Here's a list of command lines I've found useful to automate repetitive or long tasks. They are categorized by operating system.
Batch Convert UNIX Man Pages into HTML Files, With Links
'%N' is replaced for the name for the link tag, and '%S', the section number. Runs in BASH. Tested on Ubuntu. Requires mandoc to be installed.
mkdir html && for x in .; do mandoc -Thtml -O man=./html/%N.%S.html,toc $x > html/$x.html; done
Mount A file system attached to a virtual machine DISK image
I've found this to be more useful than guestmount (provided by the libguestfs-tools package for Ubuntu), because it does not require superuser privileges to execute, and is easy to unmount afterwards. Based on the answer provided on Super User: https://superuser.com/a/1548983.
mkdir -p ~/tmp/windows10-vm nbdfuse ~/tmp/windows10-vm [ qemu-nbd path_to_disk_image ] & fusermount -u ~/tmp/windows10-vm
Find Ports in use by Processes
May require superuser privileges to see info for processes for other users.