$PWD vs. pwd regarding portability

Question

I'm writing a shell script which parses the path of the current working directory (printing a like of all basenames above the current directory).

So far, I've been using the environment variable PWD to parse the path but I wonder if

• I can count on PWD to be always set
• to give the same result on every platform

Would it possibly be better to use the pwd shell-builtin? I need this script to run on as many platforms as possible, so I just wondered...

Answer 1

POSIX requires $PWD to be set in the following fashion:

PWD  

This variable shall represent an absolute pathname of the current working directory. It shall not contain any components that are dot or dot-dot. The value is set by the cd utility, and by the sh utility during initialization.

So you can rely on that being set – but do note "... an absolute path...", not the absolute path.

bash (at least recent versions) will remember what symlinks you followed when setting $PWD (and the pwd builtin). command pwd (that is, the external command) will not. So you'll get different results there, which might, or might not, be important for you. Use pwd -P if you want a path without symlinks.

Do note that the pwd documentation states:

If an application sets or unsets the value of PWD, the behavior of pwd is unspecified.

So, don't do that :)

In short, there is no winner here. The environment variable will be there in POSIX shells, as will the external command and possibly a built-in too. Choose the one that best fits your need, the important thing being whether you care about symlinks or not.