CODEWITHSUNDEEP
pythonbash
new_perl
new_perl

Reputation: 7745

How does google android repo file work?

Quote of the script:

!/bin/sh                                                                       

## repo default configuration
##
REPO_URL='https://android.googlesource.com/tools/repo'
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   
..all python from here on..

How can they put bash and python in a single script and make it run?

Upvotes: 2

Views: 358

Answers (1)

macduff
macduff

Reputation: 4685

This is how I understand it: The script is first invoked as a shell script and then calls python on line 23, so if you invoked the sync command on repo it would do the following:

"""exec" python -E "$0" "$@" """#$magic"

which I believe turns into: exec "python -E "repo" "sync" "#--calling-python-from-/bin/sh--" This then calls the script repo as a python script. You will notice that all of the syntax is legal in bash and python down to the exec line. The triple quotes on line 23 are really sweet, they work in the shell script and then work as a doc string in python. How awesome is that!

In python """ begin and end a docstring which can span multiple lines. Very roughly, it's a very special kind of comment that the python help system can read. In bash, I believe that the double quote is matched to the next double quote the interpreter encounters. So the first two quotes, quote the null string and it disappears from the interpreter. In our example, the next quotes quote the exec keyword. The last set of triple quotes, """#$magic", first quote the null string and then quote the #$magic argument. Since double quotes allow the variable $magic to be expanded, it becomes, #--calling-python-from-/bin/sh-- and passed as an argument.

If you look at the script with a text editor with syntax highlighting as a shell script and then as a python script, it's even easier to see. This is an extremely clever, way of starting the shell script and then giving control to python. Hope this helps!

Upvotes: 3

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