python - Purpose of #!/usr/bin/python3 shebang

Question

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.

Answer 1

#!/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. A shebang line could also be a bash, ruby, perl or any other scripting languages' interpreter, for example: #!/bin/bash.

Without the shebang line, the operating system does not know it's a python script, even if you set the execution flag (chmod +x script.py) on the script and run it like ./script.py. To make the script run by default in python3, either invoke it as python3 script.py or set the shebang line.

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