# top -b -n 1
# top -b -n 1 | head -n 21
# top -d 2 -n 3 -b > top.txt
将 top 的结果每隔 2 秒，打印 3 次，
-b :Batch-mode operation
Starts top in Batch mode, which could be useful for sending output from top to other programs or to a file. In this mode, top will not accept input and runs until the iterations limit you've set with the `-n' command-line option or until killed.
-d :Delay-time interval as: -d ss.t (secs.tenths)
Specifies the delay between screen updates, and overrides the corresponding value in one's personal configuration file or the startup default. Later this can be changed with the `d' or `s' interactive commands.
-n :Number-of-iterations limit as: -n number
Specifies the maximum number of iterations, or frames, top should produce before ending.
