Add a user:

useradd [username]
passwd [username]   # set user login password

List all groups on this system:

cut -d: -f1 /etc/group

List user's groups:

groups

Add a user to group:

gpasswd -a <userName> <groupName>

Remove user from group:

gpasswd -d <userName> <groupName>

Reload group without logout:

newgrp <new group name>

View disk usage:

ncdu -x -q   # ncdu packages is required

Usage:
rename [options] <expression> <replacement> <file>…

Remove file name prefix:

rename -v "[filenamePrefix-]" "" ./*

'grep' a continuous stream (e.g. tail -f)

tail -f file | grep --line-buffered my_pattern

• Users and groups - ArchWiki
• How to 'grep' a continuous stream?