Bash Cheat Sheet

Bash Cheat Sheet

This is not just another Bash syntax list. It’s a hands-on, scenario-driven cheat sheet for:

• Running and writing Bash scripts
• Handling variables, arrays, and conditions
• Performing file, directory, and network operations
• Automating routine sysadmin tasks
• Interacting with users and input
• Scheduling with cron
• Mixing Bash with other languages (like Perl)

1. Run a Script

bash myscript.sh         # Run directly
chmod +x myscript.sh     # Make it executable
./myscript.sh            # Then run it

Runs a shell script with or without making it executable.

2. Shebang Basics

#!/usr/bin/bash

Tells the OS which interpreter to use for your script.

3. Hello World + Parameters

echo "Hello $1"

Prints a personalized greeting using the first input argument.

4. Variables and Strings

name="kemal"
echo "${name^^}"   # Uppercase
echo "${name,,}"   # Lowercase

Store and manipulate text easily in scripts.

5. Arrays

arr=("postgres" "oracle")
arr+=("mysql")
unset arr[1]

Useful for managing lists and collections dynamically.

6. Maps (Associative Arrays)

declare -A db
db[user]="kemal"
echo "${db[user]}"

Key-value storage in Bash scripts.

7. Loops

for i in {1..5}; do echo "Item $i"; done

Repeat tasks without redundancy.

8. Conditions

if [ "$x" -gt 10 ]; then echo "x > 10"; fi

Decision making in scripts.

9. Files & Directories

mkdir -p /tmp/folder
touch /tmp/file.txt
rm -rf /tmp/folder

Automate file system operations.

10. Text Processing

grep "error" logs.txt
sed 's/old/new/g' file.txt
awk '{ print $1 }' file.txt

Search, replace, and parse text content efficiently.

11. Functions

say_hello() { echo "Hi, $1"; }
say_hello Kemal

Reuse logic and simplify your scripts.

12. Argument Checks

if [ -z "$1" ]; then echo "No input"; exit 22; fi

Prevent errors by validating input early.

13. Add a New User (Script Example)

sudo useradd "$1"

Automate user creation on Linux systems.

14. Cron Jobs

crontab -e

Automate script execution at scheduled intervals.

15. File Transfer

scp file.txt user@host:/path/

Move files between machines securely.

16. Package Management

sudo dnf update && sudo dnf upgrade

Keep systems up to date via terminal.

17. Disk Cleanup

du -sh * | sort -h

Analyze and free up space.

18. Network Utilities

ping 8.8.8.8
curl -I https://example.com

Quick checks for connectivity and HTTP responses.

19. Infinite Loop

while true; do echo "Running..."; sleep 1; done

Useful for background or long-running processes.

20. Exit Codes

exit 0    # Success
exit 22   # Custom error

Control script flow and return statuses to other tools.

21. Other Languages

#!/usr/bin/perl
print "Hello from Perl\n";

Run non-Bash scripts using correct interpreters.