 0
            ;;
        *) echo "bad status" >&2; return 1
            ;;
    esac


    ## here we update the branch
    if ! git update-ref "$branch" "$upstream"; then
        echo "error: failed to update ref '$branch' to '$upstream'" >&2
        return 1
    fi
}

gitff_all() {
    git for-each-ref --shell --format='ref=%(refname); up=%(upstream);' \
        refs/heads/ | while read entry; do
        eval "$entry"
        if [[ -n "$up" ]]; then
            gitff "$ref" || true
        fi
    done
}



gitff_all_status() {
    git for-each-ref --shell --format='ref=%(refname); up=%(upstream);' \
        refs/heads/ | while read entry; do
        eval "$entry"
        if [[ -n "$up" ]]; then
            local status=$(git_merge_status "$ref" "$up")
            printf "%-20s %-25s : %-s\n" \
                "${ref#refs/}" "[${up#refs/}]" "$status"
        fi
    done
}


if [[ "$BASH_SOURCE" == "$0" ]]; then
    arg="${1:-}"
    if [[ "$arg" == "-h" ]] || [[ "$arg" == "--help" ]]; then
        echo "usage: $0 <branch> || --all || --status" >&2
        echo "  fast-forward branch <branch>" >&2
        exit 1
    fi

    case "$arg" in
        "-s" | "--status") gitff_all_status;;
        "-a" | "--all")    gitff_all;;
        -*)
            echo "error: unknown option '$arg'" >&2
            exit 1;;
        *)                 gitff "$arg";;
    esac
fi