To use the grep command to count the number of times a word appears in a file on a Unix or Linux system, you can use the -c option.
Here is the basic syntax for using the grep command to count the number of times a word appears in a file:
grep -c word file
The word argument is the word that you want to count. The file argument is the name of the file to search. The -c option tells grep to count the number of lines that contain the word.
For example, to count the number of times the word "hello" appears in the file file.txt, you can use the following command:
grep -c hello file.txt
This will count the number of lines in file.txt that contain the word "hello", and display the count.
You can use other options and arguments to customize the search, such as the -i option to ignore case, or the -w option to match the word as a whole word.
You can also use the wc command to count the number of words in a file, and pipe the output of the grep command to the wc command to count the number of lines that contain the word.
For example:
grep hello file.txt | wc -l
This will search for the word "hello" in file.txt, and pipe the output to the wc command, which will count the number of lines and display the count.