Re: SECONDS=0 does not reset SECONDS, or I'm missing something

[Andreas Kusalananda Kähäri]

On Thu, Jun 04, 2020 at 02:39:40PM +0200, Pier Paolo Grassi wrote:
>      [[ SECONDS -gt 0 ]] && sleep "$SECONDS"
> 
> you are missing a $ sigil here, it should be:
> 
>      [[ $SECONDS -gt 0 ]] && sleep "$SECONDS"


No, sorry, I don't.  The [[ -gt ]] test is performed in an arithmetic
context.  In arithmetic contexts, variables don't generally need an $ on
them:

        $ n=12
        $ [[ n -gt 10 ]] && echo greater than 10
        greater than 10
        $ [[ n -gt 15 ]] && echo greater than 15


> 
> Il giorno gio 4 giu 2020 alle 12:14 Andreas Kusalananda Kähäri <
> andreas.kahari@abc.se> ha scritto:
> 
> > I wrote a small script to delete many hundreds of thousands of files
> > off a very slow disk.  The script is executed from find(1) and is
> > given pathnames of directories to delete as arguments.  If a directory
> > takes a second or more to delete a directory, the script sleeps for
> > the amount of time it took to delete that directory before proceeding,
> > in an attempt to let the filesystem sync metadata (this may have been
> > misguided but it's not really important).
> >
> >         #!/usr/local/bin/bash -x
> >
> >         for name do
> >                 SECONDS=0
> >                 rm -r -f "$name"
> >                 [[ SECONDS -gt 0 ]] && sleep "$SECONDS"
> >         done
> >
> > What I noticed was that the script would go to sleep for a second
> > every second, even if the deletion of directories was quick.  This
> > indicates that SECONDS=0 didn't properly reset the SECONDS timer.  Or,
> > it indicates that the timer is set by the system's clock (the current
> > second according to the current time), and that it is not a real "timer"
> > at all.  Or, that I have misunderstood something about how SECONDS is
> > supposed to work.  Or, all of the above (please confirm).
> >
> > A sample script (see below) reproduces this with a tight loop on macOS
> > Catalina running "5.0.17(1)-release (x86_64-apple-darwin19.4.0)"
> > (bash from Homebrew) and on OpenBSD 6.7 running "5.0.17(1)-release
> > (x86_64-unknown-openbsd6.7)".
> >
> >         while true; do SECONDS=0; [[ SECONDS -gt 0 ]] && echo hi; done
> >
> > (will print "hi" more or less once a second, and possibly slightly less
> > often occasionally)
> >
> > So, I don't know whether this is an error in the documentation, my
> > understanding of it, or in how the SECONDS variable is set/managed
> > internally by the shell.  I'm assuming I'm misunderstanding.
> >
> > For reference (from the bash manual):
> >
> >         SECONDS
> >                Each time this parameter is referenced, the number
> >                of seconds since shell invocation is returned.  If a
> >                value is assigned to SECONDS, the value returned upon
> >                subsequent references is the number of seconds since
> >                the assignment plus the value assigned.  If SECONDS is
> >                unset, it loses its special properties, even if it is
> >                subsequently reset.
> >
> >
> >
> > --
> > Andreas (Kusalananda) Kähäri
> > SciLifeLab, NBIS, ICM
> > Uppsala University, Sweden
> >
> > .
> >
> > --
> Pier Paolo Grassi

-- 
Andreas (Kusalananda) Kähäri
SciLifeLab, NBIS, ICM
Uppsala University, Sweden

.