gistfile1.sh

#!/bin/sh
#
# e.g. mkString : a b c   becomes    a:b:c
# A single separator argument means the strings to join are on stdin.
#
# mkString "\n" < /some/file does get the newline
# through the obstacle course.

mkString () {
  local sep="$1"
  local first=1

  while read line; do
    if (( $first )); then
      first=0
      printf "%s" "$line"
    else
      printf "${sep}%s" "$line"
    fi
  done

  echo ""
}

if [[ $# -gt 1 ]]; then
  sep="$1" && shift && ( IFS="$sep" ; echo "$*"; )
else
  mkString "$@"
fi