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'.)