Shell Programming and Scripting

Hi stomp,
You are correct in noting that some shells (including recent versions of bash and ksh93) can use ((cnt++)) as a shortcut for cnt=$((cnt+1)) and MadeInGermany's suggestion of cnt=$((cnt+=1)) and can also use last=${var: -1} as a shortcut for RudiC's suggestion of last=${var#${var%?}} to get the last character from the expansion of $var, but more shells (in fact any POSIX-conforming shells) support arithmetic expansions ($((expression))), and the ${var#pattern} and ${var%pattern} parameter expansions. The arithmetic command (((expression))) and the substring parameter expansions (${var:offset} and ${var:offset:length}) are extensions allowed, but not defined by the standards, and are not provided by as many shells.

But, I don't know of any shell where cnt=$((cnt++)) is required to be the same as cnt=$((cnt+1)) because the order in which a post-increment operation on a variable and an assignment to the same variable happens is unspecified. In both ksh (version: (AT&T Research) 93u+ 2012-08-01) and bash (version: 3.2.57(1)-release (x86_64-apple-darwin15)), the commands:

Code:

cnt=1
cnt=$((cnt++))
echo $cnt

produce the output:

Code:

1

Imagine the sequence of operations of cnt=$((cnt++)) as:But, it could also be processed as:
You have exactly the same problem in C and C++ with:

Code:

#include <stdio.h>
int main() {
	int	i = 1;
	int	j;

	for(j = 1; j <= 5; j++)
		printf("%d\n", i = i++);
}

Building it on some systems gives the warning:

Code:

cc     x.c   -o x
x.c:7:23: warning: multiple unsequenced modifications to 'i' [-Wunsequenced]
                printf("%d\n", i = i++);
                                 ~  ^
1 warning generated.

and, when run, producing the output:

Code:

1
1
1
1
1

or:

Code:

1
2
3
4
5

(But, on the system I'm currently using, it always produces just 1s as the output.)

Sorrry, I wanted to suggest

Code:

cnt=$((cnt+1))

Added the = without seeing the double assignment.
BTW, only the other assignment, should be possible as well

Code:

: $((cnt+=1))