$ strjoin --- foo bar baz quux
foo---bar---baz---quux

$ echo foo bar baz quux | sed "s/ /---/g"

$ echo foo bar baz quux | ex +"s/ /---/gp" -cq! /dev/stdin
$ ex +"s/ /---/gp" -scq! <(echo foo bar baz quux)

$ printf "%s---" foo bar baz quux

join_by { local IFS="$1"; shift; echo "$*"; }

$ join_by '---' foo bar baz quux

echo foo bar baz quux | tr ' ' '-'

function join(a, start, end, sep, result, i) {
    sep = sep ? sep : " "
    start = start ? start : 1
    end = end ? end : sizeof(a)
    if (sep == SUBSEP) # magic value
       sep = ""
    result = a[start]
    for (i = start + 1; i <= end; i++)
        result = result sep a[i]
    return result
}

$ gawk -f join.awk --source 'BEGIN { split("foo bar quux",a); print join(a,1,3,"---") }'
foo---bar---quux

function join() {
    for i in "$@"; do
        echo -n "$i""---"
    done
    echo
}

join foo bar baz quux

$ ./join.sh | sed 's/---$//'
foo---bar---baz---quux

$ perl -e 's/ /---/g'

$ SEP="---"; lam <(echo foo) -s$SEP <(echo bar) -s$SEP <(echo baz) -s$SEP <(echo quux)
foo---bar---baz---quux

$ printf "%sn" foo bar baz quux | paste -sd-
foo-bar-baz-quux