It would certainly be possible to whip together something in Python to query a URL to see when it was last modified, using the HTTP headers, but I wondered if there is an existing tool that can do that for me? I'd imagine something like:

% checkurl http://unix.stackexchange.com/questions/247445/
Fri Dec  4 16:59:28 EST 2015

or maybe:

% checkurl "+%Y%m%d" http://unix.stackexchange.com/questions/247445/

as a bell and/or whistle. I don't think that wget or curl have what I need, but I wouldn't be surprised to be proven wrong. Is there anything like this out there?

  • 1
    curl --head url seems to report the headers to me. Assuming a Last-Modified header does come through, curl --header url | awk '/Last-Modified/{print $2}' should be able to extract the value – iruvar Dec 4 '15 at 22:26
  • Note, though, that Last-Modified headers are completely useless on many sites because the pages are generated dynamically from a database and will always return a LM header of approximately now. This is often done deliberately as a cache-busting technique and to force re-fetches (and thus marketable page views) when the client requests the page with an If-Modified-Since request header. – cas Dec 5 '15 at 0:49
  • That's true enough. My particular use case is to monitor Web-based downloads of databases, so that's less of an issue. Still something to be mindful of, though. – Scott Deerwester Dec 5 '15 at 1:10

This seems to fit your requirements (updated to use '\r\n' as record separator for response data):


    curl --silent --head "${1:?URL ARG REQUIRED}" | 
    awk -v RS='\r\n' '
        /Last-Modified:/ {
            gsub("^[^ ]*: *", "")

unset date_format
case $1 in

url_date="$(get_url_date "${1:?URL ARG REQUIRED}")"

if [ -z "$url_date" ]
    exit 1

if [ "$date_format" != "" ]
    date "$date_format" -d"$url_date"
    echo "$url_date"

A Perl one-liner:

% perl -MLWP::Simple -MDate::Format -e 'print time2str "%C\n", (head $ARGV[0])[2]' http://example.com
Sat Aug 10 02:54:35 EEST 2013

On a modern Linux or FreeBSD system the modules it requires are likely to be already installed.


It turns out that curl and wget can both do this, but it's probably worth doing in Python after all. Here's what I ended up writing:

#!/usr/bin/env python3

import sys, dateutil.parser, subprocess, requests
from getopt import getopt

errflag = 0
gTouch = None
gUsage = """Usage: lastmod [-t file] url
-t file     Touches the given file to make its modification date
            the same as the URL modification date.
url         A URL to be retrieved

opts, args = getopt(sys.argv[1:], "t:v?")

for k, v in opts:
    if k == "-t":           # File to touch
        gTouch = v
    elif k == "-?":         # Write out usage and exit
        errflag += 1

if len(args) != 1:
    errflag += 1

if errflag:

res = requests.head(args[0])

if res.status_code != 200:
    sys.stderr.write("Failed to retrieve URL\n")

if not 'Last-Modified' in res.headers:
    sys.stderr.write("Headers has no last-modified date\n")

dt = dateutil.parser.parse(res.headers['Last-Modified'])

if gTouch:
    subprocess.call(["touch", "-t", dt.strftime("%Y%m%d%H%m"), gTouch])
    sys.stdout.write("%s\n" % dt.ctime())
  • You really want the [requests] (docs.python-requests.org/en/latest) module – iruvar Dec 4 '15 at 23:48
  • I agree in principle, but the above only grabs the header, which is what I really wanted, and it looks like request.get() grabs the whole document, which I really don't want. In fact, using requests does seem to take a LOT longer. If I've got it wrong, would you mind posting a snippet that illustrates how just to check the header? – Scott Deerwester Dec 5 '15 at 0:04
  • Sure. requests.get('http://unix.stackexchange.com/questions/247445/').headers['Last-Modified'] – iruvar Dec 5 '15 at 1:03
  • But does the call to get() retrieve the whole document as well as the headers, or is the retrieval of the document itself postponed until it's required? – Scott Deerwester Dec 5 '15 at 1:08
  • Good point..although I'm approaching the limit of my http knowledge, this should do the job requests.head('http://unix.stackexchange.com/questions/247445/').headers['Last-Modified'] – iruvar Dec 5 '15 at 1:14

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