top - Interactive view of system resources and processes

The top program is one of few interactive system inspection commands. It shows real time data about running processes, memory consumption and a basic summary about the system. It really useful to inspect what is currently going on a system ans provides a starter investigate resource consumption. It a interactive program, so after invocation it will show a lot of data which is updated every 3 seconds and meanwhile it waits for further user input.

There are a lot of short cuts to control a running top, most often a single key stroke is enough to trigger an action. Most importantly hitting q quits the top program.

A simple invocation of top shows a header with basic system information and below a table of running processes.

Below is a typical header as displayed by top. Breaking it down line by line already reveals a lot of information.

• top - 12:34:56 up 34 days,  2:51,  1 user,  load average: 60.96, 60.96, 60.80

  • the current system time is 12:34:56

  • the systems is running (uptime) for 34 days and 2h 50m.

  • 1 user is currently logged in

  • the load average is

    • 60.96 over the last minute

    • 60.96 over the last 5 minutes

    • 60.80 over the last 15 minutes

• Tasks: 1409 total,  61 running, 1347 sleeping,   1 stopped,   0 zombie

  • There are 1409 processes of which

  • 61 are running, i.e. are executed right now

  • 1347 are inactive, usually waiting for data or disk I/O, etc.

  • 1 is stopped, usually by a user

  • There are no zombie processes, which is good

• %Cpu(s):  0.0 us, 20.6 sy, 26.2 ni, 52.9 id,  0.0 wa,  0.0 hi,  0.2 si,  0.0 st

  • This a break down of how many time (in percent) the CPUs are occupied which different tasks

    • us: userland processes (with default nice value)

    • sy: system processes

    • ni: userland processes (with non-default nice value)

    • id: idle, i.e. non of the above, \(100 - (\text{us} + \text{sy} + \text{ni})\)

  • The next values are for system administrators indicating time spent on handling interrupts (hi, si) and waiting for I/O (wa, st)

top - 12:34:56 up 34 days,  2:50,  1 user,  load average: 60.96, 60.96, 60.80
Tasks: 1409 total,  61 running, 1347 sleeping,   1 stopped,   0 zombie
%Cpu(s):  0.0 us, 20.6 sy, 26.2 ni, 52.9 id,  0.0 wa,  0.0 hi,  0.2 si,  0.0 st
MiB Mem : 2052001.+total, 1514273.+free, 233328.6 used, 304399.2 buff/cache
MiB Swap: 210215.0 total, 210206.5 free,      8.5 used. 1808182.+avail Mem 

There is a more sophisticated version of top called htop the functionality is similar but the user interface is a bit simpler. Also nvtop is similar but for GPUs.