unix crontab and longest interval
simple brain teaser:
given standard unix crontab what is the longest possible interval between calls to the same program?
for example:
15 */2 * * * /bin/echo x
has interval 2 hours.
what is the longest possible? and how to make it?
March 21st, 2008 at 15:17
A year (@yearly).
March 21st, 2008 at 15:18
just in case - i know the answer. or at least i think i know.
March 21st, 2008 at 15:27
4 years? (29 feb)
March 21st, 2008 at 15:34
if i’m right, this is the correct answer:
perl -MMIME::Base64 -le ‘print decode_base64″MjggeWVhcnMuIGNyb246IDAgMCAyOSAyIDI=”‘
run this, it will print answer - i “coded” it not to spoil teaser for others.
March 21st, 2008 at 22:18
I don’t understand
Why you place “2″ on year column?
What it gives you?
March 21st, 2008 at 22:30
I would guess something like this:
python -c ‘import base64; print base64.decodestring(”MCAwIDI5IDIgNCAtPiBnaXZlcyAyOCB5ZWFycywgbmV4dDogMjAzNg==\n”)’
On the other hand remember about leap seconds! Crontab doesn’t support second precision, but if it did: 61st second, monday, 30th june
I think nobody can tell me whan it would happen
March 22nd, 2008 at 03:51
python -c “print ‘IyEvdXNyL2Jpbi9weXRob24KCmZyb20gZGF0ZXRpbWUgaW1wb3J0IGRhdGV0aW1lCgptYXhfeWVh\ncnMgPSAwCnByZXZfdHVlMl8yOSA9IHN0YXJ0ID0gMTk4NAoKZm9yIHllYXIgaW4gcmFuZ2UoIHN0\nYXJ0LCAzMDAwICk6Cgl0cnk6CgkJaWYgZGF0ZXRpbWUoIHllYXIsIDIsIDI5ICkud2Vla2RheSgp\nID09IDI6CgkJCXllYXJzID0geWVhciAtIHByZXZfdHVlMl8yOQoJCQlpZiB5ZWFycyA+IG1heF95\nZWFyczoKCQkJCW1heF95ZWFycyA9IHllYXJzCgkJCXByZXZfdHVlMl8yOSA9IHllYXIKCWV4Y2Vw\ndCBWYWx1ZUVycm9yOgoJCXBhc3MKCnByaW50IG1heF95ZWFycwoK’.decode(’base64′)” | python
March 22nd, 2008 at 04:11
I see you liked python coded answers
Now I know that last “column” is day of week, not a year
my mistake
It changes a lot
March 22nd, 2008 at 11:13
@Evolic:
tuesday. cron means: every 29th of february, which is tuesday (i could use any day, but tuesday looks nice)
@majek:
great - actually - it’s practically the same solution - it shouldn’t matter whether there is “2″, “4″or anything else on last position.
March 22nd, 2008 at 11:16
apparently i was mistaken, and my solution for 28year long period is not valid (this also applies for majek solution).
reason is in manual to crontab:
…
Note: The day of a command’s execution can be specified by two fields — day of month, and day of week. If both fields are restricted (i.e., aren’t *), the command will be run when either field matches the current time. For example,
“30 4 1,15 * 5″ would cause a command to be run at 4:30 am on the 1st and 15th of each month, plus every Friday.
…
so, actually cezio is right - the longest period would be on 0 0 29 2 *.