eml2mbox

#!/bin/sh
help() { cat <</help
Convert a list of files (via STDIN or command line) to mbox format on STDOUT
Usage: eml2mbox [OPTIONS] [FILE...]

  --rfc822      FILEs are all RFC 822 (RFC 5322, .eml) (default: check 1st FILE)
  --email=EMAIL Use EMAIL in \`From\` delimiter instead of the default address:
                $delim

  FILEs may be either .eml (RFC 5322) file or it may be a list of such files.
  eml2mbox will assume everything is the same format as the first input file.

Part of net-scripts: https://github.com/adamhotep/net-scripts
eml2mbox 0.4.20170408.1, Copyright 2010+ by Adam Katz, GPLv2+
/help
  exit
}

die() {
  echo "$*" >&2
  exit 2
}

mkmbox() {
  awk -v DELIM="$(date +"From $delim  %c")" '
    FNR == 1 { print DELIM }
    { print }' "$@"
}

delim='MAILER-DAEMON@fake-data.delimiter.eml2mbox'

while getopts h-: opt; do
  case "$opt" in
    ( h )		help ;;
    ( - )		case "$OPTARG" in
      ( email=?* )	delim="$OPTARG" ;;
      ( email* )	die "No arg for --email option ... Try --help" ;;
      ( help*  )	help ;;
      ( rfc*22 )	RFC822=1 ;;
      ( * )		die "Illegal option --$OPTARG ... Try --help" ;; esac ;;
    ( \? )		die "Try --help" ;;
  esac
done
shift $((OPTIND-1))

if [ -n "$RFC822" ] || [ -r "$1" ] && file "$1" |grep -liw mail >/dev/null; then
  mkmbox "$@"
elif [ -n "$RFC822" ]; then
  echo "Not sure what to do; your input file(s) were supposed to contain" >&2
  echo "either .eml files (rfc822) or else a list of .eml file paths." >&2
  echo "Since your input(s) were not .eml, we parsed them line by line" >&2
  echo "and tried to read each full line as a file, but that didn't work." >&2
  exit 2
else
  # Assume one file per line, null-delimit to preserve spaces
  # (Using  xargs -d "\n"  requires GNU xargs, so this is more reliable)
  awk '{printf "%s%c", $0, 0}' "$@" |xargs -0 \
    "$(which "$0" 2>/dev/null || echo "$0")" --rfc822
fi