How does /usr/bin/env work in a Linux shebang line?

Question

I know shebang line like this: ``` #!/bin/sh ``` but I found out I can also use shebang line like this: ``` #!/usr/bin/env python3 ``` This confuses me, can someone explain to me how Linux will process this one?

Accepted Answer

`env` is the name of a Unix program. If you read the manual (`man env`) you can see that one way to use it is `env COMMAND`, where in your case, `COMMAND` is `python3`.

According to the manual, this will

Set each NAME to VALUE in the environment and run COMMAND.

Running `env` alone will show you what NAMEs and VALUEs are set:

```
$ env
TERM=xterm-256color
SHELL=/bin/bash
PATH=/usr/local/bin:/usr/bin:/bin:/usr/local/sbin:/usr/sbin:/sbin
…
```

Therefore, `/usr/bin/env python3` is an instruction to set the `PATH` (as well as all the other NAME+VALUE pairs), and then run `python3`, using the first directory in the `PATH` that contains the `python3` executable.

How does /usr/bin/env work in a Linux shebang line?

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