CPU Utilization Monitoring

The top command provides a dynamic, real-time view of system processes and resource usage. Press q or Ctrl+C to exit.

The initial five lines present overall system statistics.

Line 1: System Summary This line mirrors the output of uptime.

• Current time (e.g., 10:59:33).
• Uptime: System runtime in days, hours:minutes (e.g., up 88 days, 14:39).
• Logged-in users: Number of users currently logged in.
• Load averages: Average system load over the last 1, 5, and 15 minutes.

Lines 2 & 3: Process and CPU Statistics

• Tasks: Total number of processes.
• Running: Processes currently executing.
• Sleeping: Processes in a sleep state.
• Stopped: Processes that have been halted.
• Zombie: Defunct (zombie) processes.

CPU usage percentages are broken down as follows:

• us: Percentage of CPU time spent in user space.
• sy: Percentage of CPU time spent in kernel space.
• ni: Percentage of CPU time used by processes with altered priority (nice value).
• id: Percentage of idle CPU time.
• wa: Percentage of CPU time spent waiting for I/O operations.
• hi: Percentage of CPU time spent handling hardware interrupts.
• si: Percentage of CPU time spent handling software interrupts.
• st: Steal time percentage, indicating CPU time a virtual machine waits for the hypervisor. High values may suggest resource contention on virtualized systems.

Lines 4 & 5: Memory Information

• KiB Mem: Physical memory statistics.
  • total: Total installed memory.
  • free: Unused memory.
  • used: Memory currently in use.
  • buff/cache: Memory used by kernel buffers and page cache.
• KiB Swap: Swap space statistics.
  • total: Total swap space.
  • free: Unused swap space.
  • used: Swap space currently utilized.
  • avail Mem: An estimate of memory available for starting new applications without swapping.

Process List Details The lower section lists individual processes with columns including:

• PID: Process identifier.
• USER: Process owner.
• PR/NI: Priority and nice value.
• VIRT: Total virtual memory used (VIRT = SWAP + RES).
• RES: Resident (non-swapped) physical memory used.
• SHR: Shared memory size.
• S: Process state (e.g., R=Running, S=Sleeping, Z=Zombie).
• %CPU/%MEM: CPU and physical memory usage percentages.
• TIME+: Total CPU time consumed.
• COMMAND: Command name or line.

Checking Memory Usage

The free command displays system memory usage.

free

Key output fields:

• total: Total physical RAM.
• used: Memory currently in use.
• free: Unallocated memory.
• shared: Memory used by tmpfs (shared memory).
• buff/cache: Sum of memory used by kernel buffers and the page cache.
• available: Estimate of memory available for new applications.

Use flags for human-readable output:

free -h  # Auto-scales units (KB, MB, GB)
free -g  # Shows values in gigabytes
free -m  # Shows values in megabytes

Monitoring Disk Space

The df command reports filesystem disk space usage.

df -h

Output columns:

• Filesystem: The disk partition or filesystem.
• Size: Total capacity of the partition.
• Used: Amount of space consumed.
• Avail: Free space available.
• Use%: Percentage of used space.
• Mounted on: The mount point directory.

Common df and du (disk usage) command variations:

df -hl          # Lists free disk space

du -sh /path/to/dir  # Shows total size of a specific directory

du -sm /path/to/dir  # Shows directory size in megabytes

du -h /path/to/dir   # Shows sizes of all items within a directory recursively

Port and Network Socket Inspection

To check if a specific port is in use:

netstat -anp | grep :3306

To list all listening TCP and UDP ports:

netstat -nultp

If a port is occupied, you can terminate the associated process using its PID.

Viewing System Processes

The ps command provides a snapshot of current processes.

To see all running processes:

ps aux

Common ps options for process discovery:

• ps a: Shows processes from all users on the current terminal.
• ps -A or ps -e: Displays all process on the system.
• ps -f or ps -H: Displays a forest (tree) view showing parent-child process relationships.
• ps u: Outputs in a user-oriented format.
• ps x: Includes processes not attached to a terminal (daemons).
• ps -p <PID>: Shows details for a specific process ID.