shebang

About that first line of some python files that looks like this: #!/usr/bin/env python

The first line of many Python files looks like this:

#!/usr/bin/env python

OR to specify python3:

#!/usr/bin/env python3

This is called a “shebang” (pronounced “shuh-bang”). The name comes from the fact that the exclamation point (!) is sometimes read aloud as the word “bang”, and the syntax here comes from the Unix Shell, hence “shell-bang”, contracted into “shebang”.

But what is it for? What it does is allow you to run a Python Script directly in a Unix Shell environment (also in a MacOS terminal session, which is a Unix shell) by just typing its name.

That is, if hello.py looks like this:

print ("Hello World")

then to run it, you need to do:

python hello.py

But if it has a shebang as its first line:

#!/usr/bin/env python3

print ("Hello World")

and you change its permissions to executable (chmod u+x hello.py) then you can run it directly like this:

./hello.py

More about the shebang