grep recursive filename matching (grep -ir "xyz" *.cpp) does not work - grep

Question

while
grep -ir "xyz" * recursively searches through the directories and tell me that the text is present in ./x/y/z/abc.cpp
However ,
grep -ir "xyz" *.cpp offers no result.
Isn't the second command supposed to recursively grep all cpp files inside the directory ?
What am I missing here?

Answer 1

Grep will recurse through any directories you match with your glob pattern. (In your case, you probably do not have any directories that match the pattern "*.cpp") You could explicitly specify them: grep -ir "xyz" *.cpp */*.cpp */*/*.cpp */*/*/*.cpp, etc. You can also use the --include option (see the example below)
If you are using GNU grep, then you can use the following:
grep -ir --include "*.cpp" "xyz" .
The command above says to search recursively starting in current directory ignoring case on the pattern and to only search in files that match the glob pattern "*.cpp".
OR if you are on some other Unix platform, you can use this:
find ./ -type f -name "*.cpp" -print0 | xargs -0 grep -i "xyz"
If you are sure that none of your files have spaces in their names, you can omit the -print0 argument to find and the -0 to xargs
The command above says the following: find all files (-type f) under the current directory (./) that match the name glob/wildcard "*.cpp" (-name "*.cpp") and then print them out delimited by a null (-print0). That list of files found should be written to the stdin of the next command: xargs. xargs should read from stdin (default behavior) and split its input on nulls (-0) and then call the grep command with the specified options (grep -i "xyz") on that list of files.
If you are interested in learning more about why grep -ir "xyz" *.cpp does not work the way you think it should, you should search for "shell globbing" (here is a good first article on the subject). I'll also try to provide a quick explanation. When you type in the command grep -ir "xyz" *.cpp and hit enter, there are two programs that are involved in executing your command. The first program is your shell (and unless you've done something to customize things, you are probably usually the bash shell - if you've never heard of a shell or bash, that's where you should start looking, there are tons of good articles). Suffice it say that a shell is just a program that is designed to let you navigate the filesystem on your computer and run other programs. (In Windows, when you double click on an icon to launch a program, or open a folder to access a file, the program that you are running is explorer.exe and it is the Windows graphical shell). So, when you type the command grep -ir "xyz" *.cpp, before grep is run, the shell handles reading your command and does a few things. One of the things is does is expand glob patterns (things like *.txt or [0-9]+.pdf). Like I said, if you want to understand it, go read more about it, but the thing you should take away is that the grep command never sees the *.cpp. What happens is, the shell looks in the current directory for any files or directories with a name that match the pattern *.cpp and then replaces them on the command line BEFORE it runs the grep command. (If it doesn't find anything that matches, then it will leave the *.cpp there and grep will see it, but grep because doesn't normally do glob matching, this doesn't do anything for you).
Alternatively, when you type in grep -ir "xyz" *, what happens is that the shell replaces the * with the name of every file and directory in the current directory (because * matches anything). Let's say you had a directory that contained file1, file2, and dir1, and dir2, then the shell would perform its replacements and then execute a command that looked like this grep -ir "xyz" file1 file2 dir1 dir2, which means grep would search file1 and file2 for a line with the string xyz, and because of the -ir it also search recursively through dir1 and dir2 and search any files found for that string as well. Lastly, if you've followed everything I've said so far, then it will make sense to you that grep does have a way to use glob patterns on recursive searches, and that is to use the --include option, as in the command I described earlier: grep -ir --include "*.cpp" "xyz" ., and the reason why we put the *.cpp in quotes in that command is to prevent the shell from trying to expand the glob pattern before we run the command.