Date: Tue, 19 Jul 2022 15:08:12 +0100 From: Frank Leonhardt <freebsd-doc@fjl.co.uk> To: questions@freebsd.org Subject: Re: How do get elapsed time in milliseconds in a shell script? Message-ID: <b764d6fb-0e14-ad7f-4d70-25d066591d4c@fjl.co.uk> In-Reply-To: <CAFMGiz8NcH1BMiRkftz67qd=6jicvPzrfagr40hSN94i87jxVw@mail.gmail.com> References: <4c98bb6d-a128-d10b-a896-6af11c06d41c@fjl.co.uk> <CAFMGiz8NcH1BMiRkftz67qd=6jicvPzrfagr40hSN94i87jxVw@mail.gmail.com>
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On 18/07/2022 14:45, Tom Browder wrote:
>
>
> On Tue, Jul 12, 2022 at 03:09 Frank Leonhardt <freebsd-doc@fjl.co.uk>
> wrote:
>
> I thought this would be easy, but it's not!
>
> I can get the time since the epoch to the nearest second (date
> +%s), but
> nothing more accurate. This was probably good enough in the 1970s
> but I
> think it's reasonable to want a better resolution even in a shell
> script.
>
>
> I’m a wannabe FreeBSD user lurking here. Can you tell us more about
> your use case?
>
> (Spoiler alert: I’m a long time Unix/Linux user who now relies mostly
> on Raku for other than very simple bash scripts.)
>
There is no use case here, other than trying to figure out a truly
portable way of getting a high resolution tick count.
Unix isn't a real-time OS. When it was written, one second was a pretty
short unit of time, and the smallest unit handled in many cases (e.g.
sleep, date and the strftime() subroutine). With Unix computers now
10,000 times faster, where one second resolution was adequate in the
past, something a bit higher would make sense now. It seems like there
isn't a clean way to do it using the widely supported Bourne shell (of
which bash is a clone with backward compatibility). I was hoping there
was a trick I wasn't aware of, but it seems not.
Linux has an extra strftime() macro %N (IIRC) that somehow gets the
fractions of a second, although it's hard to see how as the input is a
Unix time_t (i.e. one second resolution). As Steve O'Hara-Smith
<steve@sohara.org> pointed out, there's a FreeBSD-specific tick count
available as a kernel state variable, which allows a solution but
requires conditional code in the script.
Newer versions of BSD have a replacement function called gettimeofday(),
that passes a timeval structure containing a time_t and the time into
the current second to microsecond resolution. Before that there was an
ftime() library routine from about Unix Version 7, which used a timeb
structure that included milliseconds instead (16-bit, see?). I'm pretty
sure Linux supports gettimeofday() too.
So - if you're writing in 'C' you can get good resolution; if you're
using a shell script it seems like you're out of luck if you want
something portable. Unless there is a trick we've all missed.
Regards, Frank.
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<div class="moz-cite-prefix">On 18/07/2022 14:45, Tom Browder wrote:<br>
</div>
<blockquote type="cite"
cite="mid:CAFMGiz8NcH1BMiRkftz67qd=6jicvPzrfagr40hSN94i87jxVw@mail.gmail.com">
<meta http-equiv="content-type" content="text/html; charset=UTF-8">
<div><br>
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<div><br>
<div class="gmail_quote">
<div dir="ltr" class="gmail_attr">On Tue, Jul 12, 2022 at
03:09 Frank Leonhardt <<a
href="mailto:freebsd-doc@fjl.co.uk" moz-do-not-send="true"
class="moz-txt-link-freetext">freebsd-doc@fjl.co.uk</a>>
wrote:<br>
</div>
<blockquote class="gmail_quote" style="margin:0 0 0
.8ex;border-left:1px #ccc solid;padding-left:1ex">I thought
this would be easy, but it's not!<br>
<br>
I can get the time since the epoch to the nearest second
(date +%s), but <br>
nothing more accurate. This was probably good enough in the
1970s but I <br>
think it's reasonable to want a better resolution even in a
shell script.</blockquote>
<div dir="auto"><br>
</div>
<div dir="auto">I’m a wannabe FreeBSD user lurking here. Can
you tell us more about your use case?</div>
<div dir="auto"><br>
</div>
<div dir="auto">(Spoiler alert: I’m a long time Unix/Linux
user who now relies mostly on Raku for other than very
simple bash scripts.)</div>
<div dir="auto"><br>
</div>
</div>
</div>
</blockquote>
<p>There is no use case here, other than trying to figure out a
truly portable way of getting a high resolution tick count.</p>
<p>Unix isn't a real-time OS. When it was written, one second was a
pretty short unit of time, and the smallest unit handled in many
cases (e.g. sleep, date and the strftime() subroutine). With Unix
computers now 10,000 times faster, where one second resolution was
adequate in the past, something a bit higher would make sense now.
It seems like there isn't a clean way to do it using the widely
supported Bourne shell (of which bash is a clone with backward
compatibility). I was hoping there was a trick I wasn't aware of,
but it seems not.<br>
</p>
<p>Linux has an extra strftime() macro %N (IIRC) that somehow gets
the fractions of a second, although it's hard to see how as the
input is a Unix time_t (i.e. one second resolution). As Steve
O'Hara-Smith <a class="moz-txt-link-rfc2396E" href="mailto:steve@sohara.org"><steve@sohara.org></a> pointed out, there's a
FreeBSD-specific tick count available as a kernel state variable,
which allows a solution but requires conditional code in the
script.</p>
<p>Newer versions of BSD have a replacement function called
gettimeofday(), that passes a timeval structure containing a
time_t and the time into the current second to microsecond
resolution. Before that there was an ftime() library routine from
about Unix Version 7, which used a timeb structure that included
milliseconds instead (16-bit, see?). I'm pretty sure Linux
supports gettimeofday() too.<br>
</p>
<p>So - if you're writing in 'C' you can get good resolution; if
you're using a shell script it seems like you're out of luck if
you want something portable. Unless there is a trick we've all
missed.</p>
<p>Regards, Frank.</p>
<p><br>
</p>
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