Phase 5 — Processes and Services

Previous: Phase 4 — Users and Permissions | Next: Phase 6 — Networking

5.1 What Is a Process?

When the kernel runs a program, it creates a process. A process is the running instance of a program. A single program can have many processes running simultaneously (for example, multiple browser tabs as separate processes).

Every process has:

A unique PID (Process ID)
A PPID (Parent Process ID) — the process that started it
An owner (UID) — the user it runs as
A state: running, sleeping, stopped, zombie
File descriptors: open files, sockets, pipes

5.2 Viewing Processes

# ps: snapshot of current processes
ps                          # processes in current terminal session
ps aux                      # all processes, all users, with details
# a = all users, u = user-friendly format, x = include processes without terminal
Understanding ps aux output:
USER    PID %CPU %MEM    VSZ   RSS TTY   STAT START   TIME COMMAND
root      1  0.0  0.3 167916  6480 ?     Ss   Jan10   0:03 /sbin/init
alice  1823  2.1  1.5 456320 30240 pts/0 S+   10:22   0:01 python3 server.py
VSZ = virtual memory size, RSS = resident (actual RAM used)
STAT: S=sleeping, R=running, Z=zombie, T=stopped, I=idle
The + in S+ means it is in the foreground of a terminal
Filter for a specific process
ps aux | grep nginx
ps -p 1823                  # show process with specific PID
top: live, updating view
top
Keys inside top:
q         quit
k         kill a process (enter PID)
u         filter by user
M         sort by memory
P         sort by CPU
1         show per-CPU statistics
htop: improved version of top (install if not present)
sudo apt install htop
htop

5.3 Signals

Signals are notifications sent to processes. A process can catch and handle most signals, but SIGKILL and SIGSTOP cannot be caught.

Signal	Number	Meaning
SIGTERM	15	Polite request to terminate (process can clean up)
SIGKILL	9	Force kill, cannot be caught or ignored
SIGHUP	1	Reload configuration (many daemons respond to this)
SIGINT	2	Interrupt (same as Ctrl+C in terminal)
SIGSTOP	19	Pause the process
SIGCONT	18	Resume a paused process

# Send a signal by PID
kill 1823              # SIGTERM by default (polite)
kill -9 1823           # SIGKILL (force)
kill -HUP 1823         # SIGHUP (reload config)

# Kill by name (more convenient)
pkill nginx            # SIGTERM to all nginx processes
pkill -9 firefox       # force kill firefox

# Kill all processes of a user
pkill -u baduser

# Send SIGHUP to nginx to reload config without restart
sudo pkill -HUP nginx

5.4 Foreground and Background Jobs

# Start a process in the background
python3 server.py &
# Output: [1] 2847     (job number and PID)
List background jobs in current shell
jobs
[1]+  Running    python3 server.py &
Bring a background job to foreground
fg              # bring most recent job to foreground
fg %1           # bring job number 1 to foreground
Send current foreground process to background
First: Ctrl+Z (suspends it)
Then:
bg              # resume it in background
bg %1           # resume job 1 in background
nohup: run a command that survives terminal close
nohup python3 server.py > server.log 2>&1 &
Output goes to server.log, process continues if you log out

5.5 systemd — The Init System

Modern Linux systems use systemd as PID 1. It starts all services on boot and manages them throughout the system's life.

# Service management
sudo systemctl start nginx
sudo systemctl stop nginx
sudo systemctl restart nginx
sudo systemctl reload nginx         # reload config without full restart
Enable/disable on boot
sudo systemctl enable nginx         # start on boot
sudo systemctl disable nginx        # do not start on boot
sudo systemctl enable --now nginx   # enable AND start right now
Check status
systemctl status nginx
Output looks like this:
nginx.service - A high performance web server
Loaded: loaded (/lib/systemd/system/nginx.conf; enabled)
Active: active (running) since Mon 2024-01-15 10:22:00 UTC
Process: 1823 ExecStart=/usr/sbin/nginx (code=exited, status=0/SUCCESS)
Main PID: 1824 (nginx)
Tasks: 2 (limit: 4915)
Memory: 6.2M
CPU: 42ms
CGroup: /system.slice/nginx.service
├─1824 nginx: master process /usr/sbin/nginx
└─1825 nginx: worker process
List all services
systemctl list-units --type=service
systemctl list-units --type=service --state=failed
View service logs
journalctl -u nginx                      # all logs for nginx
journalctl -u nginx --since "1 hour ago"
journalctl -u nginx -f                   # follow live logs
journalctl -b                            # all logs since last boot
journalctl -p err                        # only error and above

Writing a simple systemd service:

# Create /etc/systemd/system/myapp.service
sudo nano /etc/systemd/system/myapp.service

[Unit]
Description=My Python Application
After=network.target
[Service]
Type=simple
User=alice
WorkingDirectory=/home/alice/myapp
ExecStart=/usr/bin/python3 /home/alice/myapp/server.py
Restart=on-failure
RestartSec=5
[Install]
WantedBy=multi-user.target

# Reload systemd to pick up new unit file, then start
sudo systemctl daemon-reload
sudo systemctl enable --now myapp

5.6 Scheduling with cron

cron runs commands at scheduled times. The schedule is defined in a crontab file.

# Edit your crontab
crontab -e
List your crontab
crontab -l
Edit another user's crontab (as root)
sudo crontab -e -u alice

Crontab format — five time fields followed by the command:

# MIN  HOUR  DAY_OF_MONTH  MONTH  DAY_OF_WEEK  COMMAND
# 0-59 0-23  1-31          1-12   0-7 (0,7=Sun)
Run every day at 2:30am
30 2 * * * /usr/local/bin/backup.sh
Run every Monday at 8am
0 8 * * 1 /usr/local/bin/weekly_report.sh
Run every 15 minutes
*/15 * * * * /usr/local/bin/check_status.sh
Run at midnight on the 1st of every month
0 0 1 * * /usr/local/bin/monthly_cleanup.sh
Redirect cron output to a log file
0 2 * * * /usr/local/bin/backup.sh >> /var/log/backup.log 2>&1

Phase 5 Exercises

Exercise 1: Run ps aux --sort=-%cpu | head -15. Identify the top three processes by CPU. What do they do?

Exercise 2: Start sleep 300 & to create a background job. Use jobs to see it. Use fg to bring it foreground. Use Ctrl+Z to suspend it. Use bg to resume in background. Use kill %1 to terminate it.

Exercise 3: Check the status of the ssh service with systemctl status ssh. Note whether it is enabled on boot.

Exercise 4: Write a crontab entry that runs the command date >> /tmp/datelog.txt every minute. Wait 3 minutes, then check /tmp/datelog.txt. Then remove the crontab entry.

Previous: Phase 4 — Users and Permissions | Next: Phase 6 — Networking

# Phase 5 — Processes and Services

Previous: [Phase 4 — Users and Permissions](Phase-4-Users-and-Permissions) | Next: [Phase 6 — Networking](Phase-6-Networking)

5.1 What Is a Process?

Every process has:

A unique PID (Process ID)
A PPID (Parent Process ID) — the process that started it
An owner (UID) — the user it runs as
A state: running, sleeping, stopped, zombie
File descriptors: open files, sockets, pipes

5.2 Viewing Processes

# ps: snapshot of current processes
ps                          # processes in current terminal session
ps aux                      # all processes, all users, with details
# a = all users, u = user-friendly format, x = include processes without terminal

# Understanding ps aux output:
# USER    PID %CPU %MEM    VSZ   RSS TTY   STAT START   TIME COMMAND
# root      1  0.0  0.3 167916  6480 ?     Ss   Jan10   0:03 /sbin/init
# alice  1823  2.1  1.5 456320 30240 pts/0 S+   10:22   0:01 python3 server.py

# VSZ = virtual memory size, RSS = resident (actual RAM used)
# STAT: S=sleeping, R=running, Z=zombie, T=stopped, I=idle
# The + in S+ means it is in the foreground of a terminal

# Filter for a specific process
ps aux | grep nginx
ps -p 1823                  # show process with specific PID

# top: live, updating view
top
# Keys inside top:
# q         quit
# k         kill a process (enter PID)
# u         filter by user
# M         sort by memory
# P         sort by CPU
# 1         show per-CPU statistics

# htop: improved version of top (install if not present)
sudo apt install htop
htop

5.3 Signals

Signals are notifications sent to processes. A process can catch and handle most signals, but SIGKILL and SIGSTOP cannot be caught.

Signal	Number	Meaning
SIGTERM	15	Polite request to terminate (process can clean up)
SIGKILL	9	Force kill, cannot be caught or ignored
SIGHUP	1	Reload configuration (many daemons respond to this)
SIGINT	2	Interrupt (same as Ctrl+C in terminal)
SIGSTOP	19	Pause the process
SIGCONT	18	Resume a paused process

# Send a signal by PID
kill 1823              # SIGTERM by default (polite)
kill -9 1823           # SIGKILL (force)
kill -HUP 1823         # SIGHUP (reload config)

# Kill by name (more convenient)
pkill nginx            # SIGTERM to all nginx processes
pkill -9 firefox       # force kill firefox

# Kill all processes of a user
pkill -u baduser

# Send SIGHUP to nginx to reload config without restart
sudo pkill -HUP nginx

5.4 Foreground and Background Jobs

# Start a process in the background
python3 server.py &
# Output: [1] 2847     (job number and PID)

# List background jobs in current shell
jobs
# [1]+  Running    python3 server.py &

# Bring a background job to foreground
fg              # bring most recent job to foreground
fg %1           # bring job number 1 to foreground

# Send current foreground process to background
# First: Ctrl+Z (suspends it)
# Then:
bg              # resume it in background
bg %1           # resume job 1 in background

# nohup: run a command that survives terminal close
nohup python3 server.py > server.log 2>&1 &
# Output goes to server.log, process continues if you log out

5.5 systemd — The Init System

Modern Linux systems use systemd as PID 1. It starts all services on boot and manages them throughout the system's life.

# Service management
sudo systemctl start nginx
sudo systemctl stop nginx
sudo systemctl restart nginx
sudo systemctl reload nginx         # reload config without full restart

# Enable/disable on boot
sudo systemctl enable nginx         # start on boot
sudo systemctl disable nginx        # do not start on boot
sudo systemctl enable --now nginx   # enable AND start right now

# Check status
systemctl status nginx

# Output looks like this:
# nginx.service - A high performance web server
#   Loaded: loaded (/lib/systemd/system/nginx.conf; enabled)
#   Active: active (running) since Mon 2024-01-15 10:22:00 UTC
#  Process: 1823 ExecStart=/usr/sbin/nginx (code=exited, status=0/SUCCESS)
# Main PID: 1824 (nginx)
#   Tasks: 2 (limit: 4915)
#   Memory: 6.2M
#      CPU: 42ms
#   CGroup: /system.slice/nginx.service
#           ├─1824 nginx: master process /usr/sbin/nginx
#           └─1825 nginx: worker process

# List all services
systemctl list-units --type=service
systemctl list-units --type=service --state=failed

# View service logs
journalctl -u nginx                      # all logs for nginx
journalctl -u nginx --since "1 hour ago"
journalctl -u nginx -f                   # follow live logs
journalctl -b                            # all logs since last boot
journalctl -p err                        # only error and above

Writing a simple systemd service:

# Create /etc/systemd/system/myapp.service
sudo nano /etc/systemd/system/myapp.service

[Unit]
Description=My Python Application
After=network.target

[Service]
Type=simple
User=alice
WorkingDirectory=/home/alice/myapp
ExecStart=/usr/bin/python3 /home/alice/myapp/server.py
Restart=on-failure
RestartSec=5

[Install]
WantedBy=multi-user.target

# Reload systemd to pick up new unit file, then start
sudo systemctl daemon-reload
sudo systemctl enable --now myapp

5.6 Scheduling with cron

cron runs commands at scheduled times. The schedule is defined in a crontab file.

# Edit your crontab
crontab -e

# List your crontab
crontab -l

# Edit another user's crontab (as root)
sudo crontab -e -u alice

Crontab format — five time fields followed by the command:

# MIN  HOUR  DAY_OF_MONTH  MONTH  DAY_OF_WEEK  COMMAND
# 0-59 0-23  1-31          1-12   0-7 (0,7=Sun)

# Run every day at 2:30am
30 2 * * * /usr/local/bin/backup.sh

# Run every Monday at 8am
0 8 * * 1 /usr/local/bin/weekly_report.sh

# Run every 15 minutes
*/15 * * * * /usr/local/bin/check_status.sh

# Run at midnight on the 1st of every month
0 0 1 * * /usr/local/bin/monthly_cleanup.sh

# Redirect cron output to a log file
0 2 * * * /usr/local/bin/backup.sh >> /var/log/backup.log 2>&1

Phase 5 Exercises

Exercise 1: Run ps aux --sort=-%cpu | head -15. Identify the top three processes by CPU. What do they do?

Exercise 3: Check the status of the ssh service with systemctl status ssh. Note whether it is enabled on boot.

Exercise 4: Write a crontab entry that runs the command date >> /tmp/datelog.txt every minute. Wait 3 minutes, then check /tmp/datelog.txt. Then remove the crontab entry.

Previous: [Phase 4 — Users and Permissions](Phase-4-Users-and-Permissions) | Next: [Phase 6 — Networking](Phase-6-Networking)

5. processes and services - mishraxharshit/harshitxmishra.github.io GitHub Wiki

Phase 5 — Processes and Services

5.1 What Is a Process?

5.2 Viewing Processes

Understanding ps aux output:

USER PID %CPU %MEM VSZ RSS TTY STAT START TIME COMMAND

root 1 0.0 0.3 167916 6480 ? Ss Jan10 0:03 /sbin/init

alice 1823 2.1 1.5 456320 30240 pts/0 S+ 10:22 0:01 python3 server.py

VSZ = virtual memory size, RSS = resident (actual RAM used)

STAT: S=sleeping, R=running, Z=zombie, T=stopped, I=idle

The + in S+ means it is in the foreground of a terminal

Filter for a specific process

top: live, updating view

Keys inside top:

q quit

k kill a process (enter PID)

u filter by user

M sort by memory

P sort by CPU

1 show per-CPU statistics

htop: improved version of top (install if not present)

5.3 Signals

5.4 Foreground and Background Jobs

List background jobs in current shell

[1]+ Running python3 server.py &

Bring a background job to foreground

Send current foreground process to background

First: Ctrl+Z (suspends it)

Then:

nohup: run a command that survives terminal close

Output goes to server.log, process continues if you log out

5.5 systemd — The Init System

Enable/disable on boot

Check status

Output looks like this:

nginx.service - A high performance web server

Loaded: loaded (/lib/systemd/system/nginx.conf; enabled)

Active: active (running) since Mon 2024-01-15 10:22:00 UTC

Process: 1823 ExecStart=/usr/sbin/nginx (code=exited, status=0/SUCCESS)

Main PID: 1824 (nginx)

Tasks: 2 (limit: 4915)

Memory: 6.2M

CPU: 42ms

CGroup: /system.slice/nginx.service

├─1824 nginx: master process /usr/sbin/nginx

└─1825 nginx: worker process

List all services

View service logs

5.6 Scheduling with cron

List your crontab

Edit another user's crontab (as root)

Run every day at 2:30am

Run every Monday at 8am

Run every 15 minutes

Run at midnight on the 1st of every month

Redirect cron output to a log file

Phase 5 Exercises

5.1 What Is a Process?

5.2 Viewing Processes

5.3 Signals

5.4 Foreground and Background Jobs

5.5 systemd — The Init System

5.6 Scheduling with cron

Phase 5 Exercises

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