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I came across this line in one of the shell scripts:
[-f $host_something ] && .$host_something
What are the square brackets with the -f switch supposed to do, and what is the point of ANDing it with the same environment variable?
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Square brackets, often just called brackets in American English, are a set of punctuation marks that are most often used to alter or add information to quoted material. Square brackets come in pairs as [ and ].
[[ … ]] double brackets are an alternate form of conditional expressions in ksh/bash/zsh with a few additional features, for example you can write [[ -L $file && -f $file ]] to test if a file is a symbolic link to a regular file whereas single brackets require [ -L "$file" ] && [ -f "$file" ] .
The double brackets, [[ ]], were introduced in the Korn Shell as an enhancement that makes it easier to use in tests in shell scripts. We can think of it as a convenient alternative to single brackets. It's available in many shells like Bash and zsh.
mvBASIC Syntax NotationsAnything shown enclosed within square brackets is optional unless indicated otherwise. The square brackets themselves are not typed unless they are shown in bold. | A vertical bar that separates two or more elements indicates that any one of the elements can be typed.
The [ is actually an actual binary. It's an alias for the test(1) command. It will ignore it's last argument which should be ]. Run man test for further information. It's not really shell syntax.
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