The Linux Commandline

The Linux command line (shell) is the primary interface for interacting with Linux systems — essential for administration, scripting, and DevOps workflows.

Shell Basics

The default shell on most modern systems is Bash (Bourne Again Shell). Zsh is also widely used (macOS default since Catalina).

echo $SHELL        # Show active shell
cat /etc/shells    # List available shells
chsh -s /bin/zsh   # Change default shell

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Navigation & Filesystem

pwd               # Print Working Directory
ls -lah           # Directory listing: long format, all files, human-readable
cd /etc/nginx     # Absolute path
cd ../..          # Two levels up
cd ~              # Home directory
cd -              # Previous directory (toggle)

Key Directories (FHS)

Path	Content
/etc	System-wide configuration files
/var/log	Log files
/proc	Virtual FS — kernel & process info
/sys	Virtual FS — devices & kernel subsystems
/usr/bin	User binaries
/usr/local/bin	Manually installed software
/tmp	Temporary files (cleared on reboot)

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Files & Directories

touch file.txt            # Create file / update timestamp
mkdir -p /opt/app/config  # Create directory including parent paths
cp -r src/ dst/           # Copy recursively
mv old.txt new.txt        # Rename / move
rm -rf /tmp/testdir       # Recursive delete (no confirmation!)
ln -s /opt/app/bin/tool /usr/local/bin/tool  # Create symlink

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Reading & Searching File Contents

cat /etc/hosts              # Output entire file
less /var/log/syslog        # Page through content (q = quit)
tail -f /var/log/nginx/access.log   # Live follow (e.g. logs)
head -n 20 file.txt         # First 20 lines
grep -rn "error" /var/log/  # Recursive search with line numbers
grep -E "WARN|ERROR" app.log  # Extended regex

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Pipes & Redirections

# Write / append stdout to file
echo "config=true" > config.txt
echo "debug=false" >> config.txt

# Redirect stderr separately
command 2>/dev/null          # Suppress errors
command > out.txt 2>&1       # Stdout + stderr into one file

# Pipes — pass output as input
ps aux | grep nginx | grep -v grep
cat /etc/passwd | awk -F: '{print $1}' | sort

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Processes & System

ps aux                    # All running processes
top / htop                # Interactive process monitor
kill -9 <PID>             # Force kill process (SIGKILL)
pkill nginx               # Kill processes by name
jobs                      # Background jobs in current session
nohup ./script.sh &       # Run process in background, immune to SIGHUP

# System resources
df -h                     # Disk usage
du -sh /var/log/*         # Directory sizes
free -h                   # RAM usage
uptime                    # Load average

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Permissions

ls -l file.txt            # Show permissions: -rwxr-xr--
chmod 755 script.sh       # Owner: rwx, Group: r-x, Others: r-x
chmod u+x,g-w file.txt    # Set symbolically
chown www-data:www-data /var/www/html  # Set owner:group

Octal Reference

Octal	Binary	Meaning
7	111	rwx
6	110	rw-
5	101	r-x
4	100	r--
0	000	---

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Useful Shortcuts (Bash)

Shortcut	Action
Ctrl+C	Interrupt process (SIGINT)
Ctrl+Z	Suspend process
Ctrl+R	Search command history (reverse)
Ctrl+L	Clear terminal (like clear)
!!	Repeat last command
!$	Last argument of previous command
Alt+.	Insert last argument