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test
By far, the command used most frequently with if is test. The test command per- forms a variety of checks and comparisons. It has two equivalent forms:
test expression and the more popular: [ expression ]
where expression is an expression that is evaluated as either true or false. The test com- mand returns an exit status of zero when the expression is true and a status of one when the expression is false.
It is interesting to note that both test and [ are actually commands. In bash they are builtins, but they also exist as programs in /usr/bin for use with other shells. The ex- pression is actually just its arguments with the [ command requiring that the “]” charac- ter be provided as its final argument.
The test and [ commands support a wide range of useful expressions and tests.