#!/usr/pkg/bin/bash

# logfile is $1 or ./log
f="${1:-./log}"

# if file exists we'll keep it's content
[[ -f "$f" ]] && l="$f"

# use date in UTC
d="$(env TZ=UTC date +%F)"

# Bash moment
read -rd '' m < <(
  cat <(printf "[$d] ") <(read -rep "[$d] "; echo "$REPLY") $l )

# overwrite the file only after cating the whole thing together to
# eliminate pipeline weirdness
echo "$m" > "$f"