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Objective Criteria/Requirements:

In determining whether to use an absolute or logical (/usr/bin/env) path to an interpreter in a shebang, there are two key considerations:

a) The interpreter can be found on the target system

b) The correct version of the interpreter can be found on the target system

If we AGREE that "b)" is desirable, we also agree that:

c) It's preferable our scripts fail rather than execute using an incorrect interpreter version and potentially achieve inconsistent results.

If we DON'T AGREE that "b)" matters, then any interpreter found will suffice.

Testing:

Since using a logical path — /usr/bin/env to the interpreter in the shebang — is the most extensible solution allowing the same script to execute successfully on target hosts with different paths to the same interpreter, we'll test it — using Python, due to its popularity — to determine whether it meets our criteria.

1. Does /usr/bin/env live in a predictable, consistent location on POPULAR (not "every") operating systems? Yes:

   • RHEL 7.5
   • Ubuntu 18.04
   • Raspbian 10 ("Buster")
   • OSX 10.15.02

2. Below Python script executed both inside and outside of virtual envelopes (Pipenv used) during tests:

   #!/usr/bin/env pythonX.x import sys print(sys.version) print('Hello, world!') 

3. The shebang in the script was varied by the Python version number desired (all installed on the same host):

   • #!/usr/bin/env python2
   • #!/usr/bin/env python2.7
   • #!/usr/bin/env python3
   • #!/usr/bin/env python3.5
   • #!/usr/bin/env python3.6
   • #!/usr/bin/env python3.7

4. Expected results: that print(sys.version) = env pythonX.x.  Each time ./test1.py was executed using a different installed Python version, the correct version specified in the shebang was printed.

5. Testing Notes:

   • Tests were exclusively limited to Python
   • Perl, like Python, MUST live in /usr/bin according to the FHS
   • I've not tested every possible combination on every possible number of Linuxy/Unixy Operating System and version of each Operating System.

Conclusion:

Although it's TRUE that #!/usr/bin/env python will use the first version of Python it matches in the user's Path, we can enforce an express preference by specifying a version number such as #!/usr/bin/env pythonX.x.  Indeed, developers don't care which interpreter is found "first"; all they care about is that their code is executed using the specified interpreter they know to be compatible with their code to ensure consistent results — wherever that may live in the filesystem...

In terms of portability/flexibility, using a logical — /usr/bin/env — rather than absolute path not only meets requirements a), b) & c) from my testing with different versions of Python, but also has the benefit of fuzzy-logic finding the same version interpreter even if they live at different paths on different Operating Systems.  And although MOST distros respect the FHS, not all do.

So, where a script will FAIL if the binary lives in a different absolute path than specified in the shebang, the same script using a logical path SUCCEEDS as it keeps going until it finds a match, thereby offering greater reliability & extensibility across platforms.