On Tue, Dec 22, 2015 at 04:04:16AM +0100, Ángel González wrote:
> Aren't you making things more complex than needed, with so much pipes
> and awk?
>
> date(1) is your friend:
>
> For instance:
> $ for y in {1900..2199} ; do echo -n "$y "; for m in {1..12}; do date +%A -d
> $y-$m-13; done | grep -c Friday ; done
>
> shows there are between 1 and 3 Fridays per year.
This is the most obvious approach, but it does a fair amount of forking.
Also, you're relying on GNU date. There's no portable way to do simple
date lookups in a shell script, unfortunately.
A pure-bash approach that avoids forking might look something like:
t=946702800 # Start at Sat Jan 1 12:00:00 EST 2000
endyear=2036
while true; do
printf -v year '%(%Y)T' "$t"
((year > endyear)) && break
printf -v day '%(%d)T' "$t"
printf -v dow '%(%w)T' "$t"
if [[ $day = 13 && $dow = 5 ]]; then
printf -v month '%(%m)T' "$t"
echo "$year-$month-$day"
fi
((t += 86400))
done
But just because it doesn't fork, doesn't mean it's *fast*. Bash is so
slow at everything. :( Your one-fork-per-month loop (plus one fork per
year) might end up being much faster than my zero-forks-per-day loop.
Mine is portable, though.
