emacs-diff

#!/usr/bin/env bash
#
# Requirements:
#   - emacsclient
#
# Author: James Cherti
# URL: https://github.com/jamescherti/bash-stdops
#
# Description:
# ------------
# Compare files in Emacs using emacsclient and ediff (by default).
#
# Usage:
#   emacs-diff [options] <file1> <file2>
#
# Options:
#   -v, --verbose         Print the emacsclient commands being executed
#   -t, --tool NAME       Select the diff tool: "ediff" (default) or "vdiff"
#
# Example:
#   emacs-diff file1.txt file2.txt
#
# (This will compare file1.txt with file2.txt)
#
# Notes:
# - Requires Emacs to be running as a server via the (server-start) function or
#   as an Emacs daemon.
# - Paths are automatically converted to absolute paths before comparison.
#
# License:
# --------
# Copyright (C) 2012-2026 James Cherti
#
# Distributed under terms of the GNU General Public License version 3.
#
# This program is free software: you can redistribute it and/or modify
# it under the terms of the GNU General Public License as published by
# the Free Software Foundation, either version 3 of the License, or
# (at your option) any later version.
#
# This program is distributed in the hope that it will be useful,
# but WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
# GNU General Public License for more details.
#
# You should have received a copy of the GNU General Public License
# along with this program.  If not, see <https://www.gnu.org/licenses/>.
#

set -euo pipefail

VERBOSE=0
DIFF_TOOL="ediff" # ediff or vdiff
FILES=()
NO_OPTIONS=0
while [[ $# -gt 0 ]]; do
  if [[ $NO_OPTIONS -eq 0 ]]; then
    case "$1" in
    --)
      NO_OPTIONS=1
      shift
      continue
      ;;
    -v | --verbose)
      VERBOSE=1
      shift
      continue
      ;;
    -t | --tool)
      if [[ $# -lt 2 ]]; then
        echo "Error: --tool requires an argument" >&2
        exit 1
      fi
      DIFF_TOOL="$2"
      shift 2
      continue
      ;;
    -*)
      echo "Unknown option: $1" >&2
      exit 1
      ;;
    esac
  fi

  # All remaining arguments (or after --) are treated as files
  FILES+=("$1")
  shift
done

escape_quote_elisp_string() {
  local input="$1"
  local escaped="${input//\\/\\\\}" # Escape backslashes
  escaped="${escaped//\"/\\\"}"     # Escape double quotes
  escaped="${escaped//$'\n'/\\n}"   # Escape newlines
  escaped="${escaped//$'\t'/\\t}"   # Escape tabs
  escaped="${escaped//$'\r'/\\r}"   # Escape carriage returns
  printf '"%s"' "$escaped"
}

emacsclient_eval() {
  local args=()

  args+=("emacsclient")

  for expr in "$@"; do
    args+=("-e" "$expr")
  done

  if [[ $VERBOSE -ne 0 ]]; then
    printf '[CMD] %s\n' "${args[*]}"
  fi

  "${args[@]}"
}

main() {
  if [[ ${#FILES[@]} -lt 2 ]]; then
    echo "Error: At least two files must be provided." >&2
    echo "Usage: $0 [options] <file1> <file2> [file3 ...]"
    exit 1
  fi

  local func=""
  case "$DIFF_TOOL" in
  ediff)
    func="ediff"
    ;;
  vdiff)
    func="vdiff"
    ;;
  *)
    echo "Error: Unsupported diff tool: $DIFF_TOOL" >&2
    exit 1
    ;;
  esac

  local file1="${FILES[0]}"
  local file2="${FILES[1]}"

  [[ ! -e "$file1" ]] && {
    echo "Error: no such file: $file1" >&2
    exit 1
  }

  [[ ! -e "$file2" ]] && {
    echo "Error: no such file: $file2" >&2
    exit 1
  }

  local abs_file1
  local abs_file2
  abs_file1="$(realpath "$file1")"
  abs_file2="$(realpath "$file2")"

  local cmd
  cmd="(progn (${func} $(escape_quote_elisp_string "$abs_file1")
                       $(escape_quote_elisp_string "$abs_file2")))"

  emacsclient_eval --suppress-output --no-wait "$cmd"
}

main "$@"