How to Use the Linux Top Command fo Show Running Processes

The Linux top command shows the running processes within your Linux environment that consume the most system resources.

In its basic form all you need to do to show the current processes is type the following in a Linux terminal:

top

The top command runs in the foreground and continuously updates itself. Its results show on five lines, plus the main table.

The first line offers some basic high-level info about the system:

• The time
• How long the computer has been running
• Number of users
• Load average

The load average shows the system load time for the last 1, 5 and 15 minutes.

The second line summarizes the number of ongoing, concurrent tasks:

• Total number of tasks
• Number of running tasks
• Number of sleeping tasks
• Number of stopped tasks
• Number of zombie tasks

The third line summarizes CPU performance:

• CPU usage as a percentage by the user
• CPU usage as a percentage by system
• CPU usage as a percentage by low priority processes
• CPU usage as a percentage by idle processes
• CPU usage as a percentage by io wait
• CPU usage as a percentage by hardware interrupts
• CPU usage as a percentage by software interrupts
• CPU usage as a percentage by steal time

The fourth line emphasizes memory:

• Total system memory
• Free memory
• Memory used
• Buffer cache

The fifth line highlights available swap space and total memory inclusive of swap:

• Total swap available
• Total swap free
• Total swap used
• Available memory

The main table lists running processes:

• Process ID
• User
• Priority
• Nice level
• Virtual memory used by process
• Resident memory used by a process
• Shareable memory
• CPU used by process as a percentage
• Memory used by process as a percentage
• Time process has been running
• Command

Although you invoke top just by typing the name in a shell session, a few switches modify the utility's behavior:

• -h: Show the current version
• -c: This toggles the command column between showing command and program name
• -d: Specify the delay time between refreshing the screen
• -o: Sorts by the named field
• -p: Only show processes with specified process IDs
• -u: Show only processes by the specified user
• -i: Do not show idle tasks

Let's explore several ways of using top with various switches.

To specify a delay between the screen refreshes whilst using top type the following:

top -d n

To refresh every 5 seconds type top -d 5

To get a list of the columns with which you can sort the top command by type the following:

top -O

There are a lot of columns so you might wish to pipe the output to less as follows:

top -O | less

Use the previous section to find a column to sort by and then use the following syntax to sort by that column:

top -o

To sort by %CPU type the following:

top -o %CPU

To show only the processes that a specific user is running use the following syntax:

top -u

For example to show all the processes that the user gary is running type the following:

top -u gary

The default top view can seem cluttered and if you want to see only active processes (i.e those that are not idle) then you can run the top command using the following command:

top -i

Whilst running top you can press the 'F' key which shows the list of fields that can be displayed in the table:

Use the arrow keys to move up and down the list of fields.

To set a field so that it is displayed on the screen press the 'D' key. To remove the field press "D" on it again. An asterisk (*) will appear next to displayed fields.

You can set the field to sort the table by simply by pressing the "S" key on the field you wish to sort by.

Press the enter key to commit your changes and press "Q" to quit.

Whilst running top press the A key to toggle between the standard display and an alternate display.

Press the Z key to change the colors of the values within top.

There are three stages required to change the colors:

1. Press either S for summary data, M for messages, H for column headings or T for task information to target that area for a color change
2. Choose a color for that target, 0 for black, 1 for red, 2 for green, 3 for yellow, 4 for blue, 5 for magenta, 6 for cyan and 7 for white
3. Enter to commit.

Press the B key to make text bold.

While the command runs in the foreground, toggle many features on and off by pressing relevant keys.

The following table shows the key to press and the function it provides:

It's not installed on most distributions by default, but the htop utility functions similarly to top but it streamlines the display and ties behavior to realtime function key presses rather than a complex menu of letter-and-switch behavior. Significantly, htop shows all running processes whereas top focuses on the top processes that consume the most system resources.