a sponge replacement in case it is too difficult to install moreutils

sponge

#!/usr/bin/env bash

function main() {
  local APPEND=0
  
  while (( "$#" )); do
    # Replace --parm=arg with --parm arg
    [[ $1 == --*=* ]] && set -- "${1%%=*}" "${1#*=}" "${@:2}"
    case $1 in
      -a|--append)
        APPEND=1
      ;;
    esac
    shift
  done
  # Set all positional arguments back in the proper order
  eval set -- "${ARGS[@]}"

  out=$1
  temp=$(mktemp "${out%%/*}/tmp-sponge.XXXXXXXX") &&
    cat > "$temp" &&
    if (( APPEND )) then
      cat "$temp" >> "$out"
    else
      if [[ -f $out ]]; then
        chmod --reference="$out" "$temp"
        mv "$temp" "$out"
      elif [[ -n $out ]]; then
        cat $temp > $out
      else
        cat $temp
      fi
    fi &&
    rm -f "$temp"
}

main "$@"