This directory contains the source code of Ion's builtin commands and documentation for their usage.
## Variables
The **variables.rs** module contains commands relating to setting and removing aliases, variables, and exports. The shell stores aliases and variables within two separate `BTreeMap` structures inside the same `Variables` structure, which is contained within the `Shell` structure.
### Alias
The `alias` command is used to set an alias for running other commands under a different name. The most common usages of the `alias` keyword are to shorten the keystrokes required to run a command and it's specific arguments, and to rename a command to something more familiar.
```ion
alias ls = 'ls --color'
```
If the command is executed without any arguments, it will simply list all available aliases.
The `unalias` command performs the reverse of `alias` in that it drops the value from existence.
```ion
unalias ls
```
### Let
The `let` command sets a variable to the value of the expression that follows. These variables are stored as local values within the shell, so other processes many not access these values.
```ion
// TODO: Ion Shell does not yet implement stderr redirection.
let git_branch = $(git rev-parse --abbrev-ref HEAD 2> /dev/null)
```
If the command is executed without any arguments, it will simply list all available variables.
To drop a value from the shell, the `drop` keyword may be used:
```sh
drop git_branch
```
### Export
The `export` command works similarly to the `let` command, but instead of defining a local variable, it defines a global variable that other processes can access.
