text: #!/usr/bin/env python3
  
  # Change the #! line occurring in Python scripts.  The new interpreter
  # pathname must be given with a -i option.
  #
  # Command line arguments are files or directories to be processed.
  # Directories are searched recursively for files whose name looks
  # like a python module.
  # Symbolic links are always ignored (except as explicit directory
  # arguments).  Of course, the original file is kept as a back-up
  # (with a "~" attached to its name).
  #
  # Undoubtedly you can do this using find and sed or perl, but this is
  # a nice example of Python code that recurses down a directory tree
  # and uses regular expressions.  Also note several subtleties like
  # preserving the file's mode and avoiding to even write a temp file
  # when no changes are needed for a file.
  #
  # NB: by changing only the function fixfile() you can turn this
  # into a program for a different change to Python programs...
  
  import sys
  import re
  import os
  from stat import *
  import getopt
  
  err = sys.stderr.write
  dbg = err
  rep = sys.stdout.write
  
  new_interpreter = None
  preserve_timestamps = False
  
  def main():
      global new_interpreter
      global preserve_timestamps
      usage = ('usage: %s -i /interpreter -p file-or-directory ...\n' %
               sys.argv[0])
      try:
          opts, args = getopt.getopt(sys.argv[1:], 'i:p')
      except getopt.error as msg:
          err(str(msg) + '\n')
          err(usage)
          sys.exit(2)
      for o, a in opts:
          if o == '-i':
              new_interpreter = a.encode()
          if o == '-p':
              preserve_timestamps = True
      if not new_interpreter or not new_interpreter.startswith(b'/') or \
             not args:
          err('-i option or file-or-directory missing\n')
          err(usage)
          sys.exit(2)
      bad = 0
      for arg in args:
          if os.path.isdir(arg):
              if recursedown(arg): bad = 1
          elif os.path.islink(arg):
              err(arg + ': will not process symbolic links\n')
              bad = 1
          else:
              if fix(arg): bad = 1
      sys.exit(bad)
  
  ispythonprog = re.compile(r'^[a-zA-Z0-9_]+\.py$')
  def ispython(name):
      return bool(ispythonprog.match(name))
  
  def recursedown(dirname):
      dbg('recursedown(%r)\n' % (dirname,))
      bad = 0
      try:
          names = os.listdir(dirname)
      except OSError as msg:
          err('%s: cannot list directory: %r\n' % (dirname, msg))
          return 1
      names.sort()
      subdirs = []
      for name in names:
          if name in (os.curdir, os.pardir): continue
          fullname = os.path.join(dirname, name)
          if os.path.islink(fullname): pass
          elif os.path.isdir(fullname):
              subdirs.append(fullname)
          elif ispython(name):
              if fix(fullname): bad = 1
      for fullname in subdirs:
          if recursedown(fullname): bad = 1
      return bad
  
  def fix(filename):
  ##  dbg('fix(%r)\n' % (filename,))
      try:
          f = open(filename, 'rb')
      except IOError as msg:
          err('%s: cannot open: %r\n' % (filename, msg))
          return 1
      line = f.readline()
      fixed = fixline(line)
      if line == fixed:
          rep(filename+': no change\n')
          f.close()
          return
      head, tail = os.path.split(filename)
      tempname = os.path.join(head, '@' + tail)
      try:
          g = open(tempname, 'wb')
      except IOError as msg:
          f.close()
          err('%s: cannot create: %r\n' % (tempname, msg))
          return 1
      rep(filename + ': updating\n')
      g.write(fixed)
      BUFSIZE = 8*1024
      while 1:
          buf = f.read(BUFSIZE)
          if not buf: break
          g.write(buf)
      g.close()
      f.close()
  
      # Finishing touch -- move files
  
      mtime = None
      atime = None
      # First copy the file's mode to the temp file
      try:
          statbuf = os.stat(filename)
          mtime = statbuf.st_mtime
          atime = statbuf.st_atime
          os.chmod(tempname, statbuf[ST_MODE] & 0o7777)
      except OSError as msg:
          err('%s: warning: chmod failed (%r)\n' % (tempname, msg))
      # Then make a backup of the original file as filename~
      try:
          os.rename(filename, filename + '~')
      except OSError as msg:
          err('%s: warning: backup failed (%r)\n' % (filename, msg))
      # Now move the temp file to the original file
      try:
          os.rename(tempname, filename)
      except OSError as msg:
          err('%s: rename failed (%r)\n' % (filename, msg))
          return 1
      if preserve_timestamps:
          if atime and mtime:
              try:
                  os.utime(filename, (atime, mtime))
              except OSError as msg:
                  err('%s: reset of timestamp failed (%r)\n' % (filename, msg))
                  return 1
      # Return success
      return 0
  
  def fixline(line):
      if not line.startswith(b'#!'):
          return line
      if b"python" not in line:
          return line
      return b'#! ' + new_interpreter + b'\n'
  
  if __name__ == '__main__':
      main()
  