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I want a way to run a command randomly, say 1 out of 10 times. Is there a builtin or GNU coreutil to do this, ideally something like:
chance 10 && do_stuff
where do_stuff is only executed 1 in 10 times? I know I could write a script, but it seems like a fairly simple thing and I was wondering if there is a defined way.
In ksh, Bash, Zsh, Yash or BusyBox sh:
[ "$RANDOM" -lt 3277 ] && do_stuff
The RANDOM special variable of the Korn, Bash, Yash, Z and BusyBox shells produces a pseudo-random decimal integer value between 0 and 32767 every time it’s evaluated, so the above gives (close to) a one-in-ten chance.
You can use this to produce a function which behaves as described in your question, at least in Bash:
function chance {
[[ -z $1 || $1 -le 0 ]] && return 1
[[ $RANDOM -lt $((32767 / $1 + 1)) ]]
}
Forgetting to provide an argument, or providing an invalid argument, will produce a result of 1, so chance && do_stuff will never do_stuff.
This uses the general formula for “1 in n” using $RANDOM, which is [[ $RANDOM -lt $((32767 / n + 1)) ]], giving a (⎣32767 / n⎦ + 1) in 32768 chance. Values of n which aren’t factors of 32768 introduce a bias because of the uneven split in the range of possible values.
$((32767/parts+1)) be bigger than 1 or we run into the risk of having the number of parts increase in 1 while the result of the division (and therefore the limit) be the same. (cont..)
– Isaac
Sep 12 '19 at 22:13
(32767/n-32767/(n+1))>=1, solving for n that gives a limit at ~181.5. In fact the number of parts could run up to 194 without issue. But at 195 parts, the resulting limit is 151, the same result as with 194 parts. That is incongruent and should be avoided. In short, the upper limit for the number of parts (n), should be 194. You could make the limit tests: [[ -z $1 || $1 -le 1 || $1 -ge 194 ]] && return 1
– Isaac
Sep 12 '19 at 22:13
Non-standard solution:
[ $(date +%1N) == 1 ] && do_stuff
Check if the last digit of the current time in nanoseconds is 1!
[ $(date +%1N) == 1 ] && do_stuff does not occur at regular intervals, otherwise the randomness is spoiled. Think of while true; do [ $(date +1%N) == 1 ] && sleep 1; done as an abstract counterexample. Nonetheless, the idea of playing with the nanoseconds is really good, I think I will use it, hence +1
– XavierStuvw
Sep 1 '19 at 0:15
clock_getres()), 2) The scheduler is not prohibited from, for example, always starting a timeslice on a 100-nanosecond boundary (which would introduce bias even if the clock is right), 3) The supported way of obtaining random values is (usually) reading from /dev/urandom or inspecting the value of $RANDOM.
– Kevin
Sep 1 '19 at 21:32
An alternative to using $RANDOM is the shuf command:
[[ $(shuf -i 1-10 -n 1) == 1 ]] && do_stuff
will do the job. Also useful for randomly selecting lines from a file, eg. for a music playlist.
Improving upon the first answer and making it much more obvious what you are trying to achieve:
[ $(( $RANDOM % 10 )) == 0 ] && echo "You win" || echo "You lose"
$RANDOM % 10 will have a bias, unless the $RANDOM produces exactly a multiple of 10 different values (which generally doesn't happen in a binary computer)
– phuclv
Sep 2 '19 at 0:38
"$RANDOM" -lt $((32767 / n + 1)).
– Eisenknurr
Sep 2 '19 at 4:59
I'm not sure if you want randomness or periodicity ... For periodicity:
for i in `seq 1 10 100`; do echo $i;done
1
11
21
31
41
51
61
71
81
91
You can mix it with the "$RANDOM" trick above to produce something more chaotic, for instance:
for i in seq 1 1000 $RANDOM; do echo $i;done
HTH :-)
