SHELL

grep text in files

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.
-e is the pattern used during the search

grep string in file

if grep -Fxq "pattern_to_search" file.txt; then #check if pattern string in file
  #do something
fi