This script prints a random word to stdout.

randomword.sh

#!/bin/bash

function print_random_word {
  # The dictionary file. It contains one word per line.
  dictionary=/usr/share/dict/words

  # The number of words in the dictionary file.
  num_words_in_dictionary=$(wc -l $dictionary | awk '{print $1}')

  # A random number corresponding to a line in the dictionary file.
  # This takes random data from /dev/random, converts it to an unsigned integer, and scales it by the number of words available.
  random_line_number=$(($(cat /dev/random | od -N3 -An -D) % $num_words_in_dictionary))

  # Prints the word corresponding to the random line calculated above.
  awk "NR == $random_line_number" $dictionary
}

print_random_word

Along with mkString, this can implement the correct horse battery staple xkcd password generator:

#!/bin/sh

# Default the number of words in the passphrase to 4.
num_words=${1:-4}

# Get $num_words random words and string them together.
for n in $(seq $num_words); do randomword; done | mkString -