A basic deploy script that uses two git repo clones and symlinks to deploy with zero downtime.

deploy.sh

#!/bin/bash

set -e # Fail on non-zero exit code
set -u # Fail on undefined variables

deploymentDir=$1
symlink=$2
dirA=$3
dirB=$4
branchName=$5

cd "$deploymentDir"
if [ "$dirA" = `readlink "$symlink"` ]; then
        currentDir=$dirA
        alternateDir=$dirB
else
        currentDir=$dirB
        alternateDir=$dirA
fi

cd "$currentDir"
liveCommit=`git rev-parse HEAD`

cd ../"$alternateDir"
git pull
latestCommit=`git rev-parse HEAD`

if [ "$latestCommit" == "$liveCommit" ]; then
	echo There are no new commits to deploy, exiting...
	exit
fi
echo New commits found, deploying them...

composer install
git tag "$branchName"-`date +%Y-%m-%d_%H-%M-%S`
git push --tags

cd ..
ln -s "$alternateDir"  "$symlink"-new
if [ `uname` = 'Darwin' ]; then
	# This is not atomic, but we don't care for now since we're not hosting on OSX.
	rm "$symlink"
	mv "$symlink"-new "$symlink"
else
	mv -T "$symlink"-new "$symlink" # This is atomic on Linux.
fi

test.sh

#!/bin/bash

set -e # Fail on non-zero exit code
set -u # Fail on undefined variables

testDir='deploy.sh-test-dir'

rm -rf $testDir
mkdir $testDir
cd $testDir

mkdir repo
mkdir deployment

cd repo
git init
echo '{}' > composer.json
echo 'commit 1' > test.txt
git add ./
git commit -m 'commit 1'

cd ../deployment
git clone ../repo repo-a
git clone ../repo repo-b
ln -s repo-a live

cd ../repo
echo 'commit 2' > test.txt
git add ./
git commit -m 'commit 1'

cd ../..
./deploy.sh $testDir/deployment live repo-a repo-b master

echo

if [ "`cat $testDir/deployment/live/test.txt`" != 'commit 2' ]; then
	echo 'Test Failed: The latest commit was not deployed!'
	exit
fi

if [ "`cat $testDir/deployment/repo-a/test.txt`" != 'commit 1' ]; then
	echo 'Test Failed: The previous repo version was not rotated as expected!'
	cat $testDir/deployment/repo-b/test.txt
	exit
fi

rm -rf $testDir

echo Tests completed successfully!