I’m posting this mostly for my own reference, but they might be of use to others.

Clearing a file
A quick and easy way to empty a large file, such as say, a log file, is the following:
cat > filename
-> then hit ctrl + d

Finding a recently modified file
The below command will find any files modified or created within the past three days.

find . -mtime -3

Number of files in a directory
You would be amazed how many times I wonder exactly how many files there are in a directory (and subdirectories) of say uploaded images (it was 5,843).

ls -1R | wc -l

Directory Size
Sometimes you need to know the size of the current folder’s sub directories.

du -h --max-depth=1