I have an alias alias lpr2='lpr -o sides=one-sided -o number-up=2' defined in .bashrc for easy printing. I can use it in the (emacs) shell, but when I use M-! on a pdf file in dired-mode and type in lpr2, I receive /bin/bash: lpr2: command not found. How can aliases be made available in dired-mode on M-!?
You cannot use aliases in M-!. However, you can define the variable lpr-switches so you can use all the Emacs' printing commands with requested behaviour.