Skip to main content

​
Linux Fundamentals

Linux is a multi-user operating system where everything is treated as a file. Understanding file permissions, directory structure, and basic commands is essential for any developer.

​
System Directory Structure

Linux organizes everything under the root directory /. Key directories include:
  • /bin - Essential command binaries (ls, cat, etc.)
  • /boot - Boot loader files and kernel images
  • /dev - Device files
  • /etc - System configuration files
  • /home - User home directories
  • /lib - Shared libraries
  • /opt - Optional software packages
  • /root - Root user home directory
  • /tmp - Temporary files
  • /usr - User programs and data
  • /var - Variable data (logs, databases)

​
File Permissions

Linux file permissions consist of read (r), write (w), and execute (x) for three categories:
  • Owner (u) - File owner
  • Group (g) - File group members
  • Others (o) - Everyone else
View permissions with ls -l: 
-rw-r--r-- 1 root root  13 Mar  8 15:48 file.txt
drwxr-xr-x 5 root root 4096 Sep 7 19:52 directory
The first character indicates file type:
  • d = directory
  • - = regular file
  • l = symbolic link

​
Permission Numbers

Permissions can be represented numerically:
  • Read (r) = 4
  • Write (w) = 2
  • Execute (x) = 1
# rwx = 4+2+1 = 7
# rw- = 4+2+0 = 6
# r-- = 4+0+0 = 4

chmod 755 script.sh  # rwxr-xr-x
chmod 644 file.txt   # rw-r--r--

​
Changing File Ownership

# Change owner
chown username file.txt

# Change owner and group
chown username:groupname file.txt

# Change recursively
chown -R username:groupname /directory

# Change group only
chgrp groupname file.txt

​
Essential Commands

​
File Operations

​
Viewing Files

# Display entire file
cat file.txt

# View with line numbers
cat -n file.txt

# Concatenate multiple files
cat file1.txt file2.txt > combined.txt

# Append to file
cat file1.txt >> file2.txt

​
Paging Through Files

# Page through file (space = next page, q = quit)
more filename

# Advanced paging (allows backward navigation)
less filename
# In less: space=forward, b=backward, /pattern=search, q=quit

​
Viewing File Portions

# First 10 lines
head file.txt

# First 20 lines
head -n 20 file.txt

# Last 10 lines
tail file.txt

# Last 20 lines
tail -n 20 file.txt

# Follow file updates (useful for logs)
tail -f /var/log/application.log

# Start from line 10
tail -n +10 file.txt

​
Directory Management

# Create directory
mkdir directory_name

# Create nested directories
mkdir -p parent/child/grandchild

# Remove empty directory
rmdir directory_name

# Remove directory and contents
rm -r directory_name

# Force remove without confirmation
rm -rf directory_name

​
File and Directory Operations

# Copy file
cp source.txt destination.txt

# Copy directory recursively
cp -r source_dir/ destination_dir/

# Move/rename file
mv oldname.txt newname.txt

# Move to directory
mv file.txt /path/to/directory/

# Move directory
mv old_dir/ new_location/

​
Searching

​
Find Files

# Find by name
find . -name "*.java"

# Find in specific directory
find /home/user -name "*.conf"

# Find by type (f=file, d=directory)
find . -type f -name "*.log"

​
Search File Contents

# Search for text in file
grep "pattern" file.txt

# Case-insensitive search
grep -i "pattern" file.txt

# Search all files in directory
grep -r "pattern" /path/to/dir

# Show line numbers
grep -n "pattern" file.txt

# Search multiple files
grep "error" *.log

​
Archive and Compression

# Create tar archive
tar -cvf archive.tar files/

# Create compressed archive
tar -zcvf archive.tar.gz files/

# Extract tar archive
tar -xvf archive.tar

# Extract to specific directory
tar -xvf archive.tar -C /destination/

# Extract compressed archive
tar -zxvf archive.tar.gz

# List archive contents
tar -tvf archive.tar
Flags explained:
  • -z - gzip compression
  • -c - create archive
  • -x - extract archive
  • -v - verbose output
  • -f - specify filename

​
System Information

​
CPU Information

# Display CPU details
lscpu

# Check CPU count
grep 'model name' /proc/cpuinfo | wc -l

​
System Load

# Show uptime and load average
uptime

# Current users and load
w

​
Disk Usage

# Disk space usage
df -h

# Directory size
du -sh /path/to/directory

# Show all subdirectory sizes
du -h --max-depth=1 /path/to/directory

​
Process Management

​
Viewing Processes

# List all processes
ps aux

# Filter processes
ps aux | grep java

# View process tree
ps -ef
pstree -p

# Interactive process viewer
top

# Enhanced system monitor
htop

​
Managing Processes

# Kill process by PID
kill 1234

# Force kill process
kill -9 1234

# Kill by process name
killall process_name

# Background process
command &

# Run without hangup
nohup java -jar application.jar &

# Redirect output
nohup java -jar app.jar > app.log 2>&1 &
Hard Link: Multiple names for the same file (same inode) Symbolic Link: Shortcut pointing to another file (different inode) 
# Create hard link
ln original.txt hardlink.txt

# Create symbolic link
ln -s /path/to/original.txt symlink.txt

# Create symbolic link to directory
ln -s /home/user/project shortcut

# Remove symbolic link
unlink symlink.txt
# or
rm symlink.txt

# View links
ls -li  # Shows inode numbers

​
Network Commands

# Show IP address
ip addr

# Test connectivity
ping google.com

# Show network statistics
netstat -tuln

# Download file
wget https://example.com/file.zip
curl -O https://example.com/file.zip

​
System Control

​
Shutdown and Reboot

# Sync data to disk
sync

# Shutdown now
shutdown -h now

# Shutdown in 10 minutes
shutdown -h +10

# Shutdown at specific time
shutdown -h 20:30

# Reboot
reboot
shutdown -r now

# Cancel scheduled shutdown
shutdown -c

​
Service Management (systemd)

# Start service
systemctl start service_name

# Stop service
systemctl stop service_name

# Restart service
systemctl restart service_name

# Check service status
systemctl status service_name

# Enable service at boot
systemctl enable service_name

# Disable service at boot
systemctl disable service_name

​
Scheduled Tasks

​
Crontab

Cron format: minute hour day month weekday command 
# Edit crontab
crontab -e

# List crontab entries
crontab -l

# Remove crontab
crontab -r
Common patterns: 
# Every minute
* * * * * /path/to/command

# Every hour
0 * * * * /path/to/command

# Every day at midnight
0 0 * * * /path/to/command

# Every Sunday
0 0 * * 0 /path/to/command

# Every month (1st day)
0 0 1 * * /path/to/command

# Weekdays at 5 PM
0 17 * * 1-5 /path/to/command

​
Shell Redirection

# Redirect stdout to file (overwrite)
command > output.txt

# Redirect stdout to file (append)
command >> output.txt

# Redirect stderr to file
command 2> error.txt

# Redirect both stdout and stderr
command > output.txt 2>&1
command &> output.txt

# Redirect stdin from file
command < input.txt

# Pipe output to another command
command1 | command2

​
Environment Variables

# View environment variable
echo $PATH
echo $HOME

# Set environment variable
export JAVA_HOME=/usr/lib/jvm/java-11

# Add to PATH
export PATH=$PATH:/new/path

# View all environment variables
env
printenv

​
Useful Tips

​
Command History

# View command history
history

# View last 10 commands
history 10

# Execute command from history
!123  # Execute command number 123
!!    # Execute previous command

​
File Statistics

# Count lines, words, characters
wc file.txt

# Count lines only
wc -l file.txt

# Count words only
wc -w file.txt

​
Text Processing

# Display text
echo "Hello World"

# Clear file contents
echo > file.txt
: > file.txt

# Create/update file
cat > file.txt << EOF
Line 1
Line 2
EOF

​
Best Practices

  1. Always use sync before shutdown to ensure data is written to disk
  2. Be careful with rm -rf - there’s no undo
  3. Use tab completion to avoid typos and speed up command entry
  4. Check current directory before running destructive commands
  5. Use less instead of cat for large files
  6. Quote file paths with spaces - rm "my file.txt"
  7. Test commands with echo before execution in scripts
  8. Use man command to read command documentation

​
Common Pitfalls

  • Forgetting to use sudo for privileged operations
  • Not backing up before modifying system files
  • Using kill -9 as first resort instead of graceful shutdown
  • Not checking if a process is still running before restart
  • Editing files directly in /etc without backups

Build docs developers (and LLMs) love

Get started for freeTalk to us