Linux Fundamentals
Master the essential commands and concepts that every DevOps engineer uses daily.
Filesystem Navigation
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Where am I?
pwd # Print working directory
List files
ls # Basic listing
ls -la # Long format, show hidden files
ls -lh # Human-readable file sizes
ls -lt # Sort by modification time
Navigate
cd /etc # Go to /etc
cd .. # Go up one level
cd ~ # Go to home directory
cd - # Go to previous directory
Find files
find / -name "nginx.conf" # Find by name
find /var/log -mtime -1 # Files modified in last 1 day
find . -type f -size +100M # Files larger than 100MB
locate nginx.conf # Fast search (uses database)
which nginx # Show full path of command
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File Operations
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Create
touch file.txt # Create empty file
mkdir -p /opt/app/logs # Create nested directories
echo "hello" > file.txt # Create file with content
Copy / Move / Delete
cp file.txt /tmp/ # Copy
cp -r /src /dst # Copy directory recursively
mv file.txt /tmp/ # Move (also renames)
rm file.txt # Delete file
rm -rf /tmp/testdir # Delete directory (BE CAREFUL!)
View files
cat /etc/hosts # Print entire file
less /var/log/syslog # Page through large files (q to quit)
head -20 /var/log/nginx/access.log # First 20 lines
tail -100 /var/log/app.log # Last 100 lines
tail -f /var/log/app.log # Follow log in real time
Search in files
grep "ERROR" /var/log/app.log
grep -r "database" /etc/ # Recursive search
grep -i "error" app.log # Case insensitive
grep -n "failed" auth.log # Show line numbers
grep -v "DEBUG" app.log # Exclude lines with DEBUG
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File Permissions
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Permission format: [type][owner][group][others]
Example: -rwxr-xr--
- = file, d = directory, l = symlink
rwx = read(4), write(2), execute(1)
View permissions
ls -la /etc/passwd
-rw-r--r-- 1 root root 1872 Jan 15 10:23 /etc/passwd
Change permissions
chmod 755 script.sh # rwxr-xr-x (owner=7, group=5, others=5)
chmod +x script.sh # Add execute for all
chmod u+w file.txt # Add write for owner
chmod o-r secret.txt # Remove read for others
chmod -R 644 /var/www/html # Recursive
Change ownership
chown user:group file.txt
chown -R www-data:www-data /var/www/
sudo chown root:root /etc/crontab
SUID, SGID, Sticky bit
chmod u+s /usr/bin/passwd # SUID - run as owner
chmod +t /tmp # Sticky bit - only owner can delete
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Process Management
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View processes
ps aux # All processes, detailed
ps aux | grep nginx # Find specific process
top # Interactive process viewer
htop # Better interactive viewer (install it!)
pgrep nginx # Get PID of process
pidof nginx # Same
Control processes
kill 1234 # Send SIGTERM (graceful stop)
kill -9 1234 # Send SIGKILL (force stop)
killall nginx # Kill by name
pkill -f "python app.py" # Kill by pattern
Background jobs
./long_script.sh & # Run in background
jobs # List background jobs
fg 1 # Bring job 1 to foreground
bg 1 # Send job 1 to background
nohup ./script.sh & # Keep running after logout
screen / tmux # Terminal multiplexer (learn tmux!)
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User Management
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Users
whoami # Current user
id # User and group IDs
sudo -i # Switch to root
su - username # Switch to user
Create/manage users
sudo useradd -m -s /bin/bash john # Create user with home dir
sudo passwd john # Set password
sudo usermod -aG docker john # Add to group
sudo userdel -r john # Delete user and home
Groups
groups # Show current user's groups
cat /etc/group | grep docker # Show docker group members
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Package Management
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Ubuntu/Debian (apt)
sudo apt update # Update package lists
sudo apt upgrade # Upgrade all packages
sudo apt install nginx # Install package
sudo apt remove nginx # Remove package
sudo apt autoremove # Remove unused dependencies
apt search nginx # Search packages
dpkg -l | grep nginx # Check if installed
RHEL/CentOS (yum/dnf)
sudo yum update
sudo yum install nginx
sudo dnf install nginx # Modern RHEL 8+
rpm -qa | grep nginx # Check if installed
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Disk and Storage
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df -h # Disk usage (human readable)
du -sh /var/log # Directory size
du -sh /* # Size of each top-level dir
lsblk # List block devices
fdisk -l # List disk partitions (root needed)
mount | grep sda # Show mounted filesystems
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