The key points of defining and calling functions in Bash scripts are as follows: 1. There are two forms of function definition, and it is recommended to use function_name() { ... }; 2. The function must be defined first and then called, and the naming must be legal; 3. When calling, write the function name directly, and the parameters are separated by spaces, and the functions are obtained through $1, $2, etc.; 4. The function cannot directly return the value, and the results can be passed through echo to capture the output, exit status code or global variables.

Defining and calling functions in Bash scripts is actually quite straightforward and the syntax is not complicated. You just need to remember the basic structure to organize the code and reuse logic in the script.

How to define functions

There are two ways to write functions defined in Bash, and it is recommended to use the first clearer form:

 function_name() {
    # Here is the function body}

Or add function keyword:

 function function_name {
    # Here is the function body}

Both writing methods can be used, but the first one is more common. Any legal Bash command can be written in the function body, such as echo, conditional judgment, loop, etc.

Note:

• Functions must be defined first and then called;
• Function names cannot have spaces or special characters, and it is recommended to underline lowercase;
• The function does not need to specify the return type, and the status code is returned after execution (0 means success).

How to call a function and pass a parameter

After definition, calling the function is like running a normal command, just write the function name directly:

 function_name

If you want to pass parameters in, the writing method is simple, follow the parameters after the function name and separate them with spaces:

 function_name arg1 arg2

The parameters are obtained through $1 and $2 inside the function, similar to the parameter processing method of the script itself:

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

greet "Alice" # output Hello, Alice

You can also use $@ to get all parameters and use $# to see how many parameters are passed.

How to handle the return value of the function

The Bash function does not "return" a value like other languages, but you can pass the result in several ways:

• Use echo to output the result , and then replace the capture output with a command when calling:

   get_name() {
      echo "Bob"
  }
  
  name=$(get_name)
  echo "Name is $name"

• Use the exit status code to return status information (0 means success, non-0 means error):

   check_file() {
      if [ -f "$1" ]; then
          return 0
      else
          return 1
      fi
  }
  
  if check_file "test.txt"; then
      echo "File exists"
  else
      echo "File not found"
  fi

• If you need to return multiple values, you can use global variables or arrays. Although it is not recommended, it can be seen in scripts sometimes.

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Basically that's it. Defining functions should be standardized, and pay attention to the order and parameter transfer method when calling, and just need to be flexible in the return value processing.

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