How can I use grep to find a word inside a folder?

In Windows, I would have done a search for finding a word inside a folder. Similarly, I want to know if a specific word occurs inside a directory containing many sub-directories and files. My searches for grep syntax shows I must specify the filename, i.e. grep string filename.

Now, I do not know the filename, so what do I do? A friend suggested to do grep -nr string, but I don't know what this means and I got no results with it (there is no response until I issue a Ctrl + C).

grep -nr 'yourString*' .

The dot at the end searches the current directory. Meaning for each parameter:

-n            Show relative line number in the file
'yourString*' String for search, followed by a wildcard character
-r            Recursively search subdirectories listed
.             Directory for search (current directory)

grep -nr 'MobileAppSer*' . (Would find MobileAppServlet.java or MobileAppServlet.class or MobileAppServlet.txt; 'MobileAppASer*.*' is another way to do the same thing.)

To check more parameters use man grep command.

grep -nr string my_directory

Additional notes: this satisfies the syntax grep [options] string filename because in Unix-like systems, a directory is a kind of file (there is a term "regular file" to specifically refer to entities that are called just "files" in Windows).

grep -nr string reads the content to search from the standard input, that is why it just waits there for input from you, and stops doing so when you press ^C (it would stop on ^D as well, which is the key combination for end-of-file).

GREP: Global Regular Expression Print/Parser/Processor/Program.
You can use this to search the current directory.
You can specify -R for "recursive", which means the program searches in all subfolders, and their subfolders, and their subfolder's subfolders, etc.

grep -R "your word" .

-n will print the line number, where it matched in the file.
-i will search case-insensitive (capital/non-capital letters).

grep -inR "your regex pattern" .

There's also:

find directory_name -type f -print0 | xargs -0 grep -li word

but that might be a bit much for a beginner.

find is a general purpose directory walker/lister, -type f means "look for plain files rather than directories and named pipes and what have you", -print0 means "print them on the standard output using null characters as delimiters". The output from find is sent to xargs -0 and that grabs its standard input in chunks (to avoid command line length limitations) using null characters as a record separator (rather than the standard newline) and then applies grep -li word to each set of files. On the grep, -l means "list the files that match" and -i means "case insensitive"; you can usually combine single character options so you'll see -li more often than -l -i.

If you don't use -print0 and -0 then you'll run into problems with file names that contain spaces so using them is a good habit.

grep -nr search_string search_dir

will do a RECURSIVE (meaning the directory and all it's sub-directories) search for the search_string. (as correctly answered by usta).

The reason you were not getting any anwers with your friend's suggestion of:

grep -nr string

is because no directory was specified. If you are in the directory that you want to do the search in, you have to do the following:

grep -nr string .

It is important to include the '.' character, as this tells grep to search THIS directory.