There's no disagreement there. Let's remember that `test` is older than most programming languages. I can't find an exact date, but 1979 is the earliest mention

There's no disagreement there. Let's remember that `test` is older than most programming languages. I can't find an exact date, but 1979 is the earliest mention of it being in `sh v7` and turned into a UNIX System III built-in in 1981. And, that it was created to solve a problem where there is no type system, so there can be no test for NULL The Bash-ism `[[` (Compound Conditional) is an attempt to reduce the unintuitiveness of `[` or `test`. However, you would still need to quote "$var". The biggest pain point of using an external application like `[` is that `&&` is a shell construct. As you may know, `&&` means "if the previous program succeeds, then execute this next statement. But, if you do `[ -z "$var" && ...`, then `[` will always fail because the command lacks the terminating arg `]`. To do conditionals with `[` you would have to use `-a` for "and" or `-o` for "or". You would want to do either `[ -z "$foo" ] && [ -z "$bar" ]` or `[ -z "$foo" -a "$bar" ]`. Conditional Expressions relieve those issues as `&&` and `||` will work between the braces. So, `[[ -z "$foo" && -z "$bar" ]]` is okay. (Some other pain points to `test` are `=` tests for string equality, not assignment; although, `==` is a synonym. For numerical equality testing, there are `-eq`, `-ne`, `-lt`, `-gt`, etc. Then, for those unfamiliar with the Unix way (and even, to some extend, to those who are), there are three cryptic sets of tests for File Type, Access Permission, and File Characteristics.) But, then, Conditional Expressions makes a few head scratching changes. Like `<` and `>` are for sorting strings; not numbers. And, instead of leaving `-a` and `-o` as-is, they were remapped so that `-a` is synonymous with `-e` (wat?) and that `-o` is for testing shell options. (Doing a little research tell me that `[[` comes from `ksh88`, so maybe they're there for parity.) Manpage for `test`, `[`: https://www.mankier.com/1/test https://www.mankier.com/1/test Conditional Expressions: http://www.gnu.org/software/bash/manual/html_node/Bash-Conditional-Expressions.html http://www.gnu.org/software/bash/manual/html_node/Bash-Condi...