On Tue, Sep 1, 2015 at 12:50 PM, Greg Wooledge <wool...@eeg.ccf.org> wrote:
> On Tue, Sep 01, 2015 at 12:50:23AM -0400, Clint Hepner wrote: > > Repeat-By: > > > > foo=bar > > bar=5 > > echo $(( foo )) # produces 5 > > echo $(( foo++ )) # produces 5 > > echo $foo # produces 6, not bar > > echo $bar # produces 5, not 6 > > bar was never changed from its value of 5, so I would say the final > result is correct. > > The $(( foo++ )) part is questionable. You've asked bash to increment > a variable (in a math context), but that variable doesn't have an > integer value. But it "points to" (contains the name of) a variable > that does. > > I would say that any of the following would make sense: > > 1) An error message is printed. > > 2) foo is unchanged (still contains "bar"), and bar is incremented. > > 3) foo takes on the value of bar, and is then incremented. > > Bash happens to do #3. > > If you think of $(( foo++ )) as being basically equivalent to > $(( tmp=foo, foo=foo+1, tmp )) then #3 is actually quite sensible. > > For comparison, ksh 93u+ and zsh 5.0.2 do the same as Bash. 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. Also post increment produces an error and pre increment produces no error and no action: $ echo $(($foo++)) dash: 9: arithmetic expression: expecting primary: "bar++" $ echo $(($bar++)) dash: 10: arithmetic expression: expecting primary: "5++" $ echo $((++$foo)) 5 $ echo $foo bar $ echo $bar 5 $ echo $((++$bar)) 5 $ echo $bar 5 -- Visit serverfault.com to get your system administration questions answered.
