How to run a command 1 out of N times in Bash

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.

Non-standard solution:

[ $(date +%1N) == 1 ] && do_stuff

Check if the last digit of the current time in nanoseconds is 1!

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.