git-branch-alias

#!/usr/bin/env bash
# git branch-alias
# Version 1.09-rc1
# Author: Phil S. (C) 2014
# http://stackoverflow.com/questions/549920/is-it-possible-to-alias-a-branch-in-git
# http://www.mail-archive.com/git%40vger.kernel.org/msg49171.html

# Creates branch aliases, so that you can refer to a long branch name
# by a convenient short alias. This is just a "do what I mean" wrapper
# around git symbolic-ref, but without the (considerable) risk of
# trashing a branch if you get your arguments wrong

# Examples:
# git branch-alias short some-overly-long-branch-name # creates alias
# git branch-alias short # creates alias for current branch
# git log short
# git checkout short
# git push origin short # pushes the branch, not the alias/reference
# git branch-alias --delete short

# Caveats:
# Although everything else I've tried works seamlessly, I note that
# git merge <alias> will cause the alias name to be mentioned in the
# commit message, rather than the real branch. It would be nicer if
# the branch name appeared.

# Compatibility:
# Originally developed with git version 1.7.12.4
# Tested with git versions 1.9.0, 2.54, 2.80
#
# Related git changes between versions 1.7.12.4 and 1.9.0:
#
# 1.8.0.1
#  * A symbolic ref refs/heads/SYM was not correctly removed with "git
#    branch -d SYM"; the command removed the ref pointed by SYM
#    instead.
#
# 1.8.1
#  * "git symbolic-ref" learned the "-d $symref" option to delete the
#    named symbolic ref, which is more intuitive way to spell it than
#    "update-ref -d --no-deref $symref".

# Change Log:
# v1.09:
# POSIX-compatible option handling and output.
#
# v1.08:
# Removed test git show-ref --verify --heads --quiet "refs/heads/${symref}"
# for asserting that the specified reference was valid before deleting a
# reference, as we need to permit the deletion of references to branches
# which have /already/ been deleted, and this test prevented that.
# n.b. We already had another validation test to fall back on, using
# git symbolic-ref "refs/heads/${symref}"
#
# v1.07:
# Minor tweaks. Posted as feature-request to git mailing list:
# http://www.mail-archive.com/git%40vger.kernel.org/msg49171.html
# Also appears at the following gmane.org URL, but there the code is broken
# by an email obfuscation filter automatically converting the symbol '@'
# to the string ' <at> ' (specifically, the shell positional parameter
# expansion "$@" is changed to "$ <at>"), so don't try to use this copy:
# http://permalink.gmane.org/gmane.comp.version-control.git/247581

git_branch_alias() {
#cwd=$(git rev-parse --show-toplevel)
git=$(git rev-parse --git-dir)
if [ $? -ne 0 ]; then
    return 1
fi

command=$(basename $0)
command="git ${command##git-}"

# Print argument (and newline) to stdout or stderr
stdout () {
    printf %s\\n "$1"
}
stderr () {
    printf %s\\n "$1" >&2
}

# POSIX compatible argument quoting and parameter save/restore
# http://www.etalabs.net/sh_tricks.html
save () {
    local param
    for param; do
        printf %s\\n "$param" \
            | sed "s/'/'\\\\''/g;1s/^/'/;\$s/\$/' \\\\/"
    done
    printf %s\\n " "
}
# parameters=$(save "$@")
# eval "set -- ${parameters}" # to restore the original parameters.

# Process option parameters
parameters=
while [ $# -gt 0 ]; do
    case "$1" in
        ( --          ) shift; break;;
        ( -d|--delete ) delete=1; shift;;
        ( -h|--help   ) help=1; shift;;
        ( -*          ) {
            stdout "Invalid option: $1"
            shorthelp=1
            shift
        };;
        ( * ) { # non-option parameter
            parameters="${parameters}$(save "$1")"
            shift
        };;
    esac
done

# Process non-option parameters
eval "set -- ${parameters}"
symref=$1
branch=$2

if [ -z "${symref}" ]; then
    help=1
fi

# n.b. Calling "git branch-alias --help" causes git to look for
# a man page for "git-branch-alias", so we shouldn't advertise
# the long option (although we support it if the script is called
# by its real name, rather than via git).
if [ -n "${shorthelp}" ]; then
    cat <<EOF

For help, use: ${command} -h

EOF
    return 0
fi

if [ -n "${help}" ]; then
    cat <<EOF
Usage:
${command} <alias> [<branch>]
${command} (-d | --delete) <alias>

Creates a symbolic reference <alias> referring to <branch>.
<branch> defaults to the current checked-out branch.

This symbolic reference acts as an alias for <branch>, and can be
used in its place. More specifically, it WILL be dereferenced to
its target in nearly all situations, so for any given command you
should treat every usage of <alias> as if it were actually <branch>.

To safely delete a branch alias, always use:
${command} -d <alias>

WARNING: These symbolic references appear in your branch list as:
 <alias> -> <branch>
and so you might be tempted to try to delete them like a branch:
 git branch -d <alias>

However this can cause problems. In git versions prior to 1.8.0.1
<alias> will be dereferenced and you will instead delete the
branch it refers to (git will allow this even if you currently
have that branch checked out), and the symbolic reference will
still remain (referencing a branch which is no longer available).

In later versions of git the <alias> will be deleted rather than
the branch; however git will still not check to see whether you
currently have <alias> checked out, and will not prevent you
from deleting it in that situation. This will leave your HEAD ref
in an invalid state. Using ${command} -d <alias> resolves
this situation by first switching HEAD to <alias>'s target branch
if HEAD was currently set to <alias>.

EOF
    return 0
fi

# Use the current branch by default.
if [ -z "${branch}" ]; then
    branch=$(git symbolic-ref -q HEAD)
    if [ $? -ne 0 ]; then
        stderr "Could not establish current HEAD."
        return 1
    fi
fi

# We expect plain branch names, but also accept the fully-qualified
# (refs/heads/NAME) paths needed by git symbolic-ref; so strip that
# refs/heads/ prefix if it is specified.
branch=${branch##refs/heads/}
symref=${symref##refs/heads/}

# Deleting a symref.
if [ -n "${delete}" ]; then
    if [ ! -f "${git}/refs/heads/${symref}" ]; then
        stderr "Symbolic reference refs/heads/${symref} does not exist."
        return 1
    fi

    # Verify that it IS a symbolic reference
    if ! git symbolic-ref "refs/heads/${symref}" >/dev/null; then
        stderr "Error validating refs/heads/${symref} as symbolic reference."
        return 1
    fi

    # If we currently have <symref> checked out, deleting it is bad
    # (as HEAD would no longer be a valid reference). I believe we do
    # need to inspect the file here, as attempting to read the HEAD
    # reference via git dereferences it to its target branch, and thus
    # we are unable to distinguish between the branch and the symref.
    if grep -q "^ref: refs/heads/${symref}\$" "${git}/HEAD"; then
        stdout "Cannot delete the currently checked out symbolic reference."
        branch=$(git symbolic-ref -q HEAD)
        if [ $? -ne 0 ]; then
            stderr "Could not establish current HEAD."
            return 1
        fi
        stdout "Switching HEAD to target branch ${branch}"
        # By using git symbolic-ref HEAD to find the target ref
        # and setting HEAD to that target, nothing really changes,
        # but we can now delete the reference safely.
        if ! git symbolic-ref HEAD "${branch}"; then
            stderr "Error updating HEAD from ${symref} to ${branch}"
            stderr "Aborting."
            return 1
        fi
    fi

    # Delete the reference.
    # git 1.8.1+ provides: git symbolic-ref --delete <symref>
    # but older versions do not include that option, so we use
    # the backwards-compatible command.
    stdout "Deleting symbolic reference refs/heads/${symref}"
    git update-ref -d --no-deref "refs/heads/${symref}"
    return $?
fi

# Creating a new symbolic reference.

# Error checking. git symbolic-ref doesn't really do any, and will
# happily mess up your branches; particularly if you get the arguments
# the wrong way around (treating it like ln -s is a really bad idea).
if [ ! -f "${git}/refs/heads/${branch}" ]; then
    stderr "Target refs/heads/${branch} does not exist."
    return 1
fi
if [ -f "${git}/refs/heads/${symref}" ]; then
    target=$(git symbolic-ref "refs/heads/${symref}")
    if [ $? -eq 0 ]; then
        stderr "Symbolic reference refs/heads/${symref} already exists:"
        stderr "  ${symref} -> ${target##refs/heads/}"
        stderr "To remove it, use: ${command} --delete ${symref}"
    else
        stderr "File refs/heads/${symref} already exists"
        stderr "(and is not a symbolic reference!)"
    fi
    return 1
fi
if git show-ref --verify --heads --quiet "refs/heads/${symref}"; then
    # n.b. I'm pretty sure this is unreachable, given the previous block.
    stderr "refs/heads/${symref} is a valid reference without a file!?"
    return 1
fi

# The parameters are good.
# Generate the reference and display the confirmed result.
if git symbolic-ref "refs/heads/${symref}" "refs/heads/${branch}"; then
    target=$(git symbolic-ref "refs/heads/${symref}")
    stdout "  ${symref} -> ${target##refs/heads/}"
else
    stderr "Failed to create branch alias."
    return 1
fi
}

git_branch_alias "$@"