Word Frequency

Solution

Using xargs, uniq and sort

cat: displays the contents of the file
xargs: applies the printf function to every line in the output of cat
sort: Sorts the current buffer piped to it
uniq -c: displays the counts of each line in the buffer
sort -nr: reverse sorts the output via the counts in the previous operation
awk: prints the 2nd column and then the 1st column based on the problem requirement

could also replace the 2nd part w/ tr -s ' ' '\n'.

cat words.txt | xargs -n 1 | sort | uniq -c | sort -nr | awk '{print $2, $1}'

Using awk

awk '
{
    for (i=1; i<=NF; i++) {
        cnt[$i]++
    }
}
END {
    for (i in cnt)
        print i, cnt[i]
}
' words.txt | sort -nr -k 2