#!/bin/bash

_help() {
  echo '
Usage: file-age {file} [units]

  Checks file modification date, and the current time. Prints the age of
  the file in [units].
  
  [units] is one of S,M,H,d,w,m,y
          seconds is the default. All values floored.
          month = 30 days
          year = 365 days
'
exit
}
for (( i=1; i<=$#; i++ ))
do  [[ "${!i}" = "-h" ]] && _help
done

STATFILE="$1"
[[ -z "$STATFILE" ]] && _help
test -e "$STATFILE" || { echo File "$STATFILE" does not exist >&2; exit 1; }

UNITS=${2:-S}
case $OSTYPE in
  darwin*) created=$( stat -f %c -t %s "$STATFILE" ) ;;
  *) created=$( stat -c %Y "$STATFILE" ) ;;
esac
now=$( date +%s )
age=$(( $now - $created ))

case $UNITS in
  S)
    echo $age;;
  M)
    echo $(( $age / 60 ));;
  H)
    echo $(( $age / 3600 ));;
  d)
    echo $(( $age / 86400 ));;
  w)
    echo $(( $age / 604800 ));;
  m)
    echo $(( $age / 2592000 ));;
  y)
    echo $(( $age / 31536000 ));;
  *)
    echo Unit $UNITS not recognized.
    _help;;
esac