Git Aliases can be set as following:

git config --global alias.unstage 'reset HEAD --'

This makes the following two commands equivalent:

$ git unstage fileA
$ git reset HEAD -- fileA

To use a non-git command, prefix it with a !:

git config --global alias.visual '!gitk'

To list all aliases, we can use

git config --get-regexp ^alias

Some aliases that I use

aliascommand
unstage <file>reset HEAD -- <file>
amendgit stage -A && git commit --amend --no-edit
force-pushgit push --force-with-lease