grep can simply be invoked: $ grep 'STRING' filename
This is OK but it does not show the true power of grep. First this only looks at one file. A cool example of using grep with multiple file would be to find all files in a directory that contains the name of a person. This can be easily accomplished using a grep in the following way :
$ grep 'Nikesh J' *
Notice the use of single quotes; This are not essential but in this example it was required since the name contains a space. Double quotes could also have been used in this example.
Grep Regular Expression
grep can search for complicated pattern to find what you need. Here is a list of some of the special characters used to create a regular expression:
`.' The period `.' matches any single character.
`?' The preceding item is optional and will be matched at most once.
`*' The preceding item will be matched zero or more times.
`+' The preceding item will be matched one or more times.
So an example of a regular expression search would be: $ grep "\<[A-Za-z].*" file
This will search for any word which begins with a letter upper or lower case.
For more details check: $ man grep