Using top and htop

Using top and htop

top and htop display real-time process information with auto-refreshing output. They show CPU usage, memory, load average, and per-process statistics in a continuously updated view. top is always available on any Ubuntu system; htop is a more user-friendly alternative that must be installed but offers color, mouse support, and easier process management.

top: the built-in monitor

# Start top (updates every 3 seconds by default)
top

# Start with a specific user's processes
top -u mysql

# Start sorted by memory instead of CPU
top -o %MEM

# Run for a specified number of iterations and exit
top -b -n 3    # Batch mode, 3 iterations — useful for scripting

Understanding the top header

top header — every field matters

top - 14:22:33 up 5 days,  2:10,  2 users,  load average: 0.85, 0.72, 0.68
Tasks: 187 total,   2 running, 184 sleeping,   0 stopped,   1 zombie
%Cpu(s):  5.2 us,  1.0 sy,  0.0 ni, 92.5 id,  1.0 wa,  0.0 hi,  0.3 si
MiB Mem :  15855.6 total,   4282.5 free,   5320.1 used,   6253.0 buff/cache
MiB Swap:   2048.0 total,   2048.0 free,      0.0 used.   9812.3 avail Mem

top interactive commands

htop: the enhanced version

# Install htop
sudo apt install -y htop

# Start htop
htop

# Start filtered for a user
htop -u mysql

# Start in tree view (shows process hierarchy)
htop -t

htop advantages over top:

• Color-coded CPU, memory, swap bars for each core
• Mouse support: click column headers to sort, click processes to select
• Tree view: shows parent-child relationships
• F-key shortcuts prominently displayed at bottom
• F9 to kill selected process
• F5 to toggle tree view
• Space to mark multiple processes, then kill all at once

Non-interactive monitoring

# Capture top output to a file (batch mode)
top -b -n 1 > /tmp/top-snapshot.txt

# Log top output every 30 seconds
while true; do
    top -b -n 1 | head -20 >> /var/log/top-monitor.log
    sleep 30
done

# One-line CPU usage summary from top
top -b -n 1 | grep "Cpu(s)"

# Watch specific processes
watch -n 2 "ps aux | grep mysqld | grep -v grep"

Conclusion

Use top for a quick real-time overview when htop is not installed. Press 1 to see per-CPU usage, M to sort by memory, and c to see full command paths. Install htop for daily use — it is more readable, supports mouse interaction, and the tree view makes process relationships immediately visible. On a server where everything looks fine in top but something is slow, check the wa (iowait) percentage and the load average relative to the number of CPU cores.