top

Understanding the top Display

When you execute top (simply type top in your terminal and press Enter), you’re presented with a constantly updating display showing various system processes. The default view typically includes:

• Tasks: The number of running, sleeping, and stopped processes.
• CPU Usage: A breakdown of CPU utilization by user processes, system processes, idle time, etc.
• Memory Usage: Information on physical memory (RAM) usage, including free and used amounts.
• Swap Usage: Details about the usage of swap space (if enabled).
• Load Average: A measure of the system load over the past 1, 5, and 15 minutes.
• Process List: A table showing individual processes, sorted by CPU usage by default, displaying their PID (Process ID), USER, PR (Priority), NI (Nice value), VIRT (Virtual Memory), RES (Resident Memory), SHR (Shared Memory), %CPU, %MEM, TIME+, and COMMAND.

Example:

A typical top output might look like this (though the specifics will vary greatly depending on your system):

top - 11:30:12 up  1:23,  1 user,  load average: 0.80, 0.72, 0.65
Tasks: 120 total,   1 running, 119 sleeping,   0 stopped,   0 zombie
%Cpu(s):  1.6 us,  0.8 sy,  0.0 ni, 97.6 id,  0.0 wa,  0.0 hi,  0.0 si,  0.0 st
MiB Mem :   7962.2 total,   6249.1 free,    674.4 used,   1038.7 buff/cache
MiB Swap:    7999.9 total,    7999.9 free,      0.0 used.   6895.7 avail Mem 

  PID USER      PR  NI    VIRT    RES    SHR S  %CPU %MEM     TIME+ COMMAND
 2928 root      20   0  120752  8720  5668 R   5.8  0.1   0:00.23 top
    1 root      20   0  266864  3388  2680 S   0.0  0.0   0:02.96 systemd
    2 root      20   0       0      0      0 S   0.0  0.0   0:00.07 kthreadd

Interacting with top

top offers many interactive features:

• Sorting: Press a column header (e.g., ‘%CPU’, ‘%MEM’) to sort the processes by that column. Press it again to reverse the sort order.
• Filtering: You can filter the process list using the o (lowercase ‘o’) command, followed by the filtering criteria. This is less common and requires more advanced syntax.
• Killing Processes: Using the k command, you can specify a PID to send a signal to a process (often SIGTERM, which requests termination). This can be helpful for stopping runaway processes.
• Interactive Commands: Pressing h or ? will display a list of interactive commands available within top. These commands offer significant control over the displayed information and behavior.
• Exiting top: Press q to exit the top command and return to the terminal prompt.

Example: Killing a Process

Let’s say you want to kill process with PID 12345. You would follow these steps:

1. Run top
2. Locate the process with PID 12345
3. Press k
4. Enter the PID 12345 and press Enter.
5. You might be prompted to confirm which signal to send. Often the default TERM signal is acceptable.

top Command-Line Options

top also accepts various command-line options to customize its behavior:

• -b (batch mode): Runs top in batch mode, suitable for scripting and redirecting output.
• -d (delay): Specifies the delay between updates (in seconds). For instance, top -d 2 updates every 2 seconds.
• -n (iterations): Specifies the number of iterations top runs before exiting. top -n 10 will display 10 iterations.
• -p <PID> (PID): Displays only processes with the specified PIDs. top -p 1234,5678 will display processes with IDs 1234 and 5678.
• -u <user> (user): Displays only processes belonging to the specified user. top -u john shows only processes owned by user john.

Example: Customizing top

To display top output every 5 seconds for 10 iterations:

top -d 5 -n 10

To view only processes belonging to user alice:

top -u alice

This guide provides a solid foundation for using the top command. Experiment with the different options and interactive commands to gain a deeper understanding of your system’s performance characteristics. Remember to use top responsibly, as indiscriminately killing processes can have unintended consequences.