What’s the difference between ${varname} and $varname in a shell scripts

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Using {} in variable names helps get rid of ambiguity while performing variable expansion.

Consider two variables var and varname. Lets see you wanted to append the string name to the variable var. You can’t say $varname because that would result in the expansion of the variable varname. However, saying ${var}name would help you achieve the desired result.

$ var="This is var variable."
$ varname="This is varname variable."
$ echo $varname
This is varname variable.
$ echo ${var}name
This is var variable.name

Braces are also required when accessing any element of an array.

$ a=( foo bar baz )       # Declare an array
$ echo $a[0]              # Accessing first element -- INCORRECT
foo[0]
$ echo ${a[0]}            # Accessing first element -- CORRECT
foo

Quoting from info bash:

   Any  element  of  an  array may be referenced using ${name[subscript]}.
   The braces are required to avoid conflicts with pathname expansion.

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