steeldriver is correct. What you should do is either:

1. Use a command that reads in the standard input:

    #!/bin/bash
   
    fun=$(cat)
    echo "$fun"  
   

Or simply:

    #!/bin/bash
    
    cat

2. Or, to convert standard input into positional parameters, use xargs:

    $ echo 1 | xargs ./test.sh
   

3. Or, use the script the way it is supposed to be used (as coded):

    ./test.sh 1
   

steeldriver is correct. What you should do is either:

1. Use a command that reads in the standard input:

    #!/bin/bash
   
    fun=$(cat)
    echo "$fun"  
   

Or simply:

    #!/bin/bash
    
    cat

2. Or, to convert standard input into positional parameters, use xargs:

    $ echo 1 | xargs ./test.sh
   

3. Or, use the script the way it is supposed to be used (as coded):

    ./test.sh 1