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0 pointsadastra224mo ago0 comments

The kernel interprets the shebang line, not the shell.

0 comments

It is possible for the shell to handle it. From zshall(1):

> If the program is a file beginning with ‘#!', the remainder of the first line specifies an interpreter for the program. The shell will execute the specified interpreter on operating systems that do not handle this executable format in the kernel.

Taking a quick look at the source in Src/exec.c:

  execve(pth, argv, newenvp);
  // [...]
  if ((eno = errno) == ENOEXEC || eno == ENOENT) {
              // [...]
              if (ct >= 2 && execvebuf[0] == '#' && execvebuf[1] == '!') {
                                // [...]
                                (pprog = pathprog(ptr2, NULL))) {

I guess at some point someone added that `|| eno == ENOENT` and the docs weren't updated.

opello4mo ago

I did a little digging and found that the `|| eno == ENOENT` was added quite a bit earlier[1] than the actual pathprog lookup[2]. While I could find the "issue discussion" for the pathprog change[3] I wasn't able to find it for the ENOENT addition, which was kind of interesting and frustrating--[4] is the `X-Seq` mentioned in the commit but that seems to be inconsistent or incorrect for the actual cross-reference, and nearby in time wasn't helpful either.

[1] https://sourceforge.net/p/zsh/code/ci/29ed6c7e3ab32da20f528a...

[2] https://sourceforge.net/p/zsh/code/ci/29ed6c7e3ab32da20f528a...

[3] https://www.zsh.org/mla/workers/2010/msg00522.html

[4] https://www.zsh.org/mla/workers/2000/msg01168.html

tombert4mo ago

I'm not sure the reason then, but they're definitely right; it works fine with zsh, doesn't work with bash. I wrote a test script to try it myself.

I don't have fish installed and can't be bothered to go that far, but I suspect they're right about that as well.

opello4mo ago

It is strange, cursory digging for an explanation was a little more complex than I bargained for...

https://github.com/torvalds/linux/blob/v6.17/fs/binfmt_scrip...

I think it makes it to calling open_exec but there's a test for BINPRM_FLAGS_PATH_INACCESSIBLE, which doesn't seem relevant since 'bash' isn't like '/dev/fd/<fd>/..', but does provoke an ENOENT.

https://github.com/torvalds/linux/blob/v6.17/fs/exec.c#L1445

Maybe someone else can explain it, I'd enjoy the details, and ran out of steam.

anotherhue4mo ago

env bash is all well and good for normies, but if you're already on NixOS did you know you can have nix-shell be your interpreter and back flip into any reproducible interpreted environment you like?

https://nixos.wiki/wiki/Nix-shell_shebang

SAI_Peregrinus4mo ago

Or any other system with Nix installed. I use this at work to provide scripts with all their dependencies specified that work across any Linux distro & MacOS. First execution is slow since it has to fetch everything, but after that it's fast and just works.

j / k navigate · click thread line to collapse

0 comments

jolmg4mo ago

It is possible for the shell to handle it. From zshall(1):

Taking a quick look at the source in Src/exec.c:

  execve(pth, argv, newenvp);
  // [...]
  if ((eno = errno) == ENOEXEC || eno == ENOENT) {
              // [...]
              if (ct >= 2 && execvebuf[0] == '#' && execvebuf[1] == '!') {
                                // [...]
                                (pprog = pathprog(ptr2, NULL))) {

I guess at some point someone added that `|| eno == ENOENT` and the docs weren't updated.

opello4mo ago

[1] https://sourceforge.net/p/zsh/code/ci/29ed6c7e3ab32da20f528a...

[2] https://sourceforge.net/p/zsh/code/ci/29ed6c7e3ab32da20f528a...

[3] https://www.zsh.org/mla/workers/2010/msg00522.html

[4] https://www.zsh.org/mla/workers/2000/msg01168.html

tombert4mo ago

I'm not sure the reason then, but they're definitely right; it works fine with zsh, doesn't work with bash. I wrote a test script to try it myself.

I don't have fish installed and can't be bothered to go that far, but I suspect they're right about that as well.

opello4mo ago

It is strange, cursory digging for an explanation was a little more complex than I bargained for...

https://github.com/torvalds/linux/blob/v6.17/fs/binfmt_scrip...

I think it makes it to calling open_exec but there's a test for BINPRM_FLAGS_PATH_INACCESSIBLE, which doesn't seem relevant since 'bash' isn't like '/dev/fd/<fd>/..', but does provoke an ENOENT.

https://github.com/torvalds/linux/blob/v6.17/fs/exec.c#L1445

Maybe someone else can explain it, I'd enjoy the details, and ran out of steam.

anotherhue4mo ago

env bash is all well and good for normies, but if you're already on NixOS did you know you can have nix-shell be your interpreter and back flip into any reproducible interpreted environment you like?

https://nixos.wiki/wiki/Nix-shell_shebang

SAI_Peregrinus4mo ago

j / k navigate · click thread line to collapse