[tombert]

Huh. I wish I had known this before.

NixOS is annoying because everything is weird and symlinked and so I find myself fairly frequently making the mistake of writing `#!/bin/bash`, only to be told it can't find it, and I have to replace the path with `/run/current-system/sw/bin/bash`.

Or at least I thought I did; apparently I can just have done `#!bash`. I just tested this, and it worked fine. You learn something new every day I guess.

[nflekkhnnn]

Anything other than ”#!/usr/bin/env bash” is doomed to fail at some time.

[hulitu]

> Anything other than ”#!/usr/bin/env bash” is doomed to fail at some time.

if you have /usr/bin/env

[kreetx]

/usr/bin/env (and /bin/sh) are part of POSIX, that is why the above shebang is the recommended way to start a shell.

[SAI_Peregrinus]

/bin/sh is NOT required by POSIX, they explicitly warn that it may not exist[1].

> Applications should note that the standard PATH to the shell cannot be assumed to be either /bin/sh or /usr/bin/sh, and should be determined by interrogation of the PATH returned by getconf PATH , ensuring that the returned pathname is an absolute pathname and not a shell built-in.

[1] https://pubs.opengroup.org/onlinepubs/9799919799/

[saghm]

I don't know if this is still a thing, but I distinctly remember when playing around with NixOS years ago that `env` was the only thing in /usr/bin, which I assumed was pretty much exactly for this reason.

[sgarland]

If that fails, I assume the user knows enough about their environment to fix it.

[normie3000]

And is this shebang guaranteed to work always? Why isn't it more common?

[jolmg]

Because /bin is the standard location for bash. The only one that breaks that expectation is NixOS (and maybe GuixSD?), apparently. I'm surprised they didn't symlink /bin or put a stub. Last time I tried NixOS was like 10 years ago. I thought there was a /bin/bash, but maybe it was just a /bin/sh?

Other interpreters like python, ruby, etc. have more likelyhood of being used with "virtual environments", so it's more common to use /usr/bin/env with them.

[Arch-TK]

/bin is the "standard" location for bash on a subset of Linux distributions and basically no other Unix...

So it's not really a standard.

/bin/sh is a much more common convention but once again, not a standard.

There really isn't a truly portable shebang, but the same can be said about executables themselves. As part of the build or install step of whatever thing you're making, you should really be looking these up and changing them.

What's more, bash isn't a standard shell.

[jolmg]

Sorry, I should probably think more widely, but I was just considering Linux distros.

> /bin is the "standard" location for bash on a subset of Linux distributions

Considering "location" such that it includes /bin symlinks, that would be nearly all distros, I would think...

> What's more, bash isn't a standard shell.

De facto and specifically among Linux distros, it is. It's probably an underestimate that 95% of all Linux distro installations have it preinstalled.

[Arch-TK]

It's only really NixOS as far as I know that doesn't ever put bash in /bin/bash (as far as Linux distributions go). But, on the other hand, there are quite a few distros (or at least flavours of distros) which don't ship bash by default (alpine, minimal versions of most distros, and embedded-Linux focused stuff if you count it). I imagine the most common "installation" of Linux is userspace in a container (yeah I know there's no kernel there, but nobody who talks about "Linux" broadly speaking specifically cares about the kernel) and a good chunk of those will be minimal with no bash.

Bash has to be explicitly installed on OpenBSD, FreeBSD, NetBSD (I think, haven't used it in a while) and probably a bunch of others. And in all of those cases (that I know of) it doesn't end up in /bin/bash once installed.

The default bash shipped on macs is so abhorrently ancient that it would be strictly better if it didn't exist because it would reduce the number of people who think bash scripts I write are broken (they're not broken, they just inevitably depend on some bash 4+ feature). Moreover, hardcoding /bin/bash as your shebang in this case will prevent anyone from remediating this problem by installing a non-ancient bash because the old one in /bin/bash will still get used.

[latexr]

> /bin is the "standard" location for bash on a subset of Linux distributions and basically no other Unix...

You’re forgetting macOS. It has been using /bin/bash forever.

[rottingchris]

Keep in mind that the bash you get on MacOS is bash 3.2 released in 2006 so relying on it for portability might not be a good idea.

[johnisgood]

Pretty much. I will continue using "#!/usr/bin/env <language>".

[somat]

Will not wok on OpenBSD where the shell that comes with the system is ksh at /bin/ksh and /bin/sh and if you want bash it is a third party package and correspondingly gets installed as /usr/local/bin/bash

It does get awkward, especially when porting. all your third party libraries and includes are in /usr/local/lib /usr/local/include but at least it is better than freebsd which also insists on putting all third party configs under /usr/local/etc/

[tombert]

They do symlink /bin/sh to be fair, and that's very often good enough for a lot of scripts. That's what I usually do if I don't need anything bash offers.

[rmunn]

Thing is, a few years ago when Debian changed its default sh from bash to ... either ash or dash, I forget which, I got into the habit of always writing `#!/bin/bash` at the top of my scripts, in case I didn't realize that something I was using was a bashism not found in classic /bin/sh. So if I used Nix (I don't, since for my particular use cases the juice isn't worth the squeeze), I would get seriously messed up by that.

[chuckadams]

/bin/sh is part of the POSIX standard -- even NixOS puts a symlink there.

[int_19h]

It's guaranteed to work provided that Bash is in the path.

It's very common for Python. Less so for Bash for two reasons: because the person who writes the script references /bin/sh instead (which is required to be there) even when they are writing bash-isms, or because the person who writes the script assumes that Bash is universally available as /bin/bash.

[eyelidlessness]

It’s quite common, although I probably see it used more frequently to invoke other (non-shell) scripting languages.

[teo_zero]

> apparently I can just have done `#!bash`

I think you're mixing two concepts: relative paths (which are allowed after #! but not very useful at all) and file lookup through $PATH (which is not done by the kernel, maybe it's some shell trickery).

[aaronmdjones]

> and file lookup through $PATH (which is not done by the kernel, maybe it's some shell trickery)

It's libc. Specifically, system(3) and APIs like execvp(3) will search $PATH for the file specified before passing that to execve(2) (which is the only kernel syscall; all the other exec*() stuff is in libc).

[saintfire]

You can use `#!/usr/bin/env bash` on NixOS

[tombert]

I didn't know that actually. I'll start using that from this point forward.

[kreetx]

/usr/bin/env and /bin/sh are part of the POSIX standard, this is why NixOS has those available.

[jlokier]

> /usr/bin/env and /bin/sh are part of the POSIX standard, this is why NixOS has those available.

Contrary to popular belief, those aren't in the POSIX standard.

The following are not in the POSIX standard, they are just widely implemented:

  - "#!" line.
  - /bin/sh as the location of a POSIX compliant shell, or any shell.
  - /usr/bin/env as the location of an env program.
  - The -S option to env.

[Guvante]

I think you are using "not required by the POSIX standard" when you say "not in" which is not an accurate shorthand.

#! is certainly in the POSIX standard as the exact topic of "is /bin/sh always a POSIX" shell is a discussion point (it is not guaranteed since there were systems that existed at the time that had a non-POSIX shell there)

[johnisgood]

Are they in POSIX? I do not think they are. All of them is a convention from what I remember.

Shebang is a kernel feature, for example, and POSIX does define the sh shell language and utilities, but does not specify how executables are invoked by the kernel.

Similarly, POSIX only requires that sh exists somewhere in the PATH, and the /bin/sh convention comes from the traditional Unix and FHS (Filesystem Hierarchy Standard), but POSIX does not mandate filesystem layout.

... and so on.

Correct me if I am wrong, perhaps with citations?

[johnisgood]

It is good practice to be using it everywhere.

[kevincox]

Assuming bash is in $PATH. Which it often is.

[miffe]

Seems like it only works in zsh, not bash or fish

[tombert]

  [tombert@puter:~/testscript]$ ./myscript.sh
    bash: ./myscript.sh: bash: bad interpreter: No such file or directory

You are right. Appears to only work with zsh. I will resume being annoyed then.

[adastra22]

Is this UNIX?

[tombert]

This is NixOS, so no, it's Linux. I guess I just hoped it would work on Linux as well.

[adastra22]

Linux is UNIX in the context of my question. On Linux the shebang is actually handled by the kernel. When you load a binary and attempt to execute it with a syscall, the kernel reads the first few bytes of the binary. If it is an ELF header, it executes the machine code as you would expect. If the first two bytes are "#!", then it interprets it as a shebang header and loads the specified binary to interpret it.

Again, this is kernel code. I admit I'm a bit confused as to why it didn't work on your system. This shouldn't be handled at the shell level.

[adastra22]

The kernel interprets the shebang line, not the shell.

[jolmg]

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.

[opello]

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

[tombert]

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.

[opello]

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.

[anotherhue]

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_Peregrinus]

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.

[Crestwave]

`#!/usr/bin/env bash` is the most portable form for executing it from $PATH

[Ferret7446]

I raise you a https://www.felesatra.moe/blog/2021/07/03/portable-bash-sheb...

Pedantic, but "#!" and "portable" don't belong in the same sentence

[Crestwave]

The script you posted is only portable in theory and not in practice. Executing it while using a non-POSIX shell like Elvish (even without having it as a default shell) makes it immediately fail.

Meanwhile, `#!/usr/bin/env` is completely portable in the practical sense. Even systems with non-standard paths like NixOS, Termux and GoboLinux patch in support for it specifically.

[hamandcheese]

Is this meaningfully more portable than #!bash though?

[tombert]

In a sibling thread someone pointed out that #!bash doesn't actually work if you're calling it from bash, and appears to only work with zsh.

I just tried it and they were absolutely right, so `#!/usr/bin/env bash` is definitely more portable in that it consistently works.

[saurik]

This mechanism doesn't do a PATH lookup: #!bash would only work if bash was located in your current working directory.