I'll collect some popular find commands here. These are mainly mental notes for commonly occuring ad-hoc searches.
The fun starts here!
2.1. Find a particular file in this folder and sub-folders
Find a file called hosts in the folder I'm in, as well as those below. Since I'm in the root-folder, it will search all folders on the system.
$ find / -name "hosts" -type f
2.2. Find all files containing specific text
Find all files in the folder /etc containing the text cyndane2.
$ grep -rnw '/etc/' -e 'cyndane2'
/etc/hosts:4:192.168.0.8 cyndane2 srbu.se www.srbu.se
2.3. Find all files with a specific extension and recursively delete them all
This is a dangerous command to run, so let's first do a dry-run.
The first command will find all files in the current directory that have an extension .bak.
$ find . -name "*.bak" -type f
If all seems okay, run it for real this time, adding the delete parameter.
$ find . -name "*.bak" -type f -delete
2.4. Find all files with spaces and replace them recursively with periods
This will recursively search in folder foo for filenames with basename bar and replace the spaces with periods.
$ find . -depth -name '* *' | while IFS= read -r f ; do mv -i "$f" "$(foo"$f")/$(bar "$f"|tr ' ' .)" ; done