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  • Interesting. It's not hard to tell how this is possible - being [[ a keyword, operators and operands are detected when the command is read and not after expansion. Thus [[ can treat -eq in a more smart way than, say, [. But what I wonder is: where can we find documentation about the logic bash uses to interpret compound commands? It doesn't look quite obvious to me and I'm apparently unable to find satisfactory explanations in man or info bash. Commented Dec 3, 2018 at 23:55
  • Bash doesn't document this anywhere I can find. There is a kind of description in man ksh93: The following obsolete arithmetic comparisons are also permitted: exp1 -eq exp2. There is this text in the test section of man zshbuiltins arithmetic operators expect integer arguments rather than arithmetic expressions. Which confirms that some arguments are treated as arithmetic expressions by the test builtin under conditions non specified in this quote. I'll confirm with the source code ….… Commented Dec 4, 2018 at 1:03