Part 3: Permissions & Processes - System Control

Table of Contents

  1. Tutorial Index
  2. Part-00 : Setup
  3. Part-01 : Getting Started
  4. Part-02 : Reading and Searching
  5. Part-03 : Permissions and Processes ◄ You are here
  6. Part-04 : Environment Customization
  7. Part-05 : Solutions and Next Steps

Now that you can navigate and search, let’s learn to control the system: managing permissions and processes.

Understanding File Permissions

When you run ls -l, you see something like:

-rw-r--r-- 1 bakayu users 1234 Nov 07 10:30 file.txt
│││││││││  │ │      │     │    │          └─ filename
│││││││││  │ │      │     │    └─ timestamp
│││││││││  │ │      │     └─ size
│││││││││  │ │      └─ group
│││││││││  │ └─ owner
│││││││││  └─ number of hard links
└┴┴┴┴┴┴┴┴─ permissions

Permission Breakdown

-rw-r--r--
│││ │││ │││
│││ │││ └┴┴─ others: read only
│││ └┴┴──── group: read only
│└┴──────── owner: read + write
└────────── file type (- = file, d = directory, l = link)

Permission meanings:

  • r (read) = 4
    • Files: Can view content
    • Directories: Can list contents
  • w (write) = 2
    • Files: Can modify
    • Directories: Can create/delete files
  • x (execute) = 1
    • Files: Can run as program
    • Directories: Can enter (cd into)

Exercise 3.1:

cd ~/workshop-practice
ls -l greeting.txt
# Decode what you see!

Changing Permissions

chmod - Change Mode

Symbolic method (easier to remember):

chmod u+x file.sh           # Add execute for user
chmod g-w file.txt          # Remove write for group
chmod o+r file.txt          # Add read for others
chmod a+x script.sh         # Add execute for all
chmod u+rw,g+r file.txt     # Multiple changes

Legend:

  • u = user (owner)
  • g = group
  • o = others
  • a = all (ugo)
  • + = add permission
  • - = remove permission
  • = = set exactly

Numeric method (faster when you know it):

chmod 755 script.sh         # rwxr-xr-x
chmod 644 file.txt          # rw-r--r--
chmod 600 secret.txt        # rw-------
chmod 777 public.txt        # rwxrwxrwx (dangerous!)

Calculate: r=4, w=2, x=1, then add:

  • 755 = rwxr-xr-x = (4+2+1)(4+0+1)(4+0+1)
  • 644 = rw-r--r-- = (4+2+0)(4+0+0)(4+0+0)

Exercise 3.2:

cd ~/workshop-practice

# Create a script
cat > hello.sh << 'EOF'
#!/bin/bash
echo "Hello from a script!"
EOF

# Try to run it
./hello.sh                  # Permission denied!

# Make it executable
chmod +x hello.sh
ls -l hello.sh
./hello.sh                  # Now it works!

# Make it read-only
chmod 444 hello.sh
ls -l hello.sh
# Try to edit it - you'll need sudo!

Common permission patterns:

  • 755 - Scripts, executables
  • 644 - Regular files (documents, configs)
  • 600 - Private files (SSH keys, passwords)
  • 700 - Private directories

chown - Change Owner

Requires sudo (root privileges):

sudo chown user file.txt            # Change owner
sudo chown user:group file.txt      # Change owner and group
sudo chown -R user:group folder/    # Recursive

Exercise 3.3:

cd ~/workshop-practice
touch myfile.txt
ls -l myfile.txt

# This will likely fail (unless you're root):
# sudo chown root myfile.txt
# sudo chown $USER myfile.txt  # Change back

chgrp - Change Group

chgrp groupname file.txt
chgrp -R groupname folder/

See your groups:

groups              # Your current groups
id                  # Detailed user info

Process Management

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

ps - Process Status

Show running processes:

ps                  # Your processes in current terminal
ps -u username      # All processes for a user
ps aux              # All processes, detailed
ps aux | grep "firefox"  # Find specific process

Exercise 3.4:

ps
ps aux
ps aux | grep "bash"

Understanding ps aux output:

USER  PID  %CPU %MEM    VSZ   RSS TTY  STAT START   TIME COMMAND
root  1    0.0  0.1  16832  1234 ?    Ss   10:00   0:01 /sbin/init
│     │    │    │     │     │    │    │    │       │    └─ command
│     │    │    │     │     │    │    │    │       └─ CPU time
│     │    │    │     │     │    │    │    └─ start time
│     │    │    │     │     │    │    └─ state (S=sleeping, R=running)
│     │    │    │     │     │    └─ terminal
│     │    │    │     │     └─ memory (KB)
│     │    │    │     └─ virtual memory
│     │    │    └─ % of RAM
│     │    └─ % of CPU
│     └─ Process ID
└─ User

top / htop - Live Process Viewer

Interactive process monitor:

top                 # Basic monitor
htop                # Better alternative (may need to install)

Controls in top:

  • q - Quit
  • k - Kill process (prompts for PID)
  • M - Sort by memory
  • P - Sort by CPU
  • h - Help

Exercise 3.5:

top
# Press 'M' to sort by memory
# Press 'q' to quit

kill - Terminate Process

kill PID                # Polite shutdown (SIGTERM)
kill -9 PID             # Force kill (SIGKILL)
killall process_name    # Kill all instances by name
pkill pattern           # Kill by pattern match

Common signals:

  • 15 (SIGTERM) - Default, graceful shutdown
  • 9 (SIGKILL) - Force kill immediately
  • 1 (SIGHUP) - Reload config
  • 2 (SIGINT) - Interrupt (like Ctrl+C)

Exercise 3.6:

# Start a long-running process in background
sleep 300 &
echo $!                 # Shows the PID

# Find it
ps aux | grep "sleep"

# Kill it (replace 12345 with actual PID)
kill 12345

# Verify it's gone
ps aux | grep "sleep"

Background Jobs

command &               # Run in background
jobs                    # List background jobs
fg                      # Bring to foreground
bg                      # Resume in background
Ctrl+Z                  # Suspend current job

Exercise 3.7:

# Start in background
sleep 100 &

# List jobs
jobs

# Start another, then suspend it
sleep 200
# Press Ctrl+Z

# List jobs again
jobs

# Resume in background
bg

# Kill all background jobs
kill $(jobs -p)

nohup - No Hangup

Keep process running after logout:

nohup long_script.sh &
# Output goes to nohup.out

File Searching (Advanced)

Uses a database (updated nightly):

locate filename
locate "*.conf"
sudo updatedb           # Update the database manually

which - Find Executable Location

which python
which ls
which code

Exercise 3.8:

which bash
which git
which nonexistent       # No output if not found

whereis - Find Binary, Source, Manual

whereis python
whereis ls

System Information

df - Disk Free

Show disk usage:

df                      # All filesystems
df -h                   # Human-readable
df -h /home             # Specific mount

Exercise 3.9:

df -h

du - Disk Usage

Show directory sizes:

du -sh folder/          # Summary of folder
du -h folder/           # All files/folders
du -sh */               # All subdirectories
du -h --max-depth=1     # One level deep

Exercise 3.10:

cd ~
du -sh Documents/
du -sh */
du -h --max-depth=1 | sort -h

free - Memory Usage

free                    # Memory info
free -h                 # Human-readable
free -m                 # In megabytes

uname - System Info

uname -a                # All info
uname -r                # Kernel version
uname -m                # Architecture

Hands-On Challenge

Ready to practice? Complete the hands-on challenge in the workshop repository:

Part 3 Challenge on GitHub

Challenge: Set up scripts with proper file permissions and create a system information script.

The challenge includes:

  • Pre-created scripts that need permission fixes
  • A script creation task
  • Verification of both permissions and functionality
  • Detailed hints about permission numbers

Clone the repository and give it a try:

git clone https://github.com/bakayu/linux-tutorial.git
cd linux-tutorial/03-permissions-processes
./setup.sh
cat README.md

Quick Reference Card

# Permissions
ls -l                   # View permissions
chmod +x file           # Add execute
chmod 755 script.sh     # rwxr-xr-x
chmod 644 file.txt      # rw-r--r--
chmod 600 secret.txt    # rw-------
sudo chown user file    # Change owner

# Processes
ps aux                  # All processes
top                     # Live monitor
kill PID                # Terminate process
kill -9 PID             # Force kill
killall name            # Kill by name
jobs                    # Background jobs
fg / bg                 # Foreground/background

# System Info
df -h                   # Disk usage
du -sh folder/          # Folder size
free -h                 # Memory usage
which command           # Find executable
uname -a                # System info