Scripting Bits
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Scripting Bits

Figuring Out What Changed Between Package Versions

Catching up with changes to /etc

Whenever I update the packages on my Arch Linux system, I usually have to reconcile upstream changes to files in /etc with changes I’ve made to them for my system.

Usually, I just compare the new file, conveniently suffixed with .pacnew, with what I have. It’s usually pretty clear what the 3-way diff is, even when I’m not looking at one of them. But, sometimes I need to look at all three.

So, let’s work out a way to properly 3 way diff these files!

I know that Pacman caches packages in /var/cache/pacman/pkg. I want to get a specific file out of the two most recent versions of the same package.

First, I need to know about the structure of these packages; so, I run:

tar -tvf /var/cache/pacman/pkg/filesystem-2020.05.20-1-x86_64.pkg.tar.zst

This prints a list of all of the files in the package. From there, I can list the contents of one of the files:

tar -xOf /var/cache/pacman/pkg/filesystem-2020.05.20-1-x86_64.pkg.tar.zst etc/shadow

From there, starting a three way diff in Neovim looks like:

nvim -d <(tar ...) /etc/shadow <(tar ...)

From here, I can do a bunch more to massage this into a convenient script.

I can derive the name of the package from the path of the /etc file:

pacman -Qoq /etc/shadow

Then, I could list out the package versions:

ls -1 /var/cache/pacman/pkg/$(pacman -Qoq /etc/shadow)-*

I could even go as far as to pick the two most recent files by sorting by last modified date and then taking the top two:

ls -1t /var/cache/pacman/pkg/$(pacman -Qoq /etc/shadow)-* | head -n2

From there, putting everything together into a script is straightforward:

diffpkg /etc/shadow

Line 8 puts the results of ls into an array for easy access in script. Then, because $FILE starts with a /, the first character needs to be removed for the tar command. ${FILE#?} uses a regular expression to do just that!

Overall, that didn’t take very long. I like taking the time to automate daily tasks; it makes me more productive and helps keep my scripting skills sharp!

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