It simply times how long a command takes to run, and gives you 3 statistics:
- Elapsed real time (in seconds).
- Total number of CPU-seconds that the command spent in user mode.
- Total number of CPU-seconds that the command spent in kernel mode.
I use time a lot to benchmark network performance, e.g.,
$ time scp some-file firstname.lastname@example.org:/home/peter/some/location/
You can be creative with time, and run it like this:
$ time cat
What does it do? It starts a timer, and stops when you enter Control D (to terminate the input stream to the cat command). It is a quick timer.