Re: Help with "getopts"

On Wed, 29 Aug 2007 12:55:46 -0400, Douglas O'Neal wrote:

Rich Leitner wrote:
I'm writing some shell scripts and trying desperately to understand
"getopts", specifically it's intrinsic variable $OPTIND. The guidance
I've seen so far seems straightforward enough but then just doesn't
seem to jibe with what I actually see when using it. What exactly is
$OPTIND counting? There 'seems' to be a pattern, but every time I think
I've figured out what it is, it 'breaks' for another form of calling a
script. Here's an example ... a simple shell script called 'optscript':

#!/bin/bash
while getopts "abc:def:ghi" flag
do
echo "flag: $flag" "OPTIND: $OPTIND" "OPTARG: $OPTARG"
done

Now the various outputs:

bash>./optscript -abc charlie
flag: a OPTIND: 1 OPTARG:
flag: b OPTIND: 1 OPTARG:
flag: c OPTIND: 3 OPTARG: charlie

bash>./optscript -a -b -c charlie
flag: a OPTIND: 2 OPTARG:
flag: b OPTIND: 3 OPTARG:
flag: c OPTIND: 5 OPTARG: charlie

bash>./optscript -abc charlie -def foxtrot flag: a OPTIND: 1 OPTARG:
flag: b OPTIND: 1 OPTARG:
flag: c OPTIND: 3 OPTARG: charlie
flag: d OPTIND: 3 OPTARG:
flag: e OPTIND: 3 OPTARG:
flag: f OPTIND: 5 OPTARG: foxtrot

bash>./optscript -a -b -c charlie -d -e -f foxtrot flag: a OPTIND: 2
OPTARG:
flag: b OPTIND: 3 OPTARG:
flag: c OPTIND: 5 OPTARG: charlie
flag: d OPTIND: 6 OPTARG:
flag: e OPTIND: 7 OPTARG:
flag: f OPTIND: 9 OPTARG: foxtrot

Can anyone tell me what actually causes $OPTIND to increment up one (or
two), and what causes it to stay the same? Thanks ...

Rich Leitner

OPTIND is set to the index of the next argument to be processed
(straight from the man page). For your first example, -abc is index 1
and charlie is at index 2. First invocation processes argument "a" and
OPTIND is kept at 1 since there is more to be processes at that
position. Second invocation processes "b" and still keep OPTIND at 1.
Third invocation processes "c" and pulls in "charlie" as the option
argument. OPTIND is then set to be 3 to go one past "charlie".

Second example: process "-a", set OPTIND to 2 so -b can be processed
next. Process -b, set OPTIND to 3 so -c can be processed next. Process
-c and pull in "charlie", set OPTIND to 5, one past "charlie".

Every time you finish with all the arguments at a given position in the
command line, increment OPTIND to point to the next unprocessed
argument.

Doug

Okay Doug, I'm running it thru all the examples I can think of ... and it
seems to be applicable to all! I guess my confusion was with what
constituted an 'argument': options grouped together, like '-abc' are
considered 'one argument' by getopts despite the fact that for the shell
script '-abc' is three arguments after parsing by getopts. Your
explanation was excellent, and I think I finally get it ... thanks.

Rich Leitner
.

Relevant Pages

  • Help with "getopts"
    ... specifically it's intrinsic variable $OPTIND. ... flag: a OPTIND: 1 OPTARG: ... flag: c OPTIND: 3 OPTARG: charlie ...
    (alt.os.linux)
  • Re: Help with "getopts"
    ... What exactly is $OPTIND counting? ... flag: a OPTIND: 1 OPTARG: ... flag: c OPTIND: 3 OPTARG: charlie ...
    (alt.os.linux)