The Linux top command is one of the stalwarts of Unix-like operating systems. Its utilitarian display is packed with useful information about your system's running processes and resource usage. But, did you know it also supports color, highlighting, and even elementary graphs?
An Information-Packed Dashboard
The top command has been around since 1984, and there are many variants of it. For this article, we're running Ubuntu 18.04 with all patches applied, and version 3.3.12 of top. As of Ubuntu 23.04, very little has changed. We also cross-referenced everything on two other test computers, one running Fedora and the other Manjaro.
The default top display crams as much information as possible in the terminal window. Information is a prerequisite to administration, so this is a good thing. One of the traits of a good system administrator is the ability to identify emerging problems and deal with them before they affect service. top gives you a dashboard of many different system metrics that help you do just that.
The display is best described as functional, rather than intuitive, and abbreviations abound. When you first encounter top, it feels cramped, cryptic, and off-putting. With a few keypresses, though, you can tune the contents and format of the display according to what's important to you.
Check Your Version of top
To check which version of top you have, type the following a terminal window:
top -v
If your version is way behind 3.3.12, it might not support all the features we're going to cover.
The Default Display
You can start top by typing the following and hitting "Enter":
top
The default display contains two areas of information: the summary area (or dashboard), and the task area (or process list). By default, top updates its display every three seconds---you'll notice a slight flicker when it does.
The first line of numbers on the dashboard includes the time, how long your computer has been running, the number of people logged in, and what the load average has been for the past one, five, and 15 minutes. The second line shows the number of tasks and their states: running, stopped, sleeping, or zombie.
The third line displays the following central processing unit (CPU) values:
- us: Amount of time the CPU spends executing processes for people in "user space."
- sy: Amount of time spent running system "kernel space" processes.
- ni: Amount of time spent executing processes with a manually set nice value.
- id: Amount of CPU idle time.
- wa: Amount of time the CPU spends waiting for I/O to complete.
- hi: Amount of time spent servicing hardware interrupts.
- si: Amount of time spent servicing software interrupts.
- st: Amount of time lost due to running virtual machines ("steal time").
The fourth line shows the total amount (in kibibytes) of physical memory, and how much is free, used, and buffered or cached.
The fifth line shows the total amount (also in kibibytes) of swap memory, and how much is free, used, and available. The latter includes memory that's expected to be recoverable from caches.
The column headings in the process list are as follows:
- PID: Process ID.
- USER: The owner of the process.
- PR: Process priority.
- NI: The nice value of the process.
- VIRT: Amount of virtual memory used by the process.
- RES: Amount of resident memory used by the process.
- SHR: Amount of shared memory used by the process.
- S: Status of the process. (See the list below for the values this field can take).
- %CPU: The share of CPU time used by the process since the last update.
- %MEM: The share of physical memory used.
- TIME+: Total CPU time used by the task in hundredths of a second.
- COMMAND: The command name or command line (name + options).
Memory values are shown in kibibytes. The COMMAND column is off-screen, to the right---it didn't fit in the image above, but we'll see it shortly.
The status of the process can be one of the following:
- D: Uninterruptible sleep
- R: Running
- S: Sleeping
- T: Traced (stopped)
- Z: Zombie
Press Q to exit top.
Scrolling the Display
You can press the Up or Down Arrows, Home, End, and Page Up or Down keys to move up and down and access all the processes.
Press the Left or Right Arrow to move the process list sideways. This is useful to see any columns that don't fit within the confines of the terminal window.
In the image below, we've pressed the Right Arrow a few times to see the COMMAND column.
Changing the Numeric Units
Let's change the display units to sensible values. Press capital E to cycle through the units used to display memory values in these options: kibibytes, mebibytes, gibibytes, tebibytes, pebibytes, and exbibytes. The unit in use is the first item on lines four and five.
Press lowercase "e" to do the same thing for the values in the process list: kibibytes, mebibytes, gibibytes, tebibytes, and pebibytes.
We pressed E to set the dashboard memory units to gibibytes and "e" to set the process list memory units to mebibytes.
Changing the Summary Contents
You can change the display settings for the lines in the dashboard or remove them completely.
Press l to toggle the load summary line (the first line) on or off. We removed the load summary line in the image below.
If you have a multi-core CPU, press 1 to change the display and see individual statistics for each CPU. There are four CPUs on our computer. We press 1 to see how hard each of them is working.
Of course, this takes up more screen real estate within the terminal window.
You can press "t" to swap the CPU displays to simple ASCII graphs that show the percentage of usage for each CPU.
For each CPU, top displays three numbers and the graph. From left to right, the numbers are as follows:
-
The combined
usandnipercentage (user space+tasks with nonstandard nice settings). -
The
sypercentage (kernel space). - The total (rounded to an integer value).
Press "t" again to change the graph display to solid block characters.
Press "t" once more to remove the CPU display and task summary line completely.
Press "m" to cycle the memory and swap memory lines through different display options. The first press replaces the statistics with an ASCII graph.
Another press changes the graph to block characters.
Press "m" once more to remove the CPU lines altogether.
If you want, you can see CPU and memory graphs at the same time. Just press "t" and "m" until you get the combination you want.
Color and Highlighting
You can press "z" to add color to the display.
When you think of top, you probably don't think of colored displays and ASCII graphs, but they're built right in.
Press "y" to highlight running tasks in the process list. Pressing "x" highlights the column used to sort the process list. You can toggle between bold and reversed text highlighting by pressing "b."
Sorting by Columns
By default, the process list is sorted by the %CPU column. You can change the sort column by pressing the following:
-
P: The
%CPUcolumn. -
M: The
%MEMcolumn. -
N: The
PIDcolumn. -
T: The
TIME+column.
In the image below, the process list is sorted by the PID column.
See the Full Command Line
Pressing "c" toggles the COMMAND column between displaying the process name and the full command line.
To see a "tree" of processes that were launched or spawned by other processes, press V.
See Processes for a Single User
Press "u" to see the processes for a single user. You'll be prompted for the name or UID.
Type the name of the UID of the person you want to monitor. We'll type "dave" and hit "Enter." Now, the only processes in the task area belong to the user dave.
