Simon Branch <simonmbra...@gmail.com> wrote: > In an arithmetic substitution -- that is, the $(( ... )) construct in a > shell script -- one can reference variables either with ($a) or without > (a) the dollar sign. However, the latter is slightly better: the > variable is interpreted as an integer, and then the integer is used > inside the expression. With the former, the variable's contents are > spliced into the expression before it is parsed. For example: > > $ a='1+5' > $ echo $((a * 2)) > 12 > $ echo $(($a * 2)) > 11
[Speaking only about POSIX, not OpenBSD ksh and sh.] https://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_06_04 > If the shell variable x contains a value that forms a valid integer > constant, optionally including a leading <plus-sign> or <hyphen-minus>, > then the arithmetic expansions "$((x))" and "$(($x))" shall return the > same value. and before that > - Only the decimal-constant, octal-constant, and hexadecimal-constant > constants specified in the ISO C standard, Section 6.4.4.1 are > required to be recognized as constants. Technically, your example is UB. Then the description of OPTIND also makes no guarantees about whether it can be used as $((OPTIND)) or not. https://pubs.opengroup.org/onlinepubs/9699919799/utilities/getopts.html > [...] the shell variable OPTIND shall be set to the index of the first > operand, or the value "$#" +1 if there are no operands; [...] That being said, consistency is always nice, and quite surely $(($OPTIND - 1)) and $((OPTIND - 1)) behaves the same in most sane usages. -Lucas
