top command

top - display Linux processes

The top command in Linux is a real-time system monitoring tool that displays running processes, CPU, memory, and other system stats. It’s interactive, allowing you to sort, filter, or kill processes directly.

Usage: top [options]

• options: Flags to customize startup behavior (optional).
• Runs interactively by default—exit with q.

Common Fields Explained

Column	Description
PID	Process ID
USER	Process owner
%CPU	CPU usage percentage
%MEM	Memory usage percentage
VIRT	Virtual memory used
RES	Resident (physical) memory used
S	State (R=running, S=sleeping, Z=zombie)
TIME+	Total CPU time used
COMMAND	Command that started the process

Examples

• Basic Usage

  Run top to see a live view of system processes.

  top

  • Output (example):

    top - 10:00:00 up 1 day, 2:00,  2 users,  load average: 0.15, 0.20, 0.25
    Tasks: 123 total,   1 running, 122 sleeping,   0 stopped,   0 zombie
    %Cpu(s):  5.0 us,  2.0 sy,  0.0 ni, 92.0 id,  0.5 wa,  0.0 hi,  0.5 si,  0.0 st
    MiB Mem :  8192.0 total,  4096.0 free,  3072.0 used,  1024.0 buff/cache
    MiB Swap:  2048.0 total,  2048.0 free,     0.0 used
    
      PID USER      PR  NI    VIRT    RES    SHR S  %CPU  %MEM     TIME+ COMMAND
    1234 alice     20   0  500m   200m   100m S   2.0   2.4   0:10.00 firefox
        1 root      20   0  100m    10m     5m S   0.0   0.1   0:05.00 systemd

  • Top Section: System summary (uptime, load, CPU, memory).

  • Bottom Section: Process list (PID, user, CPU/memory usage, command).

• Navigation and Interaction

  top is interactive—use these keys:

  Key	Action
  q	Quit top
  Space	Refresh now
  h or ?	Show help
  k	Kill a process (enter PID)
  f	Manage fields (add/remove columns)
  Shift + P	Sort by CPU usage (default)
  Shift + M	Sort by memory usage
  u	Filter by user (enter username)

  Example (Kill):

  • Press k, enter PID (e.g., 1234), press Enter, choose signal (default 15 for SIGTERM).

• Customizing Display

  Adjust what you see in real time.

  Change Refresh Rate:

  Press d, enter seconds (e.g., 1 for 1-second updates).

  Filter by User:

  Press u, type alice, press Enter—shows only alice’s processes.

  Add Fields:

  Press f, navigate with arrows, toggle columns (e.g., %MEM), press q to save.

• Starting with Options

  Launch top with specific settings.

  Example (User Filter):

  top -u alice
  • Shows only alice’s processes.

  Example (PID Focus):

  top -p 1234
  • Monitors only PID 1234.

  Example (Batch Mode):

  top -b -n 1
  • Runs once (-n 1) in batch mode (-b), outputs to terminal (useful for scripts).

For more details, check the manual with man top