getopts - displaying warnings/errors when extraneous arguments are provided

retrovertigo · 07-17-2007, 02:32 PM

I have written several scripts that run just fine, but there has always been something that has bothered me as a programmer. When using getopts to parse through the arguments, like so:

Code:

while getopts a:b: option
do
    case $option in
        a) var1=$OPTARG;;
        b) var2=$OPTARG;;
        \?) echo Unknown argument: $option;;
    esac
done

This will still allow someone to enter the following:

Code:

myscript -a 1234 abcd -b efgh

The "abcd" will just be ignored. What I would like to do is be able to detect if multiple arguments are entered after the option, and display a warning/error if this is the case. Also, I'd like to be able to accept multiple arguments after an option, and then process them individually in a loop.

I know there has to be a relatively simple way to do this, maybe by using the value of OPTINDEX or something, but I haven't been able to figure it out yet.

Also, if anyone knows of a really good resource, online or otherwise, that I can use to round out my scripting knowledge, and maybe tackle some more advanced concepts, I'd love some suggestions. I've only recently been learning how to script in bash, over the last several months.

cfaj · 07-18-2007, 12:53 AM

Quote:

Originally Posted by retrovertigo

I have written several scripts that run just fine, but there has always been something that has bothered me as a programmer. When using getopts to parse through the arguments, like so:

Code:

while getopts a:b: option
do
    case $option in
        a) var1=$OPTARG;;
        b) var2=$OPTARG;;
        \?) echo Unknown argument: $option;;

You are looking for a literal question mark. Either remove the backslash or use:
Code:
        *) echo Unknown argument: $option; exit 1;;

Quote:

Code:

    esac
done

This will still allow someone to enter the following:

Code:

myscript -a 1234 abcd -b efgh

You cannot mix arguments and options with getopts; the options must precede the arguments.

Quote:

The "abcd" will just be ignored. What I would like to do is be able to detect if multiple arguments are entered after the option, and display a warning/error if this is the case. Also, I'd like to be able to accept multiple arguments after an option, and then process them individually in a loop.

If you are going to use getopts, you cannot have multiple arguments to an option unless you enter them as a single argument (e.g., "arg1,arg2") and parse them yourself.

Quote:

I know there has to be a relatively simple way to do this, maybe by using the value of OPTINDEX or something, but I haven't been able to figure it out yet.

You use $OPTIND to tell how many arguments to shift after they have been parsed. E.g.:
Code:
while getopts a:b: opt
do
  ....
done
shift "$(( $OPTIND - 1 ))"

Quote:

Also, if anyone knows of a really good resource, online or otherwise, that I can use to round out my scripting knowledge, and maybe tackle some more advanced concepts, I'd love some suggestions. I've only recently been learning how to script in bash, over the last several months.

I have a list of resources at http://cfaj.freeshell.org/shell/resources.shtml

retrovertigo · 07-18-2007, 06:25 AM

Thanks for the link, I'll have to look at that.

Maybe I used the wrong terminology when I mixed talking about arguments or options. I basically want to be able to detect if there was more than one thing entered after the -a, and process what was entered accordingly. From what I gather, getopts will not allow me to do this, is there a better way? Maybe within my case statement I can store the (OPTIND - 1) value to give me the location of both -a and -b, and then use those positions to determine which of the parameters I want to process?

Is there a way to reference $1, $2, $3, etc. dynamically using a value from a variable? something like this:

Code:

index=1
echo $${index}

The above doesn't work of course, but is there a way to do that?

retrovertigo · 07-18-2007, 07:10 AM

Disregard the above. I tested this, and including multiple parameters after the -a frelled up the option(s) that followed.

So, if I want to handle multiple arguments without comma-delimiting them, getopts is not the way to do it.

However, if anyone knows how to reference an argument dynamically like I was describing above, then I'd love to hear how to do so.

cfaj · 07-18-2007, 02:58 PM

Quote:

Originally Posted by retrovertigo

Is there a way to reference $1, $2, $3, etc. dynamically using a value from a variable? something like this:

Code:

index=1
echo $${index}

The above doesn't work of course, but is there a way to do that?

The standard, portable method is:
Code:
eval "echo \${$index}"
Bash has:
Code:
echo ${!$index}
... but it's not portable, so I usually avoid it.

P.S. I'll post something about option handling when I have a few spare moments.

retrovertigo · 07-18-2007, 03:44 PM

Thanks, that worked. You've been a lot of help. Any further techniques you can recommend for processing options will be much appreciated.