确认shell函数的有效输入参数数量

Question

Assume a shell function my_function that expects to receive three valid input arguments:

my_function()
{
   echo "Three common metasyntactic variables are: $1 $2 $3"
}

I would like to include a test within my_function that assesses if the function has indeed received three input arguments and that none of these input arguments is empty.

$ my_function foo bar baz
Three common metasyntactic variables are: foo bar baz

$ my_function foo bar  # By default, no error message is given, which I wish to avoid
Three common metasyntactic variables are: foo bar

How would I implement that?

Edit 1 : As emphasized above, I am looking for code that not only confirms the number of input variables, but also confirms that none of them is empty. This second aspect is relevant because the input variables may be variables themselves that are passed from other functions.

Answer 1

The bash variable $# contains the length of the command line arguments passed to the script function.

my_function() {
    (( "$#" == 3 )) || { printf "Lesser than 3 arguments received\n"; exit 1; }
}

Also if you wanted to check if any of the argument is empty in a way containing only white-spaces, you can loop over the arguments and check it.

for (( i=1; i<="$#"; i++ )); do
    argVal="${!i}"
    [[ -z "${argVal// }" ]] && { printf "Argument #$i is empty\n"; exit 2; }
done

Combining these two, if you call a function with fewer arguments

my_function "foo" "bar"
Lesser than 3 arguments received

and for empty arguments,

my_function "foo" "bar" " "
Argument #3 is empty

Answer 2

You can defensively assert that such variables are set with ${var:?} :

my_function()
{
   echo "Three common metasyntactic variables are: ${1:?} ${2:?} ${3:?}"
}

This will fail when the values are null or unset:

$ my_function foo bar baz
Three common metasyntactic variables are: foo bar baz

$ my_function foo bar
bash: 3: parameter null or not set

$ my_function foo "" baz
bash: 2: parameter null or not set

Similarly, you can use ${1?} to allow empty strings, but still fail for unset variables.