Why is this shell script calling itself as python script?

Question

Obviously this shell script is calling itself as a Python script:

#!/bin/sh
## repo default configuration
##
REPO_URL='git://android.git.kernel.org/tools/repo.git'
REPO_REV='stable'

magic='--calling-python-from-/bin/sh--'
"""exec" python -E "$0" "$@" """#$magic"
if __name__ == '__main__':
  import sys
  if sys.argv[-1] == '#%s' % magic:
    del sys.argv[-1]
del magic
:
:

(Whole script: https://android.googlesource.com/tools/repo/+/v1.0/repo)

Can anyone explain

• the purpose of calling it this way?
  Why not having #!/usr/bin/env python in the first line so it gets interpreted as Python script from the beginning?

• the purpose of adding that magic last command line argument, that is removed afterwards in the beginning of the Python code?

Answer 1

Your first question: this is done to fix unix systems (or emulations thereof) that do not handle the #! correctly or at all. The high art is to make a script that is correct in shell as well as in the other language. For perl, one often sees something like:

exec "/usr/bin/perl"
   if 0;

The exec is interpreted and executed by the shell, but the perl interpreter sees a conditional statement (.... if ...) and does nothing because the condition is false.