a++
is post-increment: it increments after the statement is evaluated. By contrast, ++a
increments before. Thus:
$ a=0 ; ((a++)) ; echo $a $?
1 1
$ a=0 ; ((++a)) ; echo $a $?
1 0
In the first case, ((a++))
, the arithmetic expression is evaluated first, while a
is still zero, yielding a value of zero (and hence a nonzero return status). Then, afterward, a
is incremented.
In second case, ((++a))
, a
is incremented to 1 and then ((...))
is evaluated. Since a
is nonzero when the arithmetic expression is evaluated, the return status is zero.
From man bash
:
id++ id--
variable post-increment and post-decrement
++id --id
variable pre-increment and pre-decrement