How to create a bash variable like $RANDOM

Question

I\'m interest in some thing : every time I echo $RANDOM , the show value difference . I guess the RANDOM is special (When I read it , it may call a function , s

Answer 1

The special behavior of $RANDOM is a built-in feature of bash. There is no mechanism for defining your own special variables.

You can write a function that prints a different value each time it's called, and then invoke it as $(func). For example:

now() {
    date +%s
}

echo $(now)

Or you can set $PROMPT_COMMAND to a command that updates a specified variable. It runs just before printing each prompt.

i=0
PROMPT_COMMAND='((i++))'

This doesn't work in a script (since no prompt is printed), and it imposes an overhead whether you refer to the variable or not.