Question

In Shell script they are declare variable like ${variable_name},Is it equal to $variable_name
what is the difference between ${variable_name} and $variable_name ?

Answer 1

The curly brace form is less ambiguous
because if the braces are omitted the
shell may not be able to distinguish
between a variable name and the text
surrounding it.

If you wrote a variable ($myvariable) with the text like this

echo foo$myvariablebar

Then shell may not be able to distinguish a variable name so you needs to use {} here like this

echo foo${myvariable}bar 

Then the shell knows myvariable is a variable here and bar is text

Checkout here is a clear explanation.