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Purpose of #!/usr/bin/python3

Posted by: admin November 30, 2017 Leave a comment

Questions:

I have noticed this in a couple of scripting languages, but in this example, I am using python. In many tutorials, they would start with #!/usr/bin/python3 on the first line. I don’t understand why we have this.

  • Shouldn’t the operating system know it’s a python script (obviously it’s installed since you are making a reference to it)
  • What if the user is using a operating system that isn’t unix based
  • The language is installed in a different folder for whatever reason
  • The user has a different version. Especially when it’s not a full version number(Like Python3 vs Python32)

If anything, I could see this breaking the python script because of the listed reasons above.

Answers:

#!/usr/bin/python3 is a shebang line.

A shebang line defines where the interpreter is located. In this case, the python3 interpreter is located in /usr/bin/python3. It could be a bash, ruby, perl or any other scripting languages’ interpreter. The operating system does not know it’s a python script if you set the execution flag on the script and run it like ./script.py, unless you run it like python3 script.py or set the shebang line.

You can use /usr/bin/env python3 for portability across different systems if they have the language interpreter installed in different locations.

Questions:
Answers:

That’s called a hash-bang. If you run the script from the shell, it will inspect the first line to figure out what program should be started to interpret the script.

A non Unix based OS will use its own rules for figuring out how to run the script. Windows for example will use the filename extension and the # will cause the first line to be treated as a comment.

If the path to the Python executable is wrong, then naturally the script will fail. It is easy to create links to the actual executable from whatever location is specified by standard convention.

Questions:
Answers:

This line helps find the program executable that will run the script. This shebang notation is fairly standard across most scripting languages (at least as used on grown-up operating systems).

An important aspect of this line is specifying which interpreter will be used. On many development-centered Linux distributions, for example, it is normal to have several versions of python installed at the same time.

Python 2.x and Python 3 are not 100% compatible, so this difference can be very important. So #! /usr/bin/python and #! /usr/bin/python3 are not the same (and neither are quite the same as #! /usr/bin/env python3 as noted elsewhere on this page.

Questions:
Answers:
  1. And this line is how.

  2. It is ignored.

  3. It will fail to run, and should be changed to point to the proper location. Or env should be used.

  4. It will fail to run, and probably fail to run under a different version regardless.