Observing Processes

Linux exposes processes through the /proc filesystem. The user-space tools below read /proc.

ps — Process Snapshot

ps prints a one-time list of processes. Common forms:

• ps — only your terminal's processes.
• ps -e or ps -A — every process.
• ps -ef — full format.
• ps aux — BSD form, with %CPU and %MEM.
• ps -o pid,ppid,user,stat,cmd — custom columns.

The STAT column codes: R running, S sleeping, D uninterruptible, Z zombie, T stopped.

top and htop — Live Monitor

top refreshes every few seconds. Inside, press P to sort by CPU, M by memory, k to kill, r to renice, q to quit.

htop is a friendlier alternative with colour bars and tree view (F5).

pstree — Process Hierarchy

pstree shows the parent-child tree, useful to understand which shell launched what.

Process Priorities

Linux schedules with two related concepts:

• Nice value — user-tunable, range -20 (highest priority) to +19 (lowest). Default 0.
• Priority — actual scheduling priority computed by the kernel.

nice — Launch with Reduced Priority

nice -n 10 ./long_job.sh starts the script with niceness 10 — friendlier to other processes. Only the superuser can request a negative niceness.

renice — Change a Running Process's Priority

renice +5 -p 1234 raises the niceness of PID 1234 by 5 (lower priority). Use -u user to renice all processes of a user.

Sending Signals

kill PID sends SIGTERM. kill -9 PID sends SIGKILL (cannot be caught). pkill name matches by name. killall name kills all matching.

Worked Example

Find the heaviest CPU consumer and lower its priority:

1. ps -eo pid,pcpu,cmd --sort=-pcpu | head
2. Note the PID, say 5678.
3. renice +10 -p 5678 — gentler scheduling.
4. top -p 5678 — confirm the change.

Summary

• ps snapshot, top/htop live, pstree hierarchy.
• nice/renice tune scheduling priority via the niceness knob.
• kill sends signals; -9 is the unstoppable SIGKILL.