1) useradd = create a new user or updates default new user information
useradd login options
# useradd usr1 — Add new user usr1 with default settings
# useradd usr1 -d /home/usr1 — Create the new user’s home dir in /home
# useradd usr1 -e 2009-04-30 — From 30/4/2009 the user acc will be disabled.
# useradd usr1 -f 6 — After passwd expires, system will allow the user to login for 6 days with a warning to change his passwd.
# useradd usr1 -g staff — Set user1 group as staff
# useradd usr1 -p $1$d8 — Create the usr1 with the given encrypted password. For No passwd, acc disabled.
# useradd usr1 -s /bin/csh — Set C Shell as the default login shell for the usr1
userdel loginname – delete the user account
userdel -r loginname - delete the user account together with user’s home directory and all files inside it
userdel -f loginname – for the deletion even the user is still log in, it will force to delete the user’s home directory and files as well, this option is dangerous use with caution
3) usermod = to modify a users account
# usermod -d /home2/usr1 usr1 — Create the new home Dir for usr1 in /home2 & Move old Dir contents to this Dir.
# usermod -e 2005-04-30 usr1 — From 30/4/2005 the usr1 acc will be disabled.
# usermod -f 6 usr1 — After passwd expires, system will allow the user to login for 6 days with a warning to change his passwd.
# usermod -g prof usr1 — Set usr1′s initial group as prof.
# usermod -p $1$d8 usr1 — Set the new passwd for the usr1
# usermod -s /bin/bash usr1 — Set Bash as the default login shell for the usr1.
# usermod -L usr1 — Lock a user’s password.
# usermod -U usr1 — Unlock a user’s password.
4) groupadd = create a new group
# groupadd sales
$ last — List all entries.
$ last tty6 — List, who are all logged thru tty6.
$ last bharathi — List all entries of bharathi (user).
$ last -6 — List only last 6 entries.
$ last -R — Don’t show the Host Name field in output.
$ last -a – Show Host Name field in last column.
$ last -i – Show IP Number for Remote host.
$ last -t 20080818120000 — List all entries upto Aug 18 12:00:00 2008
6) lastb = display a list of recent bad login attempts (from the /var/log/btmp file). Accepts the same option flags and arguments as last