Count the number of times a word appears in a file

Question

What is an easy way to count the number of times a word appears in a file?

Answer 1

This will also count multiple occurrences of the word in a single line:

grep -o 'word' filename | wc -l

Answer 2

cat filename | tr ' ' '\n' | grep 'word' | wc -l

Answer 3

fgrep "word to be counted" filename|wc -w