Visiting aliases

An alias is a shortcut to replace typing a long-command sequence. In this recipe, we will see how to create aliases using the alias command.

How to do it...

These are the operations you can perform on aliases:

1. Create an alias:

        $ alias new_command='command sequence'

This example creates a shortcut for the apt-get install command:

        $ alias install='sudo apt-get install'

Once the alias is defined, we can type install instead of sudo apt-get install.

2. The alias command is temporary: aliases exist until we close the current terminal. To make an alias available to all shells, add this statement to the ~/.bashrc file. Commands in ~/.bashrc are always executed when a new interactive shell process is spawned:

        $ echo 'alias cmd="command seq"' >> ~/.bashrc

3. To remove an alias, remove its entry from ~/.bashrc (if any) or use the unalias command. Alternatively, alias example= should unset the alias named example.
4. This example creates an alias for rm that will delete the original and keep a copy in a backup directory:

        alias rm='cp $@ ~/backup && rm $@'

When you create an alias, if the item being aliased already exists, it will be replaced by this newly aliased command for that user.

There's more...

When running as a privileged user, aliases can be a security breach. To avoid compromising your system, you should escape commands.

Escaping aliases

Given how easy it is to create an alias to masquerade as a native command, you should not run aliased commands as a privileged user. We can ignore any aliases currently defined, by escaping the command we want to run. Consider this example:

$ \command

The \ character escapes the command, running it without any aliased changes. When running privileged commands on an untrusted environment, it is always a good security practice to ignore aliases by prefixing the command with \. The attacker might have aliased the privileged command with his/her own custom command, to steal critical information that is provided by the user to the command.

Listing aliases

The alias command lists the currently defined aliases:

$ aliasalias lc='ls -color=auto'
alias ll='ls -l'
alias vi='vim'