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

? 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.