How do I convert a relative to an absolute path, portably and robustly?

Question

Suppose I'm running bash on a Unix-ish system - not necessarily Linux and not necessarily very new; and it may not have every bit of software I'd like.

Now, I have a relative path for which I want to get the absolute path. What's the most robust and portable way of doing this?

The answers here seem to mostly assume the GNU core utilities are installed, which I would rather not do.

Bonus points if your answer works on any Bourne Shell variant.

One way to be more portable than using `pwd` and changing dirs(and it's funny that you want a robust way to get absolute paths but didn't said changing dirs were allowed, and also considered this "safer" than install gnu `readlink`) is to make a little utility in `C` using `realpath()` function - https://stackoverflow.com/a/229038/2231796 — , Jun 20 '17 at 18:54
@nwildner: A C compiler is definitely not installed on every system you know... — einpoklum, Jun 20 '17 at 21:14
Maybe you should add this to your answer, that it requires a portable solution that could also range from embedded to powerfull servers... — , Jun 20 '17 at 22:39

score 3 · Accepted Answer · 2017-06-20T22:50:42.347

3

I am not sure if the environment variable $PWD was set in the bourne shell, but the command pwd exists since the old minix and is part of POSIX, so:

abs_path="$(cd "$rel_path" && pwd -P)"

edited Jun 20 '17 at 22:50

answered Jun 20 '17 at 18:19

+1 for the first suggestion, but the second suggestion is the opposite of what I need, i.e. something I can really not bank on. – einpoklum Jun 20 '17 at 21:15
Fair point @einpoklum , readlink removed. – Jun 20 '17 at 22:51

score 0 · Answer 2 · answered Jun 20 '17 at 14:06

0

What I do now is

absolute_path="$(cd "$relative_path"; echo "$PWD")"

I wonder if Bourne Shell had $PWD though.

answered Jun 20 '17 at 14:06

einpoklum

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`PWD` is in POSIX at least. To avoid the `echo`, you may use `pwd -P`: `abs_path="$( cd "$rel_path" && pwd -P )"` – Kusalananda Jun 20 '17 at 14:33

JRFerguson · Answer 3 · 2017-06-20T14:36:50.567

0

You can use the Cwd core module from Perl. This module provides functions for determining the pathname of the current working directory.

perl -MCwd -le 'print Cwd::realpath("foo")'

Perl is generally already installed on Linux and Unix. This module has been a part of Perl-5 core since 1994.

edited Jun 20 '17 at 14:36

answered Jun 20 '17 at 14:28

JRFerguson

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If I ssh'ed into some minimalistic netbsd distribution embedded on some device - do you think it would have perl 5 and this module? – einpoklum Jun 20 '17 at 21:17

How do I convert a relative to an absolute path, portably and robustly?

3 Answers3