This shows me the EXIF time/date of every image in a directory, useful when I'm sorting:

Needs the debian package 'exif'.

for i in *; do echo $i; exif -t 0x0132 $i  | grep Value; done;

Performing operations on multiple files using xargs and find:

Escapes the filenames and supplies, through xargs, as many parameters as possible to rm on the command line for efficency. Also an example using two or more parameters with mv.

find $HOME -iname "*.jpg" -print0 | xargs -0 rm
find $HOME -iname "*.jpg" -exec mv {} /dest/dir/ \;
find . -exec grep -Hn hello {} \;

Watch a file and do something to it when it changes:

In this case, watch the file 'example.php' and when it changes, show the output in another terminal.

fswatch -0 example.php | xargs -0 -n 1 -I {} php {}

Check the expiry date on a SSL certificate.

openssl x509 -enddate -noout -in cert.pem 

I'm so used to using init.d I can't keep these instructions in my head, so they're written here until I remember them.

systemctl list-unit-files --all | grep enabled

journalctl -u sshd