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Here is example in which it is used for separation of a variable $ export var='test' $ echo $_string # var is a variable that uses syntax $ with the value test $ echo $var_string # var_string is a variable that doesn't exist, echo doesn't print anything.Later this notation extended ksh93 and adopted by bash and other shells.

Modern shells such as bash 3.x or ksh93 supports most of the standard string manipulation functions, but in a very pervert, idiosyncratic way.

Essentially it makes [ ] construct obsolete except for running a program to get a return code.

In turn double round brackets ((..)) construct made The variable substitution and wildcard expansion aren't done within and , making quoting less necessary. It can act as independent operator as it produces return code.Therefore, a more usual style for shell programming is this: Unless you need to modify old scripts it does not make sense to use old ksh-style regex in bash.(partially borrowed from Bash Regular Expressions | Linux Journal) Since version 3 of bash (released in 2004) bash implements They are also called POSIX regular expressions as they are defined in IEEE POSIX 1003.2.


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