SHELL

bash if file contains string

if grep -q SomeString "$File"; then
  Some Actions # SomeString was found
fi

bash check if string in file

Just use grep with flags 'F' (fixed string), 'x' (exact match) and 'q'
(quiet output) in order to check if a word string is in a file
if grep -Fxq "string" file.txt; then #do some code...#; fi