What's one Linux command you wish you'd learned years ago?

sed, awk or even the fact that I could've used perl from command line

    sed -i 's/old/new/g'

alone would have saved me lot of time instead of fiddling with a perl script everytime

Also, I would've liked to understand shell concepts like file redirections, command substitution, parameter expansion, difference between stdout/stderr, why sometimes I got an error for too many file arguments, or even knew that there existed sites like https://mywiki.wooledge.org/BashFAQ and stackoverflow/unix.stackexchange and https://www.shellcheck.net/ and https://explainshell.com/ and so on.

fzf as command history search, coupled with an unlimited history file. It is by far my “unfair advantage”, I do things so much faster than my coworkers...

Tmux and Vim for me - they are tools and not exactly commands.

^R in bash to search command history.

Xargs or awk

watch command

man

Awk.

find ... -exec ... {}\;

pushd popd cd -

shutdown :)

tac

sed, tr.

tailf