Why should there be spaces around '[' and ']' in Bash?

Once you grasp that [ is a command, a whole lot becomes clearer!

[ is another way to spell "test".

help [

However while they do exactly the same, test turns out to have a more detailed help page. Check

help test

...for more information.

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Furthermore note that I'm using, by intention, help test and not man test. That's because test and [ are shell builtin commands nowadays. Their feature set might differ from /bin/test and /bin/[ from coreutils which are the commands described in the man pages.

From another question:

A bit of history: this is because '[' was historically not a shell-built-in but a separate executable that received the expresson as arguments and returned a result. If you didn't surround the '[' with space, the shell would be searching $PATH for a different filename (and not find it) . – Andrew Medico Jun 24 '09 at 1:13