uftrace detects shebang in the first line as follows.

#!/usr/bin/env python3

But if it has some whitespaces at the beginning, it can't detect the shebang as follows.

#! /usr/bin/env python3

For example, when there is a script as follows.

$ cat p-abc.py
#! /usr/bin/env python3

import os

def c():
    return os.getpid()

def b():
    return c()

def a():
    return b()

a()

The tracing result looks as follows.

$ uftrace p-abc.py
uftrace: /home/honggyu/work/uftrace/cmds/record.c:1580:find_in_path
 ERROR: Cannot trace '': No such executable file.

The shebang with whitespaces is legal so we should fix our shebang detection logic.