Answer by mwfearnley for What is the mkdict command and what does it do? (It came in distro but has no man page or help)

On my system, /usr/sbin/mkdict is a symlink for cracklib-format, which as pointed out in comments, is a simple shell script. It's short enough to post here.

#!/bin/sh## This preprocesses a set of word lists into a suitable form for input# into cracklib-packer#LC_ALL=Cexport LC_ALLgzip -cdf "$@" |                 # concatenate input files, gunzipping if necessary    grep -v '^\(#\|$\)' |        # Remove lines that are blank or begin with '#'    tr '[:upper:]''[:lower:]' | # transform all letters to lower case    tr -cd '\n[:graph:]' |       # strip everything except newlines and printable characters    sort -u                      # sort alphabetically, remove duplicates

I've added comments to the script.Essentially it just takes a set of (possibly gzip'd) word list files, and outputs a sorted list of lower-case words.

Note: there is a more recent version of the script that can be seen in Github.