# Basic:
grep pattern file

# case nonsensitive research:
grep -i pattern file

# Recursively grep for string <pattern> in folder:
grep -R pattern folder

# Getting pattern from file (one by line):
grep -f pattern_file file

# Find all files who contain {pattern} in the directory {directory}.
# This will show: "file:line my research"
grep -rnw 'directory' -e "pattern"