grep searches for PATTERN.

grep searches for PATTERN in each FILE. A FILE of “-” stands for standard input. If no FILE is given, recursive searches examine the working directory, and nonrecursive searches read standard input. By default, grep prints the matching lines.:

grep -rnw '/path/to/somewhere/' -e 'pattern'

• -r or -R is recursive,
• -n is line number, and
• -w stands for match the whole word.
• -l (lower-case L) can be added to just give the file name of matching files.

Along with these, --exclude, --include, --exclude-dir flags could be used for efficient searching.

• This will only search through those files which have .c or .h extensions:

  grep --include=\*.{c,h} -rnw '/path/to/somewhere/' -e "pattern"
  

• This will exclude searching all the files ending with .o extension:

  grep --exclude=*.o -rnw '/path/to/somewhere/' -e "pattern"
  

• For directories it's possible to exclude a particular directory(ies) through --exclude-dirDocuments parameter. For example, this will exclude the dirs dir1/, dir2/ and all of them matching .dst/:

  grep --exclude-dir={dir1,dir2,*.dst} -rnw '/path/to/somewhere/' -e "pattern"
  

For more options check man grep.