Understanding 'top' Command Output Fields in Linux

Real-Time Process Monitoring

The top program is a dynamic view of the system’s processes, displaying a summary header followed by a process or thread list similar to ‘ps’ information. Unlike the static ps output, top continuously refreshes at a configurable interval, and provides capabilities for column reordering, sorting, and highlighting. User configurations can be saved and made persistent.

Default output columns are recognizable from other resource tools:

• The process ID (PID).

• User name (USER) is the process owner.

• Virtual memory (VIRT) is all memory the process is using, including the resident set, shared libraries, and any mapped or swapped memory pages. (Labeled VSZ in the ps command.)

• Resident memory (RES) is the physical memory used by the process, including any resident shared objects. (Labeled RSS in the ps command

• Process state (S) displays as:

  • D = Uninterruptible Sleeping
  • R = Running or Runnable
  • S = Sleeping
  • T = Stopped or Traced
  • Z = Zombie

• CPU time (TIME) is the total processing time since the process started. May be toggled to include cumulative time of all previous children.

• The process command name (COMMAND).

Fundamental Keystrokes in top

KEY	PURPOSE
? Or H	Help for interactive keystrokes.
L,T,M	Toggles for load, threads, and memory header lines.
1	Toggle showing individual CPUs or a summary for all CPUs in header.
S	Change the refresh (screen) rate, in decimal seconds (e.g., 0.5, 1, 5).
B	Toggle reverse highlighting for Running processes; default is bold only.
B	Enables use of bold in display, in the header, and for Running processes.
Shift+H	Toggle threads; show process summary or individual threads.
U, Shift+U	Filter for any user name (effective, real).
Shift+M	Sorts process listing by memory usage, in descending order.
Shift+P	Sorts process listing by processor utilization, in descending order.
K	Kill a process. When prompted, enter PID, then signal.
R	Renice a process. When prompted, enter PID, then nice_value.
Shift+W	Write (save) the current display configuration for use at the next top restart.
Q	Quit.