How to determine the absolute path to a file in a shell script

Question

I am writing a generic script that can get passed both absolute and relative (to the current working dir) paths to existing files.

I need a fail-safe way to turn all those passed paths into absolute paths because I need to pass the paths to the "open" command, which in won't pass the current directory to the launched app.

One idea I had is to check if the path starts with a "/", and if not, then I'd prepend the current working dir's path to it.

However, I wonder if there's a smarter solution to this. Also, as I'm quite inexperienced with shell scripting and related tools, I don't even know how to test for a variable's value to start with "/". I'm looking for a portable solution to run on multiple Mac, so installing extra tools or configuration would be less than ideal.

Can someone suggest a standard script using only shipping tools that takes a variable (let's call it $path) and turns it into a absolute path if it's not already absolute?

Answer 1

One option would be to install coreutils and use greadlink -f. It resolves symlinks and it works with /Foo/ or ~/foo.txt if they don't exist, but not with /Foo/foo.txt if /Foo/ doesn't exist.

$ brew install coreutils
$ greadlink -f /etc
/private/etc
$ greadlink -f ~/Documents/
/Users/lauri/Documents
$ greadlink -f ..
/Users
$ greadlink -f //etc/..////
/private
$ greadlink -f /Foo
/Foo
$ greadlink -f /Foo/foo.txt
$ 

This doesn't resolve symlinks, and it doesn't work with /Foo/foo.txt either.

abspath() {
  if [ -d "$1" ]; then
    ( cd "$1"; dirs -l +0 )
  else
    ( cd "$(dirname "$1")"; d=$(dirs -l +0); echo "${d%/}/${1##*/}" )
  fi
}

abspath /etc # /etc
abspath /Foo/foo.txt # doesn't work
abspath /Foo # works
abspath .
abspath ./
abspath ../
abspath ..
abspath /
abspath ~
abspath ~/
abspath ~/Documents
abspath /\"\ \'
abspath /etc/../etc/
abspath /private//etc/
abspath /private//
abspath //private # //private
abspath ./aa.txt
abspath aa.tar.gz
abspath .aa.txt
abspath /.DS_Store

dirs -l performs tilde expansion. dirs +0 prints only the top directory if there are other directories in the stack. You could also replace the subshells with something like old="$PWD" ... cd "$old".

Ruby's expand_path works with all paths that don't exist, but it doesn't resolve symlinks.

$ ruby -e 'print File.expand_path ARGV[0]' ~/aa/bb
/Users/lauri/aa/bb$ 

Answer 2

I'm not sure if this answer will be comprehensive enough for your needs, but this is what I would do…

--DISCLAIMER: I haven't tested this in SH/BASH/Anything so just use this as a framework please. Feel free to edit the question after you've got it working!--

You can test if the supplied script argument begins with a character using something like if [[ $a == "/*" ]].

Since all relative paths carry with them the assumption that the working path is the parent directory, if you know the path is not already absolute you can use pwd, assuming the script or parent process hasn't changed the working directory.

If the issue of the working directory changing is significant, you'll have to use a function like find.

---

Some additional notes that might get you on the right path to solving your issue: https://unix.stackexchange.com/questions/6435/how-to-check-if-pwd-is-a-subdirectory-of-a-given-path

http://www.unix.com/shell-programming-scripting/36200-getting-full-path-relative-path.html

https://stackoverflow.com/questions/2172352/in-bash-how-can-i-check-if-a-string-begins-with-some-value

Hope this puts you in the right direction!

Answer 3

No - a path is just a string and you have to parse it to decide if it's absolute or relative to $PWD or ~

See these for more context in case my answer is too short.

• Why is my applescript for checking if a file exists failing?
• "readlink" gets the original path of a symlink; what's the equivalent for Mac aliases?
• https://superuser.com/questions/205127/how-to-retrieve-the-absolute-path-of-an-arbitrary-file-from-the-os-x