git-set-id

#!/usr/bin/env bash
#
# Set git config user.{name,email}:
#
# $ git set-id [--global] [<name> <email>]
#
# If <name>/<email> are omitted, they will be inferred from the HEAD commit of the current Git repository:
#
# $ git set-id [--global]

usage() {
  echo "Usage: $0 [-g] [-c]" >&2
  echo "Usage: $0 [-g] [-c] <email | ref>" >&2
  echo "Usage: $0 [-g] [-c] <name> <email>" >&2
  exit 1
}

name_fmt=an
email_fmt=ae
global=()
while getopts "gc" opt; do
  case "$opt" in
    c) name_fmt=cn; email_fmt=ce ;;
    g) global=("--global") ;;
    \?) exit 1 ;;
  esac
done
shift $((OPTIND - 1))

name() {
  git log -n1 "--format=%$name_fmt" "$@"
}

email() {
  git log -n1 "--format=%$email_fmt" "$@"
}

if [ $# -eq 0 ]; then
  name="$(name)"
  email="$(email)"
elif [ $# -eq 1 ]; then
  arg="$1"; shift
  if [[ "$arg" =~ ^[A-Za-z0-9._%+-]+@[A-Za-z0-9.-]+\.[A-Za-z]{2,}$ ]]; then
    email="$arg"
    name=
  else
    name="$(name "$arg")"
    email="$(email "$arg")"
  fi
elif [ $# -eq 2 ]; then
  name="$1"; shift
  email="$1"; shift
else
  usage
fi

run() {
  echo "Running: $*" >&2
  "$@"
}

if [ -z "$name" ]; then
  run git config "${global[@]}" user.email "$email"
else
  run git config "${global[@]}" user.name "$name"
  run git config "${global[@]}" user.email "$email"
fi