The grep command with the -R or --dereference-recursive flag and -e or --regexp option can be used to search every file below a certain directory for a string of data. In this example, every file below the /opt directory will be searched for the text 'foo'.

grep -R --regexp 'foo' /opt

Often, the -i or --ignore-case flag is used to make the search case insensitive.

grep -R --ignore-case --regexp 'foo' /opt

Additionally, the --exclude-dir option can be used to exclude a diretory from being searched. In this example, every file below the /opt directory will be searched for the text 'foo' except that the /opt/logs directory will be excluded from the search.

grep -R --exclude-dir=logs --ignore-case --regexp 'foo' /opt