swap_select.sh

#!/bin/bash
declare -a name Rl S R
n=0 s=0
while read LINE; do
        n=$(( n + 1 ))
        name[$n]="$LINE"
done
while [ $s -lt $n ]; do # For each participant as a sender
        s=$(( s + 1 )) x=0 y=0 Rl=( )
        while [ $x -lt $n ]; do
                x=$(( x + 1 ))
                [ $x -ne $s ] || continue
                [ "${S[$x]}" ] && continue # Recipient already has sender
                y=$(( y + 1 ))
                Rl[$y]=$x
        done
        if [ $y -gt 0 ]; then
                R[$s]=${Rl[$((((RANDOM<<15)|RANDOM)%y+1))]}
        else # Randomization left sender sending to self; swap senders
                R[$s]=${R[1]} R[1]=$s
        fi
        [ $s -eq 1 ] || echo "${name[$s]} -> ${name[${R[$s]}]}"
        S[${R[$s]}]=$s
done
[ $n -lt 1 ] || echo "${name[1]} -> ${name[${R[1]}]}"