A cheap trick to manipulate PERL5LIB


Manipulating PERL5LIB for locally installed programs.

In time, I’ve become fond of not installing Perl widely but keeping them tight in some place where they can’t interfere with others. This led to Installing Perl Modules, where each project gets its own rendition of modules.

Sometimes, though, I’d just like to install some Perl application to have it available in the shell. Which basically means I install it somewhere below my home directory - like the perl5 sub-directory that cpanm defaults to.

This creates a problem, though, because it means that I have to ensure that PERL5LIB is properly set to load modules from where they were installed (~/perl5/lib and the like). I usually can’t just set it in the shell (like in ~/.bashrc) because working in different projects will probably mean I’m going to customize it shell by shell.

This usually meant that I installed a wrapper in ~/bin like this:

# ~/bin/galook - a wrapper for ~/perl5/bin/galook
export PERL5LIB="$HOME/perl5/lib/perl5:$PERL5LIB" 
export PATH="$HOME/perl5/bin:$PATH"
exec "$HOME/perl5/bin/galook" "$@"

While I’m at it, I also ensure that multiple executables in that directory can find each other.

After a few wrappers, I got obviously tired and wanted to solve the issue once and for all, so I created this shell script at ~/perl5/bin/__target__:

target="$(basename "$0")"
bindir="$(dirname "$(readlink -f "$0")")"
rootdir="$(dirname "$bindir")"
export PATH="$bindir:$PATH"
export PERL5LIB="$rootdir/lib/perl5:$PERL5LIB"
exec "$bindir/$target" "$@"

Then, instead of one ad-hoc wrapper ~/bin/galook I just create it as a symbolic link towards __target__ above:

$ ls -l ~/bin/galook
lrwxrwxrwx 1 you you 23 Oct  8 00:26 /home/you/bin/galook -> ../perl5/bin/__target__

So… how is this solving my issue? Let’s take a closer look:

1 #!/bin/sh
2 target="$(basename "$0")"
3 bindir="$(dirname "$(readlink -f "$0")")"
4 rootdir="$(dirname "$bindir")"
5 export PATH="$bindir:$PATH"
6 export PERL5LIB="$rootdir/lib/perl5:$PERL5LIB"
7 exec "$bindir/$target" "$@"

Remember that we will call the function with just galook, and it will be resolved by the shell to be ~/bin/galook, eventually calling the above script in ~/perl5/bin/__target__, i.e. a script in the same directory as our real program ~/perl5/bin/galook.

When the script is called, $0 is simply galook. Line 2 makes sure to get just this name, so even calling it with its “full” path ~/bin/galook would just yield galook. This is important because it’s the name of the real program we are after, although in a different directory (i.e. ~/perl5/bin).

Line 2 aims at finding the directory where the real program lives. Note that there are two nested operations:

  • readlink -f "$0" resolves the program we are calling into the real script position, i.e. it gives back ~/perl5/bin/__target__;
  • calling dirname on it gives back ~/perl5/bin, i.e. exactly the directory we are after.

True, we could have just hardwired it, but it’s much more general like this 🤓

Now this is exactly what we need to manipulate PATH (line 5), but we still have to do one extra step to find where the Perl modules have been installed to manipulate PERL5LIB.

For this reason, line 4 computes the root directory for our local installations, by just getting the parent of $bindir (line 4). This ends up being ~/perl5. As modules are installed in lib/perl5 inside this directory, line 5 installs $rootdir/lib/perl5 as the initial path, so that we are fine.

Do you think I’m complicating bread? Let me know!

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