I read your entire post, and I didn't miss any of the points you make. I simply disagree with you.

Quoting from your post:

    And here is my initial solution in UNIX shell:

    # bentley_knuth.sh

    # Usage:
    # ./bentley_knuth.sh n file
    # where "n" is the number of most frequent words 
    # you want to find in "file".
    
    awk '
        {
            for (i = 1; i <= NF; i++)
                word_freq[$i]++
        }
    END     {
                for (i in word_freq)
                    print i, word_freq[i]
            }
    ' < $2 | sort -nr +1 | sed $1q

So you invoke awk, and then run the output of awk through sort and sed.

You're doing all the word counting in awk.

Yes, you're invoking awk from a shell script, but that's really not the same thing as "using shell." McIlroy’s solution is genuinely shell:

    tr -cs A-Za-z '
    ' |
    tr A-Z a-z |
    sort |
    uniq -c |
    sort -rn |
    sed ${1}q

"awk" is generally accepted as a full programming language, whereas "tr", "sort", "uniq", and "sed" are command line utilities. I don't think "awk" classes as a command line utility, so I don't class your solution as "shell".

Perhaps you don't agree, perhaps you think "awk" is a command line utility. If so, then we'll agree to disagree.