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Bash Tests

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The test application, also known as [, is an application that usually resides somewhere in /usr/bin or /bin and is used a lot by shell programmers to perform certain tests on variables. In a number of shells, including bash, [ is implemented as a shell builtin.

Difference between test, "[" and "[[":
http://mywiki.wooledge.org/BashFAQ/031

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