How to use conditional statements in bash if case

In Bash scripts, if is suitable for simple logical judgment, and case is suitable for multi-value matching. 1. if is often used for dynamic condition judgments such as file existence and numerical comparison, and supports elif and else branch expansion; 2. case is more suitable for matching fixed values ​​or patterns, such as classification processing of user input commands, supports wildcards and each branch must end with;; 3. Selection basis: use if for dynamic conditions, and case for fixed value matching.

In Bash scripts, if and case are two common ways to handle conditional judgments. They are each suitable for different scenarios: if is more suitable for simple logical judgments, while case is more suitable for matching multiple fixed values. Here are some practical uses and considerations.

Use if statements for conditional judgment

The if statement in Bash is usually used with test expressions. The basic structure is as follows:

 if [condition]; then
    # Execute command fi

You can also add else or elif to extend the logic:

 if [ condition1 ]; then
    # Execute elif when condition 1 is true [ condition2 ]; then
    # Execute else when condition 2 is true
    # Execute fi when neither is true

Common usage examples:

• Check if the file exists:

   if [ -f "filename.txt" ]; then
      echo "File exists"
  else
      echo "File does not exist"
  fi

• Compare numerical values:

   if [ "$num" -gt 10 ]; then
      echo "The value is greater than 10"
  fi

Note: To compare strings, use = , and to compare numeric values, use operators such as -eq , -ne , -lt , -gt .

Use case statements for multi-value matching

When multiple situations need to be matched based on the value of a variable, case statement will be clearer and more concise. The syntax is as follows:

 case $variable in
    pattern1)
        # Execute when pattern1 is matched;;
    pattern2)
        # Execute when pattern2 is matched;;
    *)
        #Default;;
esac

Practical application scenarios:

For example, selecting an operation based on user input:

 case $choice in
    start)
        echo "Start service"
        ;;
    stop)
        echo "stop service"
        ;;
    restart)
        echo "restart service"
        ;;
    *)
        echo "Invalid option"
        ;;
esac

Features:

• Supports wildcards, such as *.txt) that can match all file names ending in .txt .
• Each branch must be added at the end ;; to indicate the end, otherwise the execution of the next branch will continue (similar to C's switch).

How to choose between if and case?

These two statements are not substitutes for each other, but have their own applicable scenarios:

• Using if:

  • The conditions are dynamic, such as numerical comparison, file status, etc.
  • Not many logical branches and simple structure

• When using case:

  • To match multiple fixed values ​​or patterns
  • The input source is user input or configuration items with limited and unambiguous values.

For example:

• If you are determining whether the user input is yes or no , you can use if
• If you want to perform different operations based on the commands entered by the user (such as start , stop , status ), it is clearer to use case

That's basically it. Mastering the use of if and case can make your Bash script more flexible and easier to maintain.

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