The case of the supersized shebang

Posted Feb 20, 2019 11:19 UTC (Wed) by epa (subscriber, #39769)
In reply to: The case of the supersized shebang by epa
Parent article: The case of the supersized shebang

OK, according to the historical lore linked later in this discussion, it's the shell that is responsible for treating an executable as a shell script if it looks like one. Also Perl's exec() call does it. But the raw system call and C library do not: trying to execl() an executable file which just contains 'echo hello' will fail. It needs the shebang line.

The case of the supersized shebang

Posted Feb 20, 2019 11:55 UTC (Wed) by pebolle (guest, #35204) [Link] (3 responses)

Michael Kerrisk's TLPI (p. 575) points to execlp() and execvp() for that behaviour. A quick test showed that execlp() indeed treats such an executable file "as though [it] started with a line containing the string #!/bin/sh".

The case of the supersized shebang

Posted Feb 20, 2019 22:08 UTC (Wed) by foom (subscriber, #14868) [Link] (2 responses)

Some shells *also* handle this themselves.

E.g., with bash, if execve fails with -ENOEXEC, it will reset all the shell state and evaluate the #!-less script file directly in the post-fork bash process, rather than exec'ing anything at all (neither /bin/sh nor /bin/bash!)

The case of the supersized shebang

Posted Feb 20, 2019 22:32 UTC (Wed) by pebolle (guest, #35204) [Link]

I see. So it seems - or, put another way, strace showed me - that this behaviour of /bin/sh is what allows execlp() to do its, well, magic.

The case of the supersized shebang

Posted Feb 22, 2019 8:31 UTC (Fri) by epa (subscriber, #39769) [Link]

Sounds like a handy trick to speed up your shell scripts!