Utility method to checkout a given git branch or tag. Useful in deployment scripts. Requires git 1.8.x.

git_checkout.sh

#!/bin/bash

# Checkout the given branch or tag to the given directory.
# Clones the repo if a clone does not already exist.
#
# Arguments:
# $1 = Git repo URL e.g. #git@github.com:foo/bar.git" (Mandatory)
# $2 = Local directory e.g. /tmp/checkout" (Mandatory, must be absolute path)
# $3 = Branch or tag name e.g. "develop" or "0.1" (Mandatory)
function git_checkout {
    if [ $# -lt 3 ]; then
        echo "Error: Not enough arguments supplied."
        echo "Usage: git_checkout <git URL> <local dir> <branch or tag>"
        return 1
    fi

    local git_url=$1
    local checkout_dir=$2
    local branch_or_tag=$3

    # Save env vars so we can clean up after ourselves
    if [ -n "${GIT_WORK_TREE+set}" ]; then local save_git_work_tree=$GIT_WORK_TREE; fi
    if [ -n "${GIT_DIR+set}" ]; then local save_git_dir=$GIT_DIR; fi

    if [ -e $checkout_dir ]; then
        echo "Found existing clone of [$git_url] at [$checkout_dir]."
        echo

        export GIT_WORK_TREE=$checkout_dir
        export GIT_DIR=$GIT_WORK_TREE/.git

        echo "Fetching latest branches and tags from origin."
        git --no-pager fetch origin
        echo

        local previous_branch=$(git rev-parse --abbrev-ref HEAD)
        if [ "$previous_branch" != "HEAD" ]; then
            echo "Currently checked out branch: [$previous_branch]"
        else
            # We are probably in detached HEAD state,
            # most likely because we checked out a tag
            echo "Currently checked out tag: [$(git describe --always --tags)]"
        fi

        echo "HEAD:"
        echo "-----"
        git --no-pager log -1
        echo "-----"
        echo
        local previous_head=$(git rev-list --max-count=1 HEAD --)

        echo "Checking out [$branch_or_tag] ..."
        git --no-pager checkout -q $branch_or_tag
        echo

        echo "Pulling latest changes from origin..."
        git --no-pager pull origin $branch_or_tag
        echo

        echo "The following files have been updated."
        diff_cmd="git --no-pager diff --stat $previous_head HEAD"
        echo "-----"
        $diff_cmd
        echo "-----"
        echo

        echo "New HEAD:"
        echo "-----"
        git --no-pager log -1
        echo "-----"
        echo
    else
        echo "Cloning [$git_url] to [$checkout_dir]..."
        git --no-pager clone $git_url $checkout_dir
        echo

        # Don't set env vars until after clone, as it causes git to go crazy
        # and put the .git folder contents into the working tree
        export GIT_WORK_TREE=$checkout_dir
        export GIT_DIR=$GIT_WORK_TREE/.git

        echo "Checking out [$branch_or_tag] ..."
        git --no-pager checkout -q $branch_or_tag
        echo
    fi

    # Clean up env vars
    if [ -n "${save_git_work_tree+set}" ]; then export GIT_WORK_TREE=$save_git_work_tree; else unset GIT_WORK_TREE; fi
    if [ -n "${save_git_dir+set}" ]; then export GIT_DIR=$save_git_dir; else unset GIT_DIR; fi
    
    echo "Done!"
    return 0
}