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The ancient utility `find` should come installed on the majority of distributions. Technically, `find` recursively locates all files and directories under a path, and prints their full paths. It is of course possible to pipe it to other tools like `grep` (actually, `ls` can also list recursively, which you can pipe to `grep` also, but it's a pretty bad way of doing it):

```sh
find / | grep '.conf'
```

But this is somewhat inefficient and inelegant. `find` supports an elaborate set of filters (and can even do some other things like running a program on each file found). For example:

* `-name` will filter based on the filename
* `-type` can be used to filter things like directories, symbolic links, etc.
* `-mtime` filters on last modification date/time

Many others can be found in `man find`.

Note that `find` is subject to access control, so if you search system directories you may see some errors. The errors actually come on the standard error channel, while the found files come on standard output, so if you pipe the output of `find` it won't actually pipe the errors (they'll still be printed out). But the way to prevent the errors is to use `sudo find`.

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`find` can be a bit slow if you are searching through a large number of files (100k+ depending on your hardware). There is also a common tool `locate`, which also searches for files, but it maintains a database indexing them rather than directly crawling the file tree. The index must be first constructed and then maintained, but this allows `locate` to return results much faster than `find`. For details, look at `man locate`.

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I personally don't think `find` is very user friendly, and I find it hard to remember its syntax. I use [`fd`](https://github.com/sharkdp/fd) which is essentially the same thing, but has nicer defaults (like colors). One gotcha with `fd` is that by default, it will ignore some paths, like:

* "Hidden" files and folders (those that start with `.`)
* Things in `.gitignore` if you are searching from a git repo
* Internal git data in `.git`

You can prevent this behavior in various ways, like passing the `-H` flag. For the full list, see `man fd`.