How to truncate a String in the middle with ellipsis POSIX compliant?

In a shell script I would like to truncate an argument when it exceeds a length of 9 to a total length of 9 (first 4 and last 4 characters with an UTF-8 ellipsis in between). It’s crucial to me to truncate it in the middle, since the first part is the name, while the last part often includes a number that helps to identify the thing it names.

• foobarSN9 should turn into foobarSN9
• foobarSN10 should turn into foob…SN10

How can this be done POSIX compliant in as little code as possible?

>Solution :

Tested in dash

trunc() {
    if [ "${#1}" -le 9 ]; then
        echo "$1"
    else
        printf '%s…%s\n' "${1%%"${1#????}"}" "${1##"${1%????}"}"
    fi
}

$ trunc "hello"
hello
$ trunc "hello world"
hell…orld