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?

@Will: Do you mean the author had some non-technical constraint that allows only shell scripts, no Python scripts; so he wrote a Python script that is formally a shell script? — Curd
– Curd, Commented Apr 8, 2011 at 12:53
I guess I have to work on my dry sense of humour. Ingo's answer is the most likely real reason. — wlangstroth
– wlangstroth, Commented Apr 8, 2011 at 13:13

Vadim Kotov · Accepted Answer · 2017-07-24 10:44:35Z

8

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.

edited Jul 24, 2017 at 10:44

Vadim Kotov

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answered Apr 6, 2011 at 11:30

Ingo

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2 Comments

intuited Over a year ago

Could also be used on systems which do handle #! correctly but are explicitly using sh to interpret scripts in a particular directory.

Andrew Aylett Over a year ago

Or even to handle systems which don't definitely have /usr/bin/env.

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