Is space allowed between #! and /bin/bash in shebang

shebang

In a shebang, is a space or more allowed between #! and the interpreter?

For example, #! /bin/bash. It seems work, but some said that it is incorrect.

Best Answer

Yes, this is allowed.

The Wikipedia article about the shebang includes a 1980 email from Dennis Ritchie, when he was introducing kernel support for the shebang (as part of a wider package called interpreter directives) into Version 8 Unix (emphasis mine):

The system has been changed so that if a file being executed begins with the magic characters #!, the rest of the line is understood to be the name of an interpreter for the executed file. […]

To take advantage of this wonderful opportunity, put
#! /bin/sh
at the left margin of the first line of your shell scripts. Blanks after ! are OK.

So spaces after the shebang have been around for quite a while, and indeed, Dennis Ritchie’s example is using them.

Note that early versions of Unix had a limit of 16 characters in this interpreter line, so you couldn’t have an arbitrary amount of whitespace there. This restriction no longer applies in modern kernels.

Objective Criteria/Requirements:

In determining whether to use an absolute or logical (/usr/bin/env) path to an interpreter in a she-bang, there are (2) key considerations:

a) The interpreter can be found on target system

b) The correct version of interpreter can be found on 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 she-bang 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 see if it meets our criteria.

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

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!')
```
The she-bang in the script was toggled by Python version number desired (all installed on 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

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 she-bang was printed.
Testing Notes:

Tests were exclusively limited to Python
Perl: Like Python- MUST live in /usr/bin according to the FHS
I've not tried 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 finds in the user's Path, we can moderate this behaviour 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 binary lives in different absolute path then specified in 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.

Bash – Perl and Python wrongly interpreted via shebang on Linux

Your kernel was compiled without CONFIG_BINFMT_SCRIPT=y. This setting controls shebang support.

From make menuconfig:

Symbol: BINFMT_SCRIPT [=y]                                             
   Type  : tristate                                                    
   Prompt: Kernel support for scripts starting with #!                 
     Location:                                                         
   (1) -> Executable file formats / Emulations                         
     Defined at fs/Kconfig.binfmt:68

Reconfigure and recompile your kernel. (Technically, it can also be built as a module, but there's no point in doing that for something as fundamental as #! support.)

Best Answer

Related Solutions

Shebang Best Practices – Why Use ‘#!/usr/bin/env NAME’?

Objective Criteria/Requirements:

Testing:

Conclusion:

Bash – Perl and Python wrongly interpreted via shebang on Linux

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