1. Find and grep

1.1. find

The find tool, known from UNIX, is very powerful. This command not only allows you to search file names, it can also accept file size, date of last change and other file properties as criteria for a search.

The most common use is for finding file names:

find <path> -name <searchstring>

This can be interpreted as "Look in all files and subdirectories contained in a given path, and print the names of the files containing the search string in their name" (not in their content).

 find /etc -name "*.conf"

1.2. grep

grep is used for filtering input lines and returning certain patterns to the output.

grep "string" path/to/file 
 grep "root" /etc/passwd

1.3. find and grep command together

find /etc -name "*.conf" -exec grep -Hns "conf" {} \;

Explanation
-H, --with-filename
      Print the filename for each match
-n, --line-number
      Prefix each line of output with the 1-based line number within its input file
-s, --no-messages
      Suppress error messages about nonexistent or unreadable files.
This can be interpreted as
- "Look for *.conf files and subdirectories contained in /etc, and if true exec grep -Hns conf in the given file"
Very powerful in bash scripts create a file test.sh .bash script 
#!/bin/bash
STRING=$(find /etc -name "*.conf" -exec grep -Hns "conf" {} \;)
echo $STRING

exec it .bash script

chmod 700 test.sh
./test.sh