Linux

I don't see anyone mentions htop. So, I will:) Just works, could be installed in any distro. Much more friendly than top but isn't bloated with features as some other alternatives are.

On the subject of editors, joe is just awesome: lightweight, powerful, had coffee coloring and line numbers, and you can choose it with Ctrl+C:)

A really simple one but surprisingly useful is cal

Discovered about rg recently and it is cool!

As someone who has to do installs and admin at work a lot I'm constantly dealing with yum/dnf. I cry when I have to work with AIX.

awk

yq is crazy cool for converting between different text-based data formats such as yaml, json, xml, csv and others, and it has a super nice pretty-printing function as well. I use it all the time!

Just be aware that your distroy might come with a yq variant too, but possibly one that isn't as powerful as the one I linked. I know this to be true at least for Ubuntu.

The pipe (|), which if you think about it is the basis for function composition.

I think socat is a really powerful und underrated tool

I like btop Maybe not really "unknown", but hey for those that don't know about it, check it out!

miniserve

A static file server, I use it for temporary file share in company, just run miniserve . in the folder.

dua

Alternative of du command, run dua i for a text UI