The grep command search the content of all the files in a directory.
grep -inR TOFIND
The i parameter is for case insensitive, n displays the line number and the R parameter is for recursive.
This example search all the files in the current directory and all the sub directories for the value test.
$ grep -inR test dir/file3.txt:1:test file1.txt:4:test file1.txt:7:this is a test file2.txt:4:test file2.txt:7:this is a test file2.txt:11:test file2.txt:14:this is a test
The output returns the file name, then the line number where the value was found and finally the value matched. File3.txt from the subdirectory dir contains test on line 4. File2.txt contains 4 instances of the value test.