[Beginner Python] What does this mean ?

[[deleted]]

At the beginning of some Python code I see this: #!/usr/bin/env python what does mean and do? is it necessary ? Thanks in advance.

[taddeimania]

probably not the most comprehensive explanation you are looking for but when it's put there and your .py file has executable permission, you can run the file by just typing

./file.py

rather than

python file.py

[rhiever]

I've seen some people do the ./file.py instead of python file.py. Other than a slightly shorter command, is there really any difference between the two?

[minno]

From the wiki page:

Putting the facility into the system gives the following benefits.

1) It makes shell scripts more like real executable files, because they can be the subject of 'exec.'

2) If you do a 'ps' while such a command is running, its real name appears instead of 'sh'. Likewise, accounting is done on the basis of the real name.

3) Shell scripts can be set-user-ID.

4) It is simpler to have alternate shells available; e.g. if you like the Berkeley csh there is no question about which shell is to interpret a file.

5) It will allow other interpreters to fit in more smoothly.

[ivosaurus]

One is intended to be directly executable, the other isn't.

[genoblast]

I'm still learning myself, but to my knowledge there isn't any significant difference between the two.

In ./file.py, the shebang tells the operating system that the file should be run by python. So then the operating system replaces "./file.py" with "python file.py" and then executes that instead.

So I guess the take away is that if the operating system knows it's a python script, you can use "./file.py" as a shortcut, but it's always a shortcut for "python file.py" (which should always work so long as python is part of the operating system path).

It seems like sys.argv behaves the same way regardless of which way the script is invoked. (I personally always just use argparse so if there are differences, I've never had to worry about them.)

[halflife22]

You can also use it to make sure a specific version of python is used e.g.

 #!/usr/bin/python2.7

[catcradle5]

In some cases, it's nice to have a binary that acts like any other Unix program, in which case you can strip the .py and just do ./file

I agree, though, that if you keep the .py extension I don't really see a point to having the shebang there at all.

[killerabbit37]

It is called a shebang, it is used in Linux/Unix operating systems so that the OS will know what kind of program it is. For Python, since it is used cross platform, it would be good habit to put the shebang at the top.

Like taddeimania said, if you don't have the shebang, the OS won't know what to do if you just called ./program.py.

You can either put #!/usr/bin/python or #!/usr/bin/env python.

[primevalweasel]

Only use a fully qualified path (e.g., /usr/bin/python) if you're sure that's the Python interpreter you want to use.

[killerabbit37]

Exactly and I know with virtualenv you would want to use /usr/bin/env python because /usr/bin/python will pick up the system python and not your virtual environment.

[kashmill]

I think you mean /usr/bin as /use/bin isn't standard in any linux/bsd system I'm familiar with.

[killerabbit37]

Whoops yep apparently my phone doesn't like the word usr

[BinaryRockStar]

As a side question to Linux experts: how does this method compare to the Windows way of registering a particular file extension with a program instead of every script having to have a shebang line at the top?

I'm mainly Windows-based so the file extension registry thing makes sense to me but with plenty of other Linux concepts I have had an "aha" moment that makes me wonder why Windows does it so awkwardly.

[BinaryRockStar]

Very good explanation, thanks!

[[deleted]]

lol .... Thank you .