From man top
-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.
-H : Threads toggle
Starts top with the last remembered 'H' state reversed. When
this toggle is On, all individual threads will be displayed.
Otherwise, top displays a summation of all threads in a
process.
-n : Number of iterations limit as: -n number
Specifies the maximum number of iterations, or frames, top
should produce before ending.
-u : Monitor by user as: -u somebody
Monitor only processes with an effective UID or user name
matching that given.
So it means operate in batch mode (-b
), toggle thread display (-H
), for a single iteration (-n 1
), showing only processes belonging to user mail
(-u mail
) : basically an one-time snapshot of the current threads belonging to mail
, more or less equivalent to ps -Lfu mail