💻 Shell Scripting

📌 Basics

• Start scripts with #!/bin/bash (called shebang)
• # is used for single-line comments
• Multi-line comments can be done using : ' comment block '
• echo "Hello" - print output
• read name - take user input
• Command substitution: $(command) or backticks `command`

📦 Variables

• name="Pradeep" - create variable
• echo $name - access variable
• Special variables:
  • $0 - script name
  • $1 $2 ... - command-line arguments
  • $# - number of arguments
  • $@ - all arguments
  • $? - exit code of last command
  • $$ - current script PID

⚖️ Conditions

• if [ condition ]; then ... fi
• if ... elif ... else ... fi
• case $var in pattern) ... ;; esac
• Common tests:
  • -f file - file exists
  • -d dir - directory exists
  • -e file - file exists (any type)
  • -s file - file not empty
  • -r/-w/-x file - readable/writable/executable
  • -eq/-ne/-gt/-lt/-ge/-le - integer comparisons
  • = / != - string comparison

🔁 Loops

• for i in 1 2 3; do ... done
• while [ condition ]; do ... done
• until [ condition ]; do ... done
• break and continue to control loops

🧩 Functions

•             function greet() {
              echo "Hello $1"
              }
              greet "Pradeep"
              

• Use return to send exit codes (0=success)

📚 Arrays

• arr=(one two three)
• echo ${arr[0]} - first element
• echo ${arr[@]} - all elements
• for i in "${arr[@]}"; do echo $i; done

⚡ String Operations

• ${#str} - length
• ${str:0:5} - substring
• ${str/old/new} - replace text

🚨 Error Handling

• Check exit code: if [ $? -ne 0 ]; then echo "failed"; fi
• Stop on error: set -e
• Debug mode: set -x (shows commands as they run)
• Use trap to catch errors and cleanup

📝 Input/Output & Redirection

• command > file - redirect output
• command >> file - append output
• command < file - input from file
• command 2> errors.txt - redirect errors
• Pipes: cmd1 | cmd2

⏳ Date and Time

• date +"%Y-%m-%d %H:%M:%S" - current date and time
• $(date +%s) - current timestamp

📌 Best Practices

• Always quote variables: "$var" to prevent word splitting
• Add comments for readability
• Use meaningful variable and function names
• Keep scripts executable with chmod +x script.sh

💡 Example Script

          #!/bin/bash
          read -p "Enter filename: " file
          if [ -f "$file" ]; then
          echo "Content of $file:"
          cat "$file"
          else
          echo "File not found"
          fi