Grep command is very useful and powerful command to search words in text files. Here is exploitation and some examples:
grep [search word] [place to search] -> global regular expression print. It will print the lines which the [search word] is found in the [place to search]
Search word inside text.txt
grep word text.txt
Search word inside all the files in the current folder
grep word *
Find whole words only in all files from the current directory
grep -w word *
To search case insensitive
grep -i word *
To search also in the sub-directories
grep -r word *
To search file names
grep -l word *
show 3 lines before and after the find word
grep -n -C 3 word text.txt