Differences of [ ] and [[ ]] test in bash script

Hello, I would like to understand on which scenario we use [ ] and [[ ]]. What is the the difference between these test operator. Please provide some example to understand it better. Thanks a lot!
beer9Asked:
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nemws1Database AdministratorCommented:
There's a long description of here it:

  http://mywiki.wooledge.org/BashFAQ/031

Basically, if you have a script with [ as its testing operand, it'll work in bash, sh, ksh, and several other shells.  [[ is more bash specific, but it allows for more fun things (like using "&&" to mean "and" instead of the older style "-a")
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farzanjCommented:
[ ] is the old way, compatible with old Born.  The newer one is [[ ]] in both Bash and Korn.  With this you can use C++ style operators like ==, <=, &&, || instead of -eq, -le, -a, -o etc and you don't need to use quotations around a variable name.

For mathematical operations, you should use (( )) pair instead.
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