alias — command shortcuts

alias barely has any flags: it takes name='command' pairs. The useful ones are -p (print the list in a form suitable for re-execution, same as with no arguments) and unalias -a (remove every alias of the current session at once).

An alias created on the command line lives only until you close the terminal. To make it permanent, add it to ~/.bashrc (per user):

echo "alias ll='ls -lah'" >> ~/.bashrc
source ~/.bashrc              # apply in the current session (or open a new terminal)

Ubuntu's default system aliases live in ~/.bash_aliases (sourced by ~/.bashrc). More on profiles and $PATH — env-vars.

An alias does not take arguments the way a beginner expects: alias gl='git log' gets substituted before the rest of the line, so gl -5 works (it becomes git log -5), but an alias cannot insert an argument in the middle of a command — that needs a shell function (see bash-scripting).