Bash trace mode (bash -x) in the shebang

Question

How come this style works:

#!/bin/bash -x
#...

But this doesn't?

#!/usr/bin/env bash -x
#...

It gives me a not found error. Trace mode needs to be set later as set -x?

Please note that I can use #!/usr/bin/env bash.

Answer 1

The shebang-line mechanism only parses the first space and passes the rest of the line as one single argument to the executable, so writing

#!/usr/bin/env bash -x

is like calling

$ /usr/bin/env "bash -x"

on the command line.

There is no executable called "bash -x" (with a space in the command name), so this fails.

Answer 2

You can always use:

#!/usr/bin/env bash
set -x