How to determine which directory a link is pointing to using bash

Question

With Bash scripting, is there a way to determine which directory a link is pointing to, then switch it to another directory based on the result?

For example, say I have the two following directories:

/var/data/1/ and /var/data/2/

I have a link to one of them, say, data_link and it is currently linked to /var/data/1/

When the script is triggered, I want it to swap the link to /var/data/2/ (i.e. ln -s /var/data/2/ /somepath/data_link).

When the script is triggered again, the opposite will happen (i.e. ln -s /var/data/1/ /somepath/data_link), and so on. What's the easiest way to do this via a Bash script?

Answer 1

Use the readlink command to get the content of a link:

$TARGET=$(readlink "$LINK")

Then use standard if to inspect the current value, rm to remove it, and ln as you indicated to re-create with the new desired target.

Answer 2

Unfortunately, the standard set of unix utilities doesn't include an easy access to the readlink system call. Some systems include a readlink command. If it's available, there's a good chance that readlink foo prints the target of the symlink foo. If you feel tempted to use options, be warned that there are multiple versions of this command floating around; you won't get portability even between Linux distributions.

If you have perl available, perl -e 'print readlink $ARGV[0]' mylink prints the target of mylink.

If you need a portable way and don't have Perl or anything else sane, you can perhaps make do with parsing the output of ls -l mylink.

If you have a symbolic link to a directory that you have permission to change into, then $(cd mylink && command -p pwd) is the absolute path to the ultimate target of the link. For example, if foo is a symlink to /var/tmp/bar, bar is a directory in /var/tmp and /var/tmp is a symbolic link to /tmp, this command yields /tmp/bar.