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From: Keith Thompson <Keith.S.Thompson+u@gmail.com>
Newsgroups: comp.unix.shell
Subject: Re: Default PATH setting - reduce to something more sensible?
Date: Sat, 25 Jan 2025 17:36:48 -0800
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cross@spitfire.i.gajendra.net (Dan Cross) writes:
> In article <vn0bpf$29qe6$1@dont-email.me>,
> Janis Papanagnou  <janis_papanagnou+ng@hotmail.com> wrote:
[...]
>>if not a bug, looks like a Bad Design Idea if it's been done
>>deliberately. - YMMV.
>
> It's obviously being done deliberately, and is easy to find in
> the source code:
> https://git.savannah.gnu.org/cgit/bash.git/tree/findcmd.c#n533

The code in question is:

    xpath = (posixly_correct == 0 && *path == '~') ? bash_tilde_expand (path, 0) : path;

where `path` is an element of $PATH.

This is obviously deliberate, and I see similar code (without the
posixly_correct condition) in bash 1.05 from 1990.

According to Wikipedia, POSIX began in 1988, and the initial
release of bash was in 1989, just a year later.  Obviously the
authors thought that expanding literal '~'s in $PATH was a good
idea at the time, and it's not suprising that they didn't pay much
attention to POSIX.  It would have been nice if they'd documented it.

> You may consider it bad design, and you're well within your
> rights to do so, but opinions on that vary, and it doesn't mean
> yours is correct.

As far as I can tell (I haven't investigated), all Bourne-based
shells other than Bash treat $PATH in the same way when executing
commands.  Bash is the only shell I'm aware of that expands literal
'~' in $PATH.

The fact that it's not POSIX compliant behavior is at worst mildly
annoying.  Bash correctly disables it in POSIX mode (enabled by the
--posix option, by running "set -o posix", by setting $POSIXLY_CORRECT
before starting bash, or by invoking it sas "sh").

I dislike the behavior because (a) it can quietly cause unexpected
problems (it's easy to miss the fact that $HOME is expanded within
double quotes and ~ is not), and (b) it's not documented.  I certainly
consider it a misfeature.  Whether it rises to the level of a bug is
a matter of opinion.

-- 
Keith Thompson (The_Other_Keith) Keith.S.Thompson+u@gmail.com
void Void(void) { Void(); } /* The recursive call of the void */