====== Command ======

===== User and Group =====

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>
  
===== Disk =====

View disk usage:

  ncdu -x -q   # ncdu packages is required

===== rename =====

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


Remove file name prefix:

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

===== grep =====

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

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

===== Reference =====

  * [[https://wiki.archlinux.org/index.php/Users_and_groups|Users and groups - ArchWiki]]
  * [[http://stackoverflow.com/a/7162898|How to 'grep' a continuous stream?]]