how to randomly loop over an array (shuffle) in bash [duplicate]

Question

Given an array of elements (servers), how do I shuffle the array to obtain a random new array ?

inarray=("serverA" "serverB" "serverC")

outarray=($(randomize_func ${inarray[@]})

echo ${outarray[@]}
serverB serverC serverA

There is a command shuf (man page) but it does not exist on every linux.

This is my first attempt to post a self-answered question stackoverflow, if you have a better solution, please post it.

Answer 1

This is another pure Bash solution:

#! /bin/bash

# Randomly permute the arguments and put them in array 'outarray'
function perm
{
    outarray=( "$@" )

    # The algorithm used is the Fisher-Yates Shuffle
    # (https://en.wikipedia.org/wiki/Fisher%E2%80%93Yates_shuffle),
    # also known as the Knuth Shuffle.

    # Loop down through 'outarray', swapping the item at the current index
    # with a random item chosen from the array up to (and including) that
    # index
    local idx rand_idx tmp
    for ((idx=$#-1; idx>0 ; idx--)) ; do
        rand_idx=$(( RANDOM % (idx+1) ))
        # Swap if the randomly chosen item is not the current item
        if (( rand_idx != idx )) ; then
            tmp=${outarray[idx]}
            outarray[idx]=${outarray[rand_idx]}
            outarray[rand_idx]=$tmp
        fi
    done
}

inarray=( 'server A' 'server B' 'server C' )

# Declare 'outarray' for use by 'perm'
declare -a outarray

perm "${inarray[@]}"

# Display the contents of 'outarray'
declare -p outarray

It's Shellcheck-clean, and tested with Bash 3 and Bash 4.

The caller gets the results from outarray rather than putting them in outarray because outarray=( $(perm ...) ) doesn't work if any of the items to be shuffled contain whitespace characters, and it may also break if items contain glob metacharacters. There is no nice way to return non-trivial values from Bash functions.

If perm is called from another function then declaring outarray in the caller (e.g. with local -a outarray) will avoid creating (or clobbering) a global variable.

The code could safely be simplified by doing the swap unconditionally, at the cost of doing some pointless swaps of items with themselves.