Understanding variables, loops, conditionals in centos

Understanding variables, loops, conditionals, and other fundamental concepts in shell scripting is essential for writing effective scripts to automate tasks on CentOS. Here's an overview of each concept:

1. Variables:

• Variables are placeholders used to store data or values.
• Declare variables using the syntax variable_name=value.
• Access variables using the $ prefix ($variable_name).
• Examples:

  name="John"
  age=30
  echo "Hello, $name! You are $age years old."
  
  

2. Loops:

• Loops allow you to repeat a set of commands multiple times.
• Common types of loops in shell scripting are for, while, and until.
• Examples:
  • for loop:

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

  • while loop:

    count=0
    while [ $count -lt 5 ]
    do
        echo "Count: $count"
        ((count++))
    done
     

3. Conditionals:

• Conditionals allow you to execute commands based on the evaluation of a condition.
• Common conditional constructs are if, elif, and else.
• Examples:

  age=20
  if [ $age -ge 18 ]; then
      echo "You are an adult."
  else
      echo "You are not yet an adult."
  fi
   

4. Functions:

• Functions allow you to encapsulate a block of code that performs a specific task.
• Declare functions using the syntax function_name() { ... }.
• Call functions by their name.
• Example:

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

5. Input/Output:

• Read user input using the read command.
• Redirect output using redirection operators (>, >>, <, |, etc.).
• Examples:
  • Read user input:

    echo "Enter your name:"
    read name
    echo "Hello, $name!"
     

  • Redirect output to a file:

    echo "Hello, world!" > output.txt
     

6. Command Substitution:

• Command substitution allows you to capture the output of a command and use it as a value.
• Use backticks (`) or $() syntax for command substitution.
• Example:

  files_count=$(ls | wc -l)
  echo "Number of files: $files_count"
  

7. Comments:

• Comments provide explanatory notes or annotations in your script.
• Comments start with the # symbol and extend to the end of the line.
• Example:

  # This is a comment