Linux CLI 21 🐧 view and terminate processes - KandZ

Processes are the running applications that consume system resources like CPU and memory. Every process has a unique PID (Process ID), and can be viewed with ps or top, and terminated with kill or pkill.

What Are Processes?

A process is an instance of a running program. Every command you run creates a process.

Key concepts:

Term	Description
Process	A running program consuming resources
PID	Process ID — a unique number assigned at creation
Parent process	The process that started another process
Child process	A process started by another process
Resources	CPU, memory, disk I/O, network, etc.

Key point: A PID is assigned when the process is created and stays the same throughout its life.

Visual — parent and child processes:

┌──────────────────┐
│  bash (PID 100)  │  ← parent
└────────┬─────────┘
         │ starts
    ┌────┴────┐
    ▼         ▼
┌────────┐ ┌────────┐
│ls(PID  │ │vim(PID │  ← children
│ 101)   │ │ 102)   │
└────────┘ └────────┘

Commands Overview

Command	Purpose
`ps`	Snapshot of running processes
`top`	Real-time process viewer
`htop`	Interactive process viewer (better)
`kill`	Terminate a process by PID
`pkill`	Terminate processes by name

The `ps` Command

Process Status — displays information about running processes.

ps aux
ps -u kronos
ps --pid=31009

Command	Description
`ps aux`	All processes with detailed info
`ps -u kronos`	Processes owned by kronos
`ps --pid=31009`	Info about a specific process
`ps -ef`	Alternative format (System V)
`ps -e`	All processes, simple format
`ps aux \| grep nginx`	Filter by name

Understanding `ps aux` Output

$ ps aux
USER       PID %CPU %MEM    VSZ   RSS TTY      STAT START   TIME COMMAND
root         1  0.0  0.1 168336 13212 ?        Ss   Jan14   0:03 /sbin/init
root       512  0.0  0.2  41208  8108 ?        Ss   Jan14   0:00 /usr/sbin/sshd
kronos    1234  0.5  1.2 512345 45678 pts/0    S+   10:30   0:02 vim notes.txt
kronos    5678  2.3  5.6 876543 98765 pts/1    R+   10:35   0:15 firefox

Column	Meaning
USER	User who owns the process
PID	Process ID
%CPU	CPU usage percentage
%MEM	Memory usage percentage
VSZ	Virtual memory size (KB)
RSS	Resident Set Size — physical memory (KB)
TTY	Terminal the process is attached to
STAT	Process state
START	Start time/date
TIME	Total CPU time used
COMMAND	Command that started the process

The STAT Column — Process States

Symbol	Meaning
`R`	Running — actively using CPU
`S`	Sleeping — waiting for an event
`D`	Disk sleep — uninterruptible wait for I/O
`Z`	Zombie — finished but not reaped by parent
`T`	Stopped — paused (Ctrl+Z)
`I`	Idle — idle kernel thread
`<`	High priority
`N`	Low priority
`s`	Session leader
`+`	Foreground process group

Common combinations:

Ss   →  Sleeping, session leader
R+   →  Running, foreground
S+   →  Sleeping, foreground
Z    →  Zombie

Filtering `ps` Output

By user:

$ ps -u kronos
PID TTY          TIME CMD
1234 pts/0    00:00:02 bash
5678 pts/0    00:00:00 vim

By PID:

$ ps --pid=1234
PID TTY          TIME CMD
1234 pts/0    00:00:02 vim

By name (with grep):

$ ps aux | grep nginx
root       812  0.0  0.1  ...  nginx: master process
www-data   813  0.0  0.1  ...  nginx: worker process

Just the name and PID:

$ ps -eo pid,comm
  PID COMMAND
    1 systemd
  512 sshd
 1234 bash

The `top` Command

A real-time process viewer that updates continuously.

top

Sample output:

top - 10:35:22 up 2 days,  3:15,  2 users,  load average: 0.52, 0.48, 0.45
Tasks: 245 total,   1 running, 244 sleeping,   0 stopped,   0 zombie
%Cpu(s):  5.2 us,  1.1 sy,  0.0 ni, 93.5 id,  0.2 wa,  0.0 hi,  0.0 si,  0.0 st
MiB Mem :  16384.0 total,   8124.3 free,   5123.4 used,   3136.3 buff/cache
MiB Swap:   2048.0 total,   2048.0 free,      0.0 used.  10214.0 avail Mem

  PID USER      PR  NI    VIRT    RES    SHR S  %CPU  %MEM     TIME+ COMMAND
 5678 kronos    20   0  876543  98765  12345 R  15.3   5.6   1:32.45 firefox
 1234 kronos    20   0  512345  45678   5678 S   2.5   1.2   0:15.20 vim
    1 root      20   0  168336  13212   8765 S   0.0   0.1   0:03.12 systemd

Understanding the header:

load average — system load over 1, 5, 15 minutes
Tasks — total processes by state
%Cpu(s) — CPU breakdown (us=user, sy=system, id=idle, wa=wait)
MiB Mem — memory usage
MiB Swap — swap usage

Interactive Shortcuts in `top`

Key	Action
`z`	Toggle colors
`1`	Toggle between single and per-CPU view
`f`	Select which fields to show
`s`	Set update interval (seconds)
`h`	Show help
`q`	Quit top
`k`	Kill a process (prompts for PID)
`M`	Sort by memory usage
`P`	Sort by CPU usage
`T`	Sort by time
`u`	Filter by user

Examples in action:

$ top
# Press z → colors on/off
# Press 1 → see each CPU core
# Press f → choose columns (add PIDs, user, etc.)
# Press s → set refresh to 2 seconds
# Press M → sort by memory
# Press q → exit

The `kill` Command

Sends a signal to a process by PID — usually to terminate it.

kill -9 PID
kill -15 PID
kill -l

Command	Description
`kill PID`	Sends SIGTERM (default, signal 15)
`kill -9 PID`	Force kill (SIGKILL)
`kill -15 PID`	Normal terminate (SIGTERM)
`kill -l`	List all available signals
`kill -SIGKILL PID`	Same as `-9` (named signal)
`kill -SIGTERM PID`	Same as `-15`
`kill -1 PID`	SIGHUP — reload configuration
`kill -2 PID`	SIGINT — same as Ctrl+C

Common Signals

Number	Name	Description
`1`	SIGHUP	Hangup / reload config
`2`	SIGINT	Interrupt (Ctrl+C)
`9`	SIGKILL	Force kill (cannot be caught)
`15`	SIGTERM	Normal termination (default)
`18`	SIGCONT	Continue stopped process
`19`	SIGSTOP	Stop process (cannot be caught)
`20`	SIGTSTP	Stop process (Ctrl+Z)

Examples

Normal termination (graceful):

$ kill -15 5678
# or
$ kill 5678
# Process gets a chance to clean up

Force kill (immediate):

$ kill -9 5678
# Process is terminated immediately — no cleanup

Using signal names:

$ kill -SIGTERM 5678
$ kill -SIGKILL 5678

Reload config (for daemons):

$ kill -SIGHUP 812
# Many daemons reload their config on SIGHUP

List all signals:

$ kill -l
 1) SIGHUP       2) SIGINT       3) SIGQUIT      4) SIGILL
 5) SIGTRAP      6) SIGABRT      7) SIGBUS       8) SIGFPE
 9) SIGKILL     10) SIGUSR1     11) SIGSEGV     12) SIGUSR2
13) SIGPIPE     14) SIGALRM     15) SIGTERM     ...

SIGTERM vs SIGKILL

Aspect	SIGTERM (15)	SIGKILL (9)
Cleanup	✅ Process can clean up	❌ No cleanup
Can be caught?	✅ Yes	❌ No
Use case	Normal termination	Forced termination
Preferred?	✅ Try this first	⚠️ Last resort

Best practice:

# Try graceful shutdown first
$ kill -15 5678
# Wait a few seconds
# If still running, force kill
$ kill -9 5678

The `pkill` Command

Process KILL — sends signals to processes by name (or other criteria).

pkill -1 name
pkill -9 name
pkill -15 name
pkill -9 -u user name

Command	Description
`pkill -1 name`	Reload a process (SIGHUP)
`pkill -9 name`	Force kill (SIGKILL)
`pkill -15 name`	Terminate normally (SIGTERM)
`pkill -9 -u user name`	Kill by name for specific user

Examples

Kill by name:

$ pkill firefox
# Kills all processes named "firefox"

Force kill:

$ pkill -9 chrome

Reload a daemon:

$ pkill -1 nginx
# Reloads nginx configuration

Kill processes for a specific user:

$ pkill -9 -u alice firefox
# Only kills firefox processes owned by alice

Kill by pattern:

$ pkill -f "python.*script"
# -f matches against full command line

Common `pkill` Options

Option	Description
`-u user`	Match by user
`-f`	Match against full command line
`-x`	Exact name match
`-n`	Newest process
`-o`	Oldest process
`-c`	Count matching processes
`-e`	Echo what’s killed
`-l`	List signal names

Complete Example Session

# ============================================
# PART 1: VIEWING PROCESSES
# ============================================

# All processes with detail
$ ps aux
USER       PID %CPU %MEM    VSZ   RSS TTY      STAT START   TIME COMMAND
root         1  0.0  0.1 168336 13212 ?        Ss   Jan14   0:03 /sbin/init
root       512  0.0  0.2  41208  8108 ?        Ss   Jan14   0:00 sshd
kronos    1234  0.5  1.2 512345 45678 pts/0    S+   10:30   0:02 vim

# Filter by user
$ ps -u kronos
PID TTY          TIME CMD
1234 pts/0    00:00:02 vim
5678 pts/0    00:00:00 bash

# Specific PID
$ ps --pid=1234
PID TTY          TIME CMD
1234 pts/0    00:00:02 vim

# Filter by name
$ ps aux | grep firefox
kronos    5678  2.3  5.6  ...  firefox

# ============================================
# PART 2: TOP COMMAND
# ============================================

$ top
# Press z → colors
# Press 1 → per-CPU view
# Press M → sort by memory
# Press P → sort by CPU
# Press u → filter by user
# Press k → kill a process
# Press h → help
# Press q → quit

# ============================================
# PART 3: KILL COMMAND
# ============================================

# Find the PID
$ ps aux | grep firefox
kronos    5678  2.3  5.6  ...  firefox

# Try graceful termination
$ kill -15 5678

# If still running, force kill
$ kill -9 5678

# Using signal names
$ kill -SIGTERM 5678
$ kill -SIGKILL 5678

# Reload a daemon
$ sudo kill -SIGHUP 812

# List all signals
$ kill -l
 1) SIGHUP       2) SIGINT       3) SIGQUIT  ...

# ============================================
# PART 4: PKILL COMMAND
# ============================================

# Kill by name
$ pkill firefox

# Force kill all chrome processes
$ pkill -9 chrome

# Reload nginx
$ sudo pkill -1 nginx

# Kill firefox for user alice only
$ pkill -9 -u alice firefox

# Match full command line
$ pkill -f "python.*script.py"

# Count matches without killing
$ pkill -c firefox
5

# See what would be killed
$ pkill -e firefox
firefox killed (pid 5678)
firefox killed (pid 5679)

# ============================================
# PART 5: PRACTICAL SCENARIOS
# ============================================

# Scenario 1: App is frozen
$ ps aux | grep firefox
kronos    5678  99.0  5.6  ...  firefox

$ kill -9 5678
# ✅ Force killed the frozen process

# Scenario 2: Kill all by name
$ pkill -9 chrome
# Kills all chrome processes at once

# Scenario 3: Restart a service gracefully
$ sudo pkill -1 nginx
# Nginx reloads config without dropping connections

# Scenario 4: Find and kill a runaway Python script
$ ps aux | grep python
kronos    9100  95.0  10.0  ...  python data_process.py

$ kill -15 9100
# Give it a chance to save state
# If ignored:
$ kill -9 9100

# Scenario 5: Filter processes by user
$ ps -u alice
$ ps aux | grep "^alice"

# Scenario 6: Real-time monitoring
$ top
# Watch CPU, memory, and process list update

# Scenario 7: Kill a process by pattern
$ pkill -f "java -jar myapp.jar"
# Kills the specific Java app

# Scenario 8: Zombie process
$ ps aux | grep defunct
user  1234  0.0  0.0  ...  [python ] <defunct>
# Zombie — needs to be reaped by parent process

Quick Reference

`ps` Options

Option	Description
`aux`	All processes, detailed
`-ef`	All processes (Unix style)
`-u user`	Filter by user
`--pid=PID`	Specific PID
`-e`	All processes

Process States (STAT)

Symbol	Meaning
`R`	Running
`S`	Sleeping
`D`	Disk sleep
`Z`	Zombie
`T`	Stopped
`I`	Idle
`<`	High priority

`top` Shortcuts

Key	Action
`z`	Colors
`1`	Per-CPU view
`f`	Fields
`s`	Update interval
`h`	Help
`q`	Quit
`k`	Kill process
`M` / `P`	Sort by mem / CPU

Common Signals

Number	Name	Purpose
`1`	SIGHUP	Reload config
`2`	SIGINT	Interrupt (Ctrl+C)
`9`	SIGKILL	Force kill
`15`	SIGTERM	Normal terminate

Best Practices

✅ Do This:

# Try SIGTERM first, then SIGKILL
kill -15 PID
sleep 3
kill -9 PID

# Use ps or top to find the PID
ps aux | grep firefox
# Then kill the specific PID

# Use pkill by name for convenience
pkill firefox

# Reload daemons with SIGHUP
sudo pkill -1 nginx

# Filter by user to avoid killing wrong processes
pkill -u alice firefox

# Check what would be killed before doing it
pkill -c firefox    # Count matches

❌ Don’t Do This:

# Don't jump straight to -9
kill -9 PID        # ❌ Process can't clean up!
kill -15 PID       # ✅ Try this first

# Don't kill PID 1
kill -9 1          # ❌ Kills init — system crash!

# Don't pkill with common names
pkill -9 bash      # ❌ Kills your own shell!
pkill -9 python    # ⚠️ Kills ALL Python processes

# Don't use kill on your own shell
kill -9 $$         # ❌ Kills your current shell

# Don't ignore what ps tells you
ps aux | grep chrome
# Before killing, verify it's really chrome!

Common Pitfalls

Pitfall	Problem	Solution
`kill -9` first	No cleanup	Try `-15` first
Killing PID 1	System crash	Never touch PID 1
`pkill` common name	Kills unrelated processes	Use `-u` or `-f`
Wrong PID	Kills wrong process	Verify with `ps` first
Zombie won’t die	Parent hasn’t reaped	Restart parent
`kill` on systemd service	Doesn’t restart	Use `systemctl stop`

Real-World Examples

1. Kill a Frozen App

# Find the frozen process
$ ps aux | grep firefox
kronos  5678  99.0  ...  firefox

# Try graceful shutdown
$ kill -15 5678

# Force kill if needed
$ kill -9 5678

2. Stop All Chrome Processes

$ pkill chrome
$ pkill -9 chrome

3. Reload Nginx Config

$ sudo pkill -1 nginx
# Nginx re-reads its config without dropping connections

4. Kill Runaway Python Script

$ ps aux | grep python
kronos  9100  95.0  ...  python process.py

$ kill -15 9100
$ kill -9 9100

5. Find Top CPU Consumers

$ ps aux --sort=-%cpu | head -10
# Top 10 CPU hogs

6. Find Top Memory Consumers

$ ps aux --sort=-%mem | head -10
# Top 10 memory hogs

7. Kill Process for Specific User

$ sudo pkill -9 -u alice firefox

8. Live Monitoring

$ top
# Press M → sort by memory
# Press P → sort by CPU
# Press u → filter by user
# Press k → kill a process

9. Find Processes by Terminal

$ ps -t pts/0
# All processes on terminal pts/0

10. Kill Interactive

# From top:
$ top
# Press k
# Enter PID: 5678
# Enter signal: 15
# ✅ Killed

Visual: Process Lifecycle

Creation ────→ Running ────→ Sleeping ────→ Terminated
                │   ▲              │              │
                │   │              │              │
                └───┘              └──────────────┘
              (CPU use)         (waiting for event)

Signals:
  SIGTERM (15)  →  "Please stop"     (can clean up)
  SIGKILL (9)   →  "STOP NOW!"       (cannot be caught)
  SIGHUP  (1)   →  "Reload config"   (for daemons)
  SIGSTOP (19)  →  "Pause"           (cannot be caught)
  SIGCONT (18)  →  "Resume"

Summary

Command	Purpose	Example
`ps aux`	View all processes	`ps aux`
`ps -u user`	Processes by user	`ps -u kronos`
`top`	Real-time viewer	`top`
`kill -15 PID`	Graceful terminate	`kill -15 5678`
`kill -9 PID`	Force kill	`kill -9 5678`
`pkill name`	Kill by name	`pkill firefox`
`kill -l`	List signals	`kill -l`

Key takeaways:

Processes are running programs — each has a unique PID
ps gives a snapshot — use aux for detail
top shows real-time — press q to quit, M/P to sort
kill -15 is the graceful option — try this first
kill -9 is the nuclear option — last resort
pkill kills by name — convenient but be specific (-u, -f)
Signals are communication — SIGHUP reloads, SIGTERM terminates, SIGKILL forces
NEVER kill PID 1 — it’s the init system
Verify the PID before killing — a wrong PID can kill something important

Remember: The process viewer ps and top are your eyes into what’s running. kill and pkill are your hands for stopping it. The golden rule: always try SIGTERM (15) first — it gives the process a chance to save state and clean up. Only use SIGKILL (9) when the process is truly stuck. And never kill a process you don’t understand — especially not PID 1!

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What Are Processes?

Key concepts:

Commands Overview

The ps Command

Understanding ps aux Output

The STAT Column — Process States

Filtering ps Output

The top Command

Interactive Shortcuts in top

The kill Command

Common Signals

Examples

SIGTERM vs SIGKILL

The pkill Command

Examples

Common pkill Options

Complete Example Session

Quick Reference

ps Options

Process States (STAT)

top Shortcuts

Common Signals

Best Practices

Common Pitfalls

Real-World Examples

1. Kill a Frozen App

2. Stop All Chrome Processes

3. Reload Nginx Config

4. Kill Runaway Python Script

5. Find Top CPU Consumers

6. Find Top Memory Consumers

7. Kill Process for Specific User

8. Live Monitoring

9. Find Processes by Terminal

10. Kill Interactive

Visual: Process Lifecycle

Summary

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The `ps` Command

Understanding `ps aux` Output

Filtering `ps` Output

The `top` Command

Interactive Shortcuts in `top`

The `kill` Command

The `pkill` Command

Common `pkill` Options

`ps` Options

`top` Shortcuts