Correctly use gnu-sed on macos

- corrects outdated gnu-sed installation instructions on macos
- uses gsed on macos
- display error message if gnu-sed is missing

README.md

@@ -114,13 +114,11 @@ Options:
 Similar to a distributed
 `git ls-files -z -- FILENAMES | xargs -0 sed -i EXPRESSION`.
 
-_note_: this assumes GNU sed. If you're on macOS, install `gnu-sed` with Homebrew:
+_note_: this assumes GNU sed. If you're on macOS, install `gnu-sed` with
+Homebrew, which will expose `gsed` executable:
 
 ```bash
 brew install gnu-sed
-
-# Add to .bashrc / .zshrc
-export PATH="$(brew --prefix)/opt/gnu-sed/libexec/gnubin:$PATH"
 ```
 
 Arguments:

all_repos/sed.py

@@ -3,8 +3,11 @@
 import argparse
 import functools
 import os.path
+import platform
 import shlex
+import shutil
 import subprocess
+import sys
 from typing import Generator
 from typing import Sequence
 
@@ -14,6 +17,14 @@
 from all_repos.config import Config
 from all_repos.util import zsplit
 
+SED_EXEC = "sed"
+if platform.system() == 'Darwin':
+    if not shutil.which("gsed"):
+        print('MacOS specific prerequisite missing, please install it with: brew install gnu-sed')
+        sys.exit(2)
+
+    SED_EXEC = "gsed"
+
 
 def find_repos(
         config: Config,
@@ -44,7 +55,7 @@ def main(argv: Sequence[str] | None = None) -> int:
     parser = argparse.ArgumentParser(
         description=(
             'Similar to a distributed '
-            '`git ls-files -z -- FILENAMES | xargs -0 sed -i EXPRESSION`.'
+            f'`git ls-files -z -- FILENAMES | xargs -0 {SED_EXEC} -i EXPRESSION`.'
         ),
         usage='%(prog)s [options] EXPRESSION FILENAMES',
     )
@@ -62,7 +73,7 @@ def main(argv: Sequence[str] | None = None) -> int:
         '--commit-msg', '--commit-message',
         help=(
             'override the autofixer commit message.  (default '
-            '`git ls-files -z -- FILENAMES | xargs -0 sed -i ... EXPRESSION`).'
+            f'`git ls-files -z -- FILENAMES | xargs -0 {SED_EXEC} -i ... EXPRESSION`).'
         ),
     )
     parser.add_argument(
@@ -79,7 +90,7 @@ def main(argv: Sequence[str] | None = None) -> int:
         dash_r = ('-r',)
     else:
         dash_r = ()
-    sed_cmd = ('sed', '-i', *dash_r, args.expression)
+    sed_cmd = (SED_EXEC, '-i', *dash_r, args.expression)
     ls_files_cmd = ('git', 'ls-files', '-z', '--', args.filenames)
 
     msg = f'{shlex.join(ls_files_cmd)} | xargs -0 {shlex.join(sed_cmd)}'