CeWL and Parrot OS

Back with another learning moment. This time CeWL and Parrot OS are the culprits. Normally CeWL can be installed through most package managers. Sadly with the current version of Parrot (v5.1.2 Electro Ara) CeWL has a dependency problem and attempting to install it with apt will result in the error: 

ruby-mini-portile2 : Breaks: ruby-nokogiri (< 1.13.2) but 1.13.1+dfsg-2parrot1 is to be installed

Not a fan of getting too crazy on daily use systems but after a little bit of research into Ruby and reviewing CeWL's git page, a reasonable solution was discovered. I was hoping that Ruby had something similar to Python's virtualenv, which it appears Ruby does, but getting it setup/configured is apparently a headache. The alternative solution is both interesting and functional. Hopefully, Parrot's devs will have CeWL's dependencies fixes soon as well and this article will be obsolete!

Installation

The first step is to make sure git and bundler are installed on the system. Parrot should already have git but just in case it doesn't make sure to install it with apt. Bundler is likely not already installed but can be installed with 

apt install bundler

Next, create a directory where CeWL's repo should be downloaded. I tend to make a 'repos' folder on in my home directory on testing systems in order to centralize all the extra tools I place onto my testing systems.

Now with the source of CeWL obtained, the Gem tool can be use to install the dependencies for CeWL without changing the system-wide Ruby installation. To do this, create a folder called 'gems' in the CeWL folder. Then a special environment variable will be set in order to install the required Gems into the newly created folder in CeWL's source.

Assuming the above environment variable, associated folder have been created, and the current directory is the CeWL directory, the bundler command can be used to install the required Gems for CeWL. Note: that exiftool is required and there will be a warning message after Bundler completes. In Parrot, libimage-exiftool-perl, can be installed to meet this requirement.

If there weren't any errors, CeWL should run without errors. Note that since this folder is unlikely to be part of the PATH environment variable, CeWL will have to be launched from this folder currently.

Victory! There are some drawbacks to this setup but if you completely understand all the shortcomings of this band-aid fix, the feel free to crawl on your merry way (terrible pun intended!).

Caveats

As mentioned above there are some shortcomings of this fix in its current state. 

1. The GEM_HOME env will have to always be set before invoking CeWL
2. CeWL has to be run by specifying the path to the Ruby file (cewl.rb) due to this directory not being in the PATH environment variable

Most Linux user's will have their own preference on how to solve this issue but here's how I approached the situation while leaving as much of the rest of the system unchanged as possible. Leveraging the .bash_aliases file, I created a function that sets GEM_HOME, runs CeWL, and then unsets GEM_HOME. The benefit of this approach is the system defaults for Ruby are only modified when necessary but I can still run CeWL as though it were installed system-wide! 

This method, as shown, is bash specific and relies on ~/.bashrc sourcing ~/.bash_aliases so the fix is user specific but is easy to add to other user's configurations as necessary. CeWL also never needs root privileges from what I can tell so this is likely sufficient for any non-root user.

Breaking the contents of ~/.bash_aliases a bit.

1. The first line declares a BASH function called 'cewl' 
2. Sets the GEM_HOME env variable to the Gems folder where the dependencies for CeWL were installed earlier
3. Runs CeWL and passes all the command line args to the cewl.rb script
4. unsets the GEM_HOME variable so that any system-wide changes can still be made by the package manger as needed

• Note: Make sure the file paths in 2 and 3 are corrected to match the system this function is being created on!
  

Make sure that ~/.bashrc, ~/.profile, or the system-wide equivilants contains the following lines as well.

After the changes have been made to the proper files, open a new terminal and see if the new 'cewl' function is working (it is important to open a new terminal as this is a quick way to source the new changes in bash_aliases or source them in the current terminal).

Notice that GEM_HOME is blank before and after the invocation of the cewl tool. The final test is to make sure that arguments are being passed properly.

Note: There's still the slightest potential for a hiccup should the user ctrl+c while cewl is running though so the solution isn't perfect. In the event that cewl.rb is stopped early, GEM_HOME may stay set until cewl is allowed to complete or the terminal is closed. I'll continue working on this to find a better solution for traps.

Regardless, success! While not as nice as apt installing something, this process acts as a good one of for tools that have odd dependencies! Hope this can be of use to others out there.