Shell spacing in square brackets [duplicate]

Spaces are required after [ and before ].

1. [ is actually the name of a command, an alias for test. It's not a special symbol, it's just a command with an unusual name.

   $ help '['[: [ arg... ]    Evaluate conditional expression.    This is a synonym for the "test" builtin, but the last argument must    be a literal `]', to match the opening `['.

2. Because it's an ordinary command name and not a special character, a space is required after the [. If you omit the space and write [foo the shell will search the $PATH for a command named [foo.

   $ [ foo = foo ] && echo truetrue$ [foo = foo] && echo true[foo: command not found

3. For readability's sake, [ expects its last argument to be exactly ]. Being an ordinary command-line argument, ] must have a space before it. If there's no space then the bracket will become the last character of the previous argument, and [ will complain about its last argument not being ].

   $ [ foo = foo]bash: [: missing `]'$ [ foo = 'foo]'bash: [: missing `]'

[[ is a bash enhancement with more features than [, namely saner handling of unquoted variable names. It requires the a space on both ends, same as [. However [[ is in fact special shell syntax and is parsed differently. It's not an "ordinary command" the way [ is.

For a detailed explanation of the difference between [ and [[, see:

• What's the difference between [ and [[ in Bash?