Shell spacing in square brackets [duplicate]

Question

This question already has an answer here:

• Why should there be a space after '[' and before ']' in Bash? 4 answers

As a beginner, I did not find answered anywhere, the rules about spacing (grammar) and parsing.

For example.

Can I do

if [$# -eq 2] ; 
then 
    llll 
fi

or must I always have a blank or two between the objects, as

if [ $# -eq 2 ] ;
then
   llll
fi

and the second related question is about the difference between

if [[ $# -eq 2 ]] ;
then
wafwaf
fi

The concern I have is about spacing before/after [, ].

No searching has provided me with a set of rules.

Answer 1

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 true
   true
   $ [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?