On 2018-02-06 09:01:58 -0500, rhkra...@gmail.com wrote:
> Just an attempt to get a more informative subject line--maybe
> somebody can improve it.
Corrected the subject. This is not related to bash at all (I'm under
zsh, BTW). The GNU date utility comes from the coreutils.
> On Tuesday, February 06, 2018 08:49:39 AM Greg Wooledge wrote:
> > On Tue, Feb 06, 2018 at 01:48:19PM +0100, Vincent Lefevre wrote:
> > > On 2018-02-05 09:39:12 -0500, Greg Wooledge wrote:
> > > > Anyway, here's what I came up with:
> > > >
> > > > lastday() {
> > > >
> > > > date +%Y-%m-%d -d "$1 1 day ago + 1 month"
> > > >
> > > > }
> > >
> > > But the exact meaning of "month" seems undocumented, which may
> > > silently break in a future version (e.g. possibly as a consequence
> > > of a bug fix). So this is a good example of what you should *not*
> > > do.
> >
> > If I have the luxury of writing in a real language, then I prefer
> > to use that language's date/time libraries, if they exist and don't
> > suck.
> >
> > In bash, you get this, or you get to write your own date/time code
> > by hand.
> >
> > > This is completely crazy:
> > >
> > > Where is the logic behind that?
> >
> > As noted by someone else earlier in this thread, date recently learned
> > a --debug option (this is my first time hearing about it), so let's
> > try that:
> >
> > wooledg:~$ date --debug +%Y-%m-%d -d '2003-09-01 1 day ago + 1 month'
> > date: parsed date part: (Y-M-D) 2003-09-01
> > date: parsed relative part: -1 day(s)
> > date: parsed relative part: +1 month(s) -1 day(s)
> > date: input timezone: -05:00 (set from system default)
> > date: warning: using midnight as starting time: 00:00:00
> > date: starting date/time: '(Y-M-D) 2003-09-01 00:00:00 TZ=-05:00'
> > date: warning: when adding relative months/years, it is recommended to
> > specify the 15th of the months date: warning: when adding relative days,
> > it is recommended to specify 12:00pm date: after date adjustment (+0
> > years, +1 months, -1 days),
> > date: new date/time = '(Y-M-D) 2003-09-30 00:00:00 TZ=-05:00'
> > date: '(Y-M-D) 2003-09-30 00:00:00 TZ=-05:00' = 1064894400 epoch-seconds
> > date: output timezone: -05:00 (set from system default)
> > date: final: 1064894400.000000000 (epoch-seconds)
> > date: final: (Y-M-D) 2003-09-30 04:00:00 (UTC0)
> > date: final: (Y-M-D) 2003-09-30 00:00:00 (output timezone TZ=-05:00)
> > 2003-09-30
> >
> > So, it adds "a month" first, then subtracts "a day" second. I suppose
> > I could have rearranged the sub-arguments to make that clearer, but
> > honestly, I stopped tweaking it once I had a working invocation. It's
> > so fragile that I didn't want to change *anything* for fear of breaking
> > it.
> >
> > In some cases, I dealt with this stuff using a truly ancient version
> > of GNU date from the sh-utils package, before coreutils existed. Some
> > of the more modern invocations didn't work with the older program, so
> > I often had to find different tricks that would work. I don't know
> > whether this was one such case. It could have been.
--
Vincent Lefèvre <vinc...@vinc17.net> - Web: <https://www.vinc17.net/>
100% accessible validated (X)HTML - Blog: <https://www.vinc17.net/blog/>
Work: CR INRIA - computer arithmetic / AriC project (LIP, ENS-Lyon)
