In Bash, why does using getopts in a function only work once?

Question

I'm trying to create a simple function on macOS Sierra that counts the characters in a string. This works fine (added to my bashrc file):

function cchar() {
    str=$1
    len=${#str}
    echo "string is $len char long"
}

$ cchar "foo"
string is 3 char long

I'm trying to expand it with a -a option, so I added this to my function (and commented the rest out for testing):

while getopts "a:" opt; do
    case $opt in
        a)
            echo "-a param: $OPTARG" >&2
            ;;
    esac
done

After some testing while writing this, I've noticed everytime I run cchar -a "test", I have to run it without options (cchar) otherwise the next time I run it with the -a option, it doesn't recognise the option.

$ cchar

$ cchar -a "foo"
-a param: foo

$ cchar -a "foo"

$ cchar

$ cchar -a "foo"
-a param: foo

My end goal is to have the `-a` option only return the string count as an int, for use in other functions. I came upon this issue while trying to do that. — Trav Easton, Feb 20 '17 at 04:35
I tried `unset opt` after the `while` loop in case the var is remembered between after the function ends, didn't fix it. Also, different redirections had no positive effect (`&2>1`, `>&1`, [blank]) — Trav Easton, Feb 20 '17 at 04:52
Redirections are not command-line options. Why would they affect anything? — rici, Feb 20 '17 at 04:56
In the 4th line of my second code block. Since that wasn't echoing, I thought there could be a problem there. — Trav Easton, Feb 20 '17 at 05:06

score 7 · Accepted Answer · answered Feb 20 '17 at 04:55

7

You have to reset the variable OPTIND, which keeps track of the current positional argument number. It should suffice to make that variable local to your function.

answered Feb 20 '17 at 04:55

rici

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That solved my problem, thank you. – Trav Easton Feb 20 '17 at 05:09
See [getopts](https://www.gnu.org/software/bash/manual/bash.html#index-getopts) in the Bash manual. It says, in part: _Each time it is invoked, getopts places the next option in the shell variable `name`, initializing `name` if it does not exist, and the index of the next argument to be processed into the variable OPTIND. OPTIND is initialized to 1 each time the shell or a shell script is invoked. … The shell does not reset OPTIND automatically; it must be manually reset between multiple calls to getopts within the same shell invocation if a new set of parameters is to be used._ – Jonathan Leffler Feb 20 '17 at 05:16
1

Yes, `getopts` is designed for parsing script options, not function arguments. But it will work just as well with local variables (although `local OPTIND=1` is probably better, just in case.) – rici Feb 20 '17 at 05:22
What would be a better way to handle function arguments? – Trav Easton Feb 20 '17 at 05:32
@trav: getopts is just fine. I use it a lot. I was trying to explain the failure to initialize OPTIND. – rici Feb 20 '17 at 05:34
Plus one for the local variable override. – xdhmoore Jul 17 '17 at 21:32

Trav Easton · Answer 2 · 2017-02-20T06:01:57.627

0

The final code I ended up using:

# Count characters in a string
function cchar() {
    local OPTIND

    while getopts "a:" opt; do
        case $opt in
            a)
                echo ${#OPTARG}
                return
                ;;
        esac
    done

    echo "string is ${#1} characters long"
}

$ cchar "foo bar"
string is 7 characters long

$ cchar -a "foo bar"
7

Note: I had to use return in lieu of exit, when sourced from .bashrc, exit will close the current shell.

edited Feb 20 '17 at 06:01

answered Feb 20 '17 at 05:17

Trav Easton

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1

Note that you could simply use: `echo "string is ${#1} char long"` in the long form (and `echo "${#OPTARG}" >&2` in the short form). Is there a good reason to write to standard error in the short form? It would be more usual to write to standard output — you can then redirect the output to standard error when you invoke the function if that's what you need. – Jonathan Leffler Feb 20 '17 at 05:21
You're right, thanks. I was writing to stderr because [the accepted answer in this question](http://stackoverflow.com/questions/14447406/bash-shell-script-check-for-a-flag-and-grab-its-value) used it. – Trav Easton Feb 20 '17 at 05:59

In Bash, why does using getopts in a function only work once?

2 Answers2