semantics: prefix and postfix, "++" and "--" operators

[Denys Vlasenko]

On Sunday 19 April 2009 22:54, Cristian Ionescu-Idbohrn wrote:
> Walter Harms pointed it out, initially.  Consider this:
> 
> m=999
> echo initial m=$m
> echo "m++" m=$((m++))
> echo "++m" m=$((++m))
> echo "m--" m=$((m--))
> echo "--m" m=$((--m))
> 
> math.c defines this:
> 
> /* longest must be first */
> static const char op_tokens[] ALIGN1 = {
> ...
> 	'-','-',    0, TOK_POST_DEC,
> ...
> 	'+','+',    0, TOK_POST_INC,
> 
> Is it TOK_POST_... or TOK_PRE_...?
> 
> This works:
> 
> 	echo "++m" m=$((++m))
> 	echo "--m" m=$((--m))
> 
> but not this:
> 
> 	echo "m++" m=$((m++))
> 	echo "m--" m=$((m--))

It does:

# ./busybox hush -c 'm=1; echo "m++" m=$((m++)); echo m=$m'
m++ m=1
m=2

# ash -c 'm=1; echo "m++" m=$((m++)); echo m=$m'
m++ m=1
m=2

# bash -c 'm=1; echo "m++" m=$((m++)); echo m=$m'
m++ m=1
m=2

--
vda