Here's a list of command lines I've found useful to automate repetitive or long tasks. They are categorized by operating system.

GNU Linux

File Management

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

Virtual Machines

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

Networking

Find Ports in use by Processes

May require superuser privileges to see info for processes for other users.

netstat -atpn