Grabs the latest tag and sets the version number. You must only have version numbers as tags and they must be in format "X.y[.z]-B" where: X, y are major and minor versions; z (patch version) is optional; and B is the build number. Execute this with a post-build script phase similar to this: https://gist.github.com/andrewtheis/7127566

bump_and_tag.sh

#!/bin/sh

PLIST_PATH="${BUILT_PRODUCTS_DIR}/${INFOPLIST_PATH}"

# Get the current version and extract the version/build from there
LATEST_TAG=`xcrun git describe --tags --abbrev=0`
TAG_BUILD_NUM=${LATEST_TAG#*-}
TAG_VERSION=${LATEST_TAG%-*}
NEW_BUILD_NUM=$(expr $TAG_BUILD_NUM + 1)

# Set the new version
defaults write "${PLIST_PATH%.*}" 'CFBundleVersion' "${NEW_BUILD_NUM}" &> /dev/null
BUILD_NUM_SET=`defaults read "${PLIST_PATH%.*}" 'CFBundleVersion'`

echo "Set build number to ${BUILD_NUM_SET} in ${INFOPLIST_PATH}"

# Tag it!
xcrun git tag ${TAG_VERSION}-${BUILD_NUM_SET}
xcrun git push origin --tags

echo "Bumped and tagged."