Processes and signals.
Every running program is a process with a PID, a parent, an owner, and a state. Watch a process move through its five states, learn the signals that push it between them, and the difference between asking it to stop and forcing it.
Linux processes and signals animated tutorial. Process anatomy, the five process states (running, sleeping, uninterruptible, stopped, zombie), job control, the signal catalog, SIGTERM vs SIGKILL, kill/pkill/killall, and nice/renice priority. RHCSA EX200 ready.A process is a running program the kernel tracks by PID. It's always in one of five states, and signals are how you (and the kernel) push it between them.
- ·Comfortable running commands and reading output at the shell
- ·Helpful: the command-line essentials lesson (pipes, ps | grep)
- ·Know roughly what 'the CPU' and 'I/O' mean
Read the STAT column, name the five states, use job control, send the right signal (15 before 9), and adjust priority with nice/renice.
pace: 10 minutes
A process is a running program with an identity
When you launch a program, the kernel creates a process: a PID (its unique number), a PPID (its parent — every process except PID 1 has one), an owner (UID), a state, and the command line. PID 1 is the init system (systemd). `ps` and `top` show all of this. A fresh process is in the Running/Runnable state, R.
$ps aux$ps -ef$top / htop$pgrep -a nginxThe golden rule of killing things: ask nicely, then force. Send SIGTERM (plain kill) first and give the process a couple of seconds to clean up. Only escalate to SIGKILL (-9) if it's unresponsive. Reaching for -9 first is a habit that leaves corrupt files, stale locks, and half-written state behind.
$ps aux / ps -ef$ps -o pid,ppid,stat,ni,comm$top / htop$pgrep -a sshd$pstree -p$kill 1234$kill -9 1234$kill -HUP 1234$kill -l$pkill -f pattern$killall firefox$jobs / fg / bg$nohup cmd & ; disown$nice -n 10 cmd$renice -n 5 -p 1234$/proc/<pid>/statusA process is stuck in state D (uninterruptible sleep) and `kill -9` does nothing. Why?
What's the difference between SIGTERM (15) and SIGKILL (9)?
What is a zombie (Z-state) process?
As a regular (non-root) user, you try `renice -n -5 -p 1234` and it's denied. Why?
These aren't graded — they're just for active recall, which is what actually makes the lesson stick.
Process explorer
Browse a sample process tree, see PIDs, states, and resource use, and practice picking the right signal — without touching a real system.
keep going — these pair well with what you just learned.
SELinux contexts
Why Apache can't read your file. Watch the policy engine decide — and learn the workflow for fixing denials. RHCSA-ready.
Linux boot process
From power button to login prompt. Every stage in the chain — UEFI, GRUB, kernel, initramfs, systemd — with the rescue moves that save you. RHCSA-ready.
LVM stacking
Layer-by-layer build of an LVM stack: disks → PVs → VG → LVs → filesystem. Then extend live and snapshot. RHCSA-ready.