split-table.sh

#!/bin/bash

# Splits a given table into n evenly-spaced shards.
# call with ./split-table <tablename> <numshards>

table=$1
shards=$2
cluster="localhost:29015"

splits=`seq 0 $((16**4/$shards)) $((16**4+1)) \
    | tail -n +2 \
    | head -$((shards-1)) \
    | xargs printf " 'S%04x'"`

echo "Splitting at these points: $splits"

conf=`eval 'cat<<EOF
split shard $table $splits
'"$conf"'
EOF'`

echo $conf | rethinkdb admin -j $cluster

Based on code by @mlucy.