Necessary Command for Linux

http://ss64.com/bash/
http://www.computerhope.com/unix/overview.htm
http://www.mediacollege.com/linux/command/linux-command.html
http://www.tomax7.com/mcse/linux_commands2.htm

For  Show User List :

# cat /etc/passwd |grep 500*

For Linux DNS Cache Clear:

# Service nscd restart
# /etc/init.d/nscd restart 

For Showing system uptime information

ian@attic4:~$ uptime
 17:41:17 up 20:03,  5 users,  load average: 0.00, 0.00, 0.00

For showing free memory information

The free command displays the amount of free and used memory in your system. By default the display is in kilobytes, but you can override this using -b for bytes, -k for kilobytes, -m for megabytes, or -g for gigabytes. The -t option displays a total line, and the -s option along with a value refreshes the info with the frequency specified. The number is in seconds but may be a floating point value. Listing 15 shows two examples.

ian@attic4:~$ free
             total       used       free     shared    buffers     cached
Mem:       4057976    1543164    2514812          0     198592     613488
-/+ buffers/cache:     731084    3326892
Swap:     10241428          0   10241428
ian@attic4:~$ free -mt
             total       used       free     shared    buffers     cached
Mem:          3962       1506       2456          0        193        599
-/+ buffers/cache:        713       3249
Swap:        10001          0      10001
Total:       13964       1506      12457