Bash Scripting Basics Cheat Sheet

1. What is Bash?

• Bash (Bourne Again SHell) – A command-line interpreter for Unix/Linux.
• Purpose: Automate tasks, run commands, and create reusable scripts.
• Script File Extension: .sh
• Execute Script:

  bash script.sh  
  ./script.sh  (if executable)
  

2. Creating and Running a Script

1. Create the File:

   nano myscript.sh  
   

2. Add the Shebang (Interpreter Directive):

   #!/bin/bash  
   

3. Make the Script Executable:

   chmod +x myscript.sh  
   

4. Run the Script:

   ./myscript.sh  
   

3. Basic Script Structure

#!/bin/bash
# This is a comment
echo "Hello, World!"  

4. Variables

• Define:

  name="John"  
  

• Use:

  echo "Hello, $name!"  
  

• User Input:

  read user_input  
  

5. Conditional Statements

If-Else

if [ $age -ge 18 ]; then  
  echo "Adult"  
else  
  echo "Minor"  
fi  

Comparison Operators:

• -eq – Equal
• -ne – Not equal
• -gt – Greater than
• -lt – Less than
• -ge – Greater than or equal
• -le – Less than or equal

6. Loops

For Loop:

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

While Loop:

count=1  
while [ $count -le 5 ]; do  
  echo "Count: $count"  
  ((count++))  
done  

7. Functions

greet() {  
  echo "Hello, $1"  
}  
greet "Alice"  

8. Command Line Arguments

• $0 – Script name
• $1, $2… – Positional arguments
• $# – Number of arguments
• $@ – All arguments

echo "Script name: $0"  
echo "First argument: $1"  

9. Error Handling

command || echo "Command failed"  
command && echo "Command succeeded"  

10. File Operations

if [ -f "file.txt" ]; then  
  echo "File exists"  
else  
  echo "File not found"  
fi  

• -f – File exists
• -d – Directory exists
• -e – File or directory exists

11. Script Debugging

• Run with Debugging:

  bash -x script.sh  
  

• Add Debugging Inside Script:

  set -x  # Enable  
  set +x  # Disable  
  

12. Scheduling with Cron

• Open Crontab:

  crontab -e  
  

• Schedule Example (Every Day at 5 PM):

  0 17 * * * /path/to/script.sh  
  

13. Useful Shortcuts

• Ctrl + C – Stop execution
• Ctrl + Z – Suspend
• Ctrl + D – End input
• !! – Repeat last command
• !n – Run nth command from history

Example: Backup Script

#!/bin/bash
backup_dir="/backup"
mkdir -p $backup_dir
cp *.txt $backup_dir
echo "Backup completed!"