Even if you’ve only dabbled with python for a minute or two, you’ve probably already figured out that you can print variables using placeholders. You might have tried concatenation too. However, did you know you can print using %s but with the variable names directly?

You don’t need calculators or other languages and compilers to do math. You can do a lot in bash (GNU Bourne-Again SHell), and it’s really easy!

You can change the default behavior of queries using the RPM command by putting a single line in your .rpmmacros. You can also do this manually each time by including the query format in your rpm command.

The pre-install is not inside a chroot environment, so whatever variables you set or export are only available for use outside of a chroot. And that’s the problem with the post install section, by default anyway.

The following is a list of variables you can use for both yum commands and yum configuration files.

In udev rules, the %k, %b, %n variables are nice and all, but you can also use the variables you’re comparing and setting such as ID_FS_TYPE, KERNEL, SUBSYSTEM, PHYSDEVPATH, etc. When you run a command in a udev rule, there’s nothing stopping you from calling a shell and executing a few commands without actually calling a standalone script to do the dirty work. If you write out the array of environment variables from inside an external program, you can get a better understanding of just which part of the device discovery is matching your rule and getting processed.

This is so handy, I can’t believe i’ve never used or even heard of this until today! You can easily run your bash shell scripts in debug mode to watch what they’re doing behind the scenes in real time. You get to see the levels of nesting when you’re inside loops and variables get replaced with their actual contents at the time of execution.
This might come in handy if you have multiple levels of nesting in ‘for’ and ‘while’ loops or a few if/then/else statements and you want to see just what is getting passed in the comparisons.

If you check the contents of a post variable that never got passed, you get warnings. If you like keeping your verbosity set that high and want to avoid this warning, or you just want to avoid checking against a non-existent variable, try this