Shell Commands

Here’s a complete, forward-thinking guide to Linux shell commands — structured so you can understand what they do, how they work together, and how to master them for engineering and automation.
Author

Benedict Thekkel

🧭 1. What Is the Linux Shell?

A shell is the command interpreter between you and the operating system. Common shells include:

Shell Description Command to start
bash Default on most distros, scripting-friendly bash
zsh Extended Bash with better completion & themes (used in macOS) zsh
fish User-friendly, smart autosuggestions fish
sh POSIX shell, minimal and portable sh

Pro tip: Use echo $SHELL to see which shell you’re using.

⚙️ 2. Shell Command Basics

Command Description Example
pwd Print working directory pwd
ls List files ls -la
cd Change directory cd /var/www
mkdir Create a directory mkdir logs
rmdir / rm -r Remove directory / recursively rm -rf old_dir
cp Copy files or directories cp file1.txt backup/
mv Move or rename files mv main.py src/
cat Show file content cat /etc/passwd
less / more View long files less large.log
touch Create empty file or update timestamp touch app.log

🧩 3. File and Text Manipulation

Command Use Example
grep Search for text patterns grep "error" app.log
find Search for files find . -name "*.py"
awk Field-based text processing awk '{print $1,$3}' data.txt
sed Stream editor (find/replace) sed 's/foo/bar/g' file.txt
cut Extract columns cut -d',' -f2 users.csv
sort Sort lines sort data.txt
uniq Remove duplicates sort data.txt | uniq
wc Word/line/byte count wc -l access.log
diff Compare files diff file1.txt file2.txt

🔁 4. Redirection and Pipes

Operator Meaning Example
> Redirect output (overwrite) ls > files.txt
>> Append output echo "done" >> log.txt
< Take input from file sort < data.txt
| Pipe output into another command cat log.txt | grep error
2> Redirect errors python script.py 2> errors.txt
&> Redirect both stdout and stderr ./run.sh &> output.log

🧮 5. Permissions and Ownership

Command Purpose Example
chmod Change permissions chmod 755 script.sh
chown Change owner chown ben:staff project/
umask Set default permission mask umask 022
sudo Run as superuser sudo apt update

💡 chmod +x file.sh makes a file executable.

🧠 6. Process & System Management

Command Description Example
ps Show running processes ps aux | grep nginx
top / htop Interactive process viewer htop
kill Stop a process by PID kill -9 1024
jobs / fg / bg Manage background tasks sleep 60 &
df -h Disk usage by filesystem df -h
du -sh Directory size du -sh ~/Downloads
free -h Memory usage free -h
uptime System load uptime
whoami Current user whoami
uname -a Kernel info uname -a
lscpu, lsusb, lspci Hardware info lscpu

🌐 7. Networking Commands

Command Function Example
ping Test connection ping google.com
curl Fetch URLs curl https://example.com
wget Download files wget https://example.com/file.zip
ifconfig / ip addr Network interfaces ip addr show eth0
netstat / ss Show open ports ss -tuln
scp Copy over SSH scp file.txt user@host:/path
ssh Remote shell ssh user@192.168.1.10
traceroute Trace network hops traceroute 8.8.8.8
nslookup / dig DNS lookup dig recoverymetrics.com

📦 8. Package Management

Distro Commands Example
Debian/Ubuntu apt, apt-get sudo apt install vim
Fedora/RHEL dnf, yum sudo dnf update
Arch pacman sudo pacman -S neovim
Alpine apk apk add curl
openSUSE zypper sudo zypper install git

🧰 9. System and User Management

Command Purpose Example
adduser, useradd Create user sudo adduser ben
passwd Change password passwd ben
groups, usermod Manage groups sudo usermod -aG sudo ben
crontab -e Schedule jobs 0 3 * * * /usr/bin/backup.sh
systemctl Manage services sudo systemctl restart nginx
journalctl View logs sudo journalctl -u nginx
shutdown / reboot Power control sudo reboot

🧑‍💻 10. Scripting and Automation

Example: backup.sh

#!/bin/bash
SRC="/var/www"
DEST="/backup/www_$(date +%F).tar.gz"

tar -czf "$DEST" "$SRC"
echo "Backup completed: $DEST"

Make it executable:

chmod +x backup.sh

Schedule it with cron:

crontab -e
0 2 * * * /home/ben/backup.sh

🧱 11. Environment Variables

Command Description Example
export Set variable export DEBUG=True
env List variables env
unset Remove variable unset DEBUG
.env files Common in dev setups source .env

🧮 12. Compression and Archiving

Command Type Example
tar Archive multiple files tar -czf backup.tar.gz folder/
zip, unzip ZIP format zip -r project.zip .
gzip / gunzip Compress single file gzip bigfile.log
xz, bz2 High-ratio compression xz -z data.csv

🧰 13. Monitoring & Performance Tools

Command Description
htop / atop Real-time process stats
iotop Disk I/O usage
iftop Network usage per process
vmstat, mpstat, iostat CPU/memory/disk stats
sar System Activity Reporter

🧠 14. Power Shell Tricks

Trick Description
!! Re-run last command
!sudo Run last command with sudo
!ls Re-run last command starting with “ls”
Ctrl + R Search command history
history List command history
alias ll='ls -la' Create custom shortcuts
df -Th | grep /dev/nvme Check specific disk type

📈 15. Example Workflow (Data Analysis)

# Find all CSVs modified today and summarize line counts
find . -name "*.csv" -mtime -1 | while read f; do
  echo "$(wc -l < "$f") lines in $f"
done | sort -nr

📊 16. Visual Diagram: Shell Command Flow

Input -> Shell -> Command -> Kernel -> Hardware
                     |
                     v
              stdout / stderr -> Terminal / Pipe

🚀 17. Next-Level Mastery

Area Description Tools
Scripting Learn conditionals, loops, functions bash, zsh, fish
Regex mastery Combine grep, awk, sed
Automation Combine cron + systemd + scripts
Security Learn sudo, ufw, and file perms
Networking Practice with ssh, scp, nmap
Observability dmesg, journalctl, htop, ps, etc.
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