Date: Thu, 17 Sep 2009 06:46:40 +0100 From: Matthew Seaman <m.seaman@infracaninophile.co.uk> To: Martin McCormick <martin@dc.cis.okstate.edu> Cc: freebsd-questions@freebsd.org Subject: Re: A question about the date Function Message-ID: <4AB1CD40.5020307@infracaninophile.co.uk> In-Reply-To: <200909162025.n8GKP4M6095537@dc.cis.okstate.edu> References: <200909162025.n8GKP4M6095537@dc.cis.okstate.edu>
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Martin McCormick wrote:
=20
> date -j -f "%a %b %d %T %Z %Y" "`date`" "+%s" >f0
> date +%s >f1
> What does the long form of this command give us that
> date +%s fails to do?
It's a contrived example:
date -j -f "%a %b %d %T %Z %Y" "`date`" "+%s"=20
-j says "don't alter the system date" -- this is used if you want
to read and format a date/time string other than the present time.
-f says use the following format to read the input date. That's
%a -- abbreviated weekday name (localized)
%b -- abbreviated month name (localized)
%d -- day of month as decimal number, zero padded to two digits
%T -- equivalent to %H:%M:%S
%H -- Hour in 24h clock, zero padded to two digits
%M -- Minute, zero padded
%S -- Second, zero padded
%Z -- Time zone name
%Y -- Year as 4 digits including century.
(See strftime(3))
Which looks like this:
% date +"%a %b %d %T %Z %Y"
Thu Sep 17 06:31:15 BST 2009
and that just happens to be the default *output* format date produce=
s
without any arguments. Which is appropriate as the next item on the=
command
line is
"`date`" Rune the date command without arguments and substitute the ou=
tput
into the command line here as a single argument
+%s finally, says output the date that was read in as the number of se=
conds
since the epoch. This is an argument to the initial date command.
so the end result is that the command reads the current date time in the =
standard
output format, parses all of that then converts it into seconds-since-the=
-epoch,
using two invocations of the date(1) program to do so. Which is not at al=
l efficient
if all you need to do is generate the current epoch time. Just use
date +%s
for that.
On the other hand, it does show you how to convert an arbitrary date/time=
to=20
epoch time. eg.:
% date -j -f "%a %b %d %T %Z %Y" "Fri Feb 13 23:31:30 GMT 2009" +%s
1234567890
Cheers,
Matthew
--=20
Dr Matthew J Seaman MA, D.Phil. 7 Priory Courtyard
Flat 3
PGP: http://www.infracaninophile.co.uk/pgpkey Ramsgate
Kent, CT11 9PW
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