2015-09-01 16:23:08 -0400, Greg Wooledge: > On Tue, Sep 01, 2015 at 03:13:57PM -0500, Dennis Williamson wrote: > > The version of dash I have handy (0.5.7) has math support which IMHO is > > broken: > > > > $ foo=bar > > $ bar=5 > > $ echo $foo > > bar > > $ echo $((foo)) > > dash: 4: Illegal number: bar > > $ echo $(($foo)) > > 5 > > $ echo $((bar)) > > 5 > > $ echo $(($bar)) > > 5 > > > > Note the inconsistency in support of omitting the inner dollar sign. > > $foo is expanded to bar, so the following two lines are always going to > be equivalent: > > echo $(($foo)) > echo $((bar)) > > POSIX also specifies (vaguely!!) that $((x)) and $(($x)) are equivalent.
Note that while POSIX may (vaguely indeed) say $((x)) and $(($x)) are equivalent at least when x contains a litteral number, $((-x)) and $((-$x)) are not equivalent in all shells if $x contains a negative number. $ a=0 x=-1 bash -c 'echo $((a-$x))' bash: a--1: syntax error in expression (error token is "1") $ a=0 x=-1 bash -c 'echo $((a-x))' 1 (they're OK in shells that don't implement the (optional in posix) -- and ++ operators like dash. Or you can add spaces to make it more reliable: $((a - $x)) In ksh/zsh/bash, see also the difference between: $ a=1+1; echo $((a*2)) 4 $ a=1+1; echo $(($a*2)) 3 -- Stephane > >
