noticed via lsof: $ dash -c 'echo $(())' dash: 1: arithmetic expression: expecting primary: "" $ bash -c 'echo $(())' 0 $ busybox ash -c 'echo $(())' 0 not sure if this behavior is mandated by POSIX. need to ask on their mailing lists. the relevant part of the spec: http://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_01 http://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap01.html#tag_17_01_02_01
after discussion on POSIX lists, looks like both behaviors are acceptable
