watch is used to run any designated command at regular intervals. It displays its output on a console (i.e., all-text mode display) or terminal window (i.e., a window in a GUI that emulates a console) that is temporarily cleared of all other content (i.e., prompts, commands and results of commands).

This makes it easy to observe the changing output of a command over time.

Example :

watch -d free -m

watch command will observe the free command and it will highlight the difference for each 2 seconds.

Output:

Every 2.0s: free -m                                                                                                                  Sun Dec 19 01:24:29 2010

             total     used       free     shared    buffers     cached
Mem:          1498     1038        459          0         48        644
-/+ buffers/cache:        345       1152
Swap:         1398          0       1398