"python" may point to nothing (most probable), python2 or python3 (least probable)

[magnumripper]

From #4898 (comment) on:

Me:

Um, what is the current "correct" way of calling python? Shebangs like #!/usr/bin/env python doesn't work on Ubuntu (at least not my 24.04) but some of our tools (and this one) use that, while others use #!/usr/bin/env python3 and a couple even has #!/usr/bin/env python2.

Solar:

Our conversion was to use #!/usr/bin/env python for scripts that work with either Python 2 or 3, #!/usr/bin/env python3 for scripts that are Python 3 only, and #!/usr/bin/env python2 for scripts that are Python 2 only. Unfortunately, it looks like our convention for universal script is detached from distro reality.

If so, I suggest we convert them to use #!/usr/bin/env python3 because a modern distro is unlikely to have anything else available by default, but add a comment just below saying that the script also works with Python 2.

This is especially important and unfortunate for bitcoin2john.py, which supports both versions of Python but is easier to get working with Python 2 (BDB bundled) than 3. So we may want to put a more elaborate comment into that one script.

[magnumripper]

I wonder if it's possible (without jumping through hoops) to have a shebang with something along the lines of env python || env python3 ?

[exploide]

Yes, some distributions stopped shipping a python symlink by default and only have python3. They optionally offer packages like python-is-python3 or python-unversioned-command which ship that symlink but it's not guaranteed to be present.

Overall, python2 is enf-of-life for over five years now.

Personally, I would just drop python2 support. Much less maintenance work. But I think I understand that you prefer keeping support in case someone wants to run the 2john script on an ancient victim machine. (However, I never do this. I always run 2john tools on my own system.)

If so, I suggest we convert them to use #!/usr/bin/env python3 because a modern distro is unlikely to have anything else available by default, but add a comment just below saying that the script also works with Python 2.

I think this is a good way forward.

I wonder if it's possible (without jumping through hoops) to have a shebang with something along the lines of env python || env python3 ?

Never saw anythink like this. Even if this would work, it would be very untypical.

(Side note: Even when the shebang is #!/usr/bin/env python, the worst thing that happens is a command not found error. Users (who hopefully know their distribution and setup) can then just manually invoke python3 xxx2john.py. But yes, changing to a python3 shebang is probably nicer.)

[solardiz]

I wonder if it's possible (without jumping through hoops) to have a shebang with something along the lines of env python || env python3 ?

Never saw anythink like this. Even if this would work, it would be very untypical.

I think we need a shell+Python polyglot for this, and someone already came up with one for this very purpose here:

https://unix.stackexchange.com/a/66242

#!/bin/sh
''':'
if type python2 >/dev/null 2>/dev/null; then
  exec python2 "$0.real" "$@"
else
  exec python "$0.real" "$@"
fi
'''
# real Python script starts here
def …

(The whole text between ''' and ''' is a Python string at toplevel, which has no effect. For the shell, the second line is ''':' which after stripping quotes is the no-op command :.)

I wonder if we can simplify (and avoid portability issues of) the if statement, instead just trying one exec after the other (but we'd probably get an error message from the first one, which we can't redirect to /dev/null without similarly impacting the script's output when that exec succeeds).

[solardiz]

This seems to work (tried several shells):

+++ b/run/pcap2john.py
@@ -1,6 +1,12 @@
-#!/usr/bin/env python
+#!/bin/sh
 # -*- coding: utf-8 -*-
 
+''':'
+type python3 > /dev/null 2> /dev/null && exec python3 "$0" "$@"
+type python > /dev/null 2> /dev/null && exec python "$0" "$@"
+exec python2 "$0" "$@"
+'''
+
 # Combine of all of Dhiru Kholia pcap convert utilities
 # into a single program.  The pcap2john.readme lists
 # all of the original license statements. This merge was

[exploide]

I must say I don't like it much. 😅
This is so unusual it will confuse everyone looking at the code.

I prefer the clean and standard way of doing it (change shebang to python3) and wouldn't care much about (rare) issues when executing on a python2 only system. The user will get an error then, maybe look at the code, see the comment that it could work with python2 too and just run python2 xx2john.py.

Just my opinion but do as you like :D

(In the past, there were many complaints that some script is not python3 compatible, but I remember nobody insisting on running some script with python2.)

[solardiz]

These are valid points @exploide.
@magnumripper What's your preference?

For bitcoin2john.py, we could maybe do this:

+++ b/run/bitcoin2john.py
@@ -47,7 +47,7 @@ def do_import_bsddb():
             try:
                 import bsddb3.db as bsddb_db
             except:
-                sys.stderr.write("Error: This script needs bsddb3 to be installed!\n")
+                sys.stderr.write("Error: This script needs bsddb3 to be installed or run under python2!\n")

[magnumripper]

Definitely valid points, let's do that. My comment was mostly a curious thought triggered by the discussion.