2024-08-21
What a POSIX shell has to do with $PWD
It's reasonably well known about Unix people that '$PWD' is a
shell variable with the name of the current working directory. Well,
sort of, because sometimes $PWD isn't right or isn't even set
(all of this is part of the broader subject of shells and the
current directory). Until recently, I
hadn't looked up what POSIX has to say about
$PWD, and when I did I was surprised, partly because I didn't expect
POSIX to say anything about it.
(Until I looked it up, I had the vague impression that $PWD was a common but non-POSIX Bourne shell thing.)
What POSIX has to say is in 2.5.3 Shell Variables part of the overall description of the POSIX shell. To put my own summary on what POSIX says, the shell creates and maintains $PWD in basically all circumstances, and is obliged to update $PWD when it does a 'cd', even in shell scripts. The only case where $PWD's value isn't specified in the shell environment is if you don't have access permissions for the current directory for some reason.
(As far as I can tell, the complicated POSIX wording boils down to
that if you start the shell with a correct $PWD that uses symbolic
links (eg '/u/cks' instead of '/h/281/cks'), the shell is allowed
to update that to the post-symlink 'physical' version but doesn't
have to. See how 'pwd -P' is
described.)
However, $PWD is not necessarily correct when you're running a program written in C, because POSIX chdir() doesn't seem to be required to update $PWD for you (although it's a bit confusing, since Environment Variables seems to imply that POSIX utilities are entitled to believe $PWD is correct if it's in the environment). In fact I don't think that the POSIX shell is always obliged to export $PWD into the environment, which is why I called it a shell variable instead of an environment variable. I believe most actual Bourne shell implementations do always export $PWD, even if they're started in an environment with it undefined (where I believe POSIX allows it to not be exported).
(Bash, Dash, and FreeBSD's Almquist shell all allow $PWD to be unexported, although keeping it that way may be tricky in Dash and FreeBSD sh, which appear to re-export it any time you do a 'cd'.)
The upshort of this is that in a modern environment where /bin/sh is a POSIX shell, $PWD will almost always be correct. It pretty much has to be correct in your POSIX shell sessions and in your POSIX shell scripts. POSIX-compatible shells like Bash will keep it correct even in their more expansive modes, and non-Bourne shells have a strong motive to go with the show because people expect $PWD to work and be correct.
(However, this leaves me mystified about what the problem was in my specific circumstance this time around, since I'd expect $PWD to have gotten set correctly when my /bin/sh based script used 'cd'.)