# Ion Shell
[](https://travis-ci.org/redox-os/ion)
[](./LICENSE)
[](https://crates.io/crates/ion-shell)
Ion is a modern system shell that is written entirely in Rust, features a simple (and powerful) syntax, and offers performance that exceeds the level of Dash. While it is developed alongside RedoxOS as the default shell for RedoxOS, it is equally-supported on UNIX platforms (Linux/Mac/BSDs), by which it is developed and tested from. Windows support could also easily be obtained, but we currently do not have any developers that use Windows. Ion's design is influenced by many other successful shells, which can be seen in it's borrowing of ideas from Bash, Fish, and Oil; whilst also offering some unique ideas of it's own. It is still a work in progress, but most of the core functionality is complete. It is also currently significantly faster than Dash, even though it contains many more features and abilities, making it the fastest system shell to date. Finally, as it is written in Rust, we can guarantee that our codebase offers a high degree of memory safety compared to Bash, Dash, Zsh, Fish and other shells that are written in unsafe languages. That means no chance for a shellshock-like vulnerability to arise.
# Ion's Goals
Syntax and feature decisions for Ion are made based upon three specific measurements: *"is the feature useful, is it simple to use, and will it's implementation be efficient to parse and execute?"*. The language should be efficient to parse, with zero room for ambiguities, and implemented in a zero-cost manner as much as possible. In addition, we believe that as a shell is effectively a string-based language, the shell should also have first class string-manipulation capabilities, in order to eliminate the need for external utilities. The `awk` command should not be required as often when writing Ion scripts, as many basic uses of it are incorporated into Ion's syntax in a manner that is simple to use and learn.
## Ion Is Not POSIX
While Ion's foundations are heavily influenced by POSIX shell syntax, it does offer some critical features and differentiations that you won't find in a POSIX shell. The similarities only exist because POSIX syntax already had some good ideas, but it also came with a number of bad design decisions that have lead to inflexibility, and so we have taken the good ideas and implemented even better ideas on top of them, and as a replacement to the bad parts. Hence, while syntax may look familiar, it is not, nor will it ever, be compliant with POSIX.
In example, we have carried a lot of the same basic features such as strings (**$string**) and process expansions that return strings (**$(command args...)***), but we have also implemented support for first class arrays (**@array**) and array-based process expansions (**@(command args..)**), rather than compounding the string variables, and utilize the distinction between the two types to implement methods (**$join(array)**, **@split(string)**) and slicing (**$string[..5]**, **@array[..5]**). In addition, we implement better syntax for redirecting/piping stderr (**^>**, **^|**), and both stderr/stdout (**&>**, **&|**); as well as dropping the **do** keyword, and using the **end** keyword to end a block.
# Features
Below is an overview of features that Ion has either already implemented, or aims to implement in the future. If you have ideas for features that you would like to see on this list, then you are welcome to open an issue to describe the feature and what you would like to use it for / why you think it's useful.
- [x] Expansions
- [x] String Expansions
- [x] Array Expansions
- [x] Glob Expansions
- [x] Brace Expansions
- [x] Ranges
- [x] Permutations
- [x] Nested Braces
- [x] Process Expansions
- [x] String-based Command Substitution (**$()**)
- [x] Array-based Command Substitution (**@()**)
- [ ] Arithmetic Expansions
- [x] Flow Control
- [x] For Loops
- [ ] Foreach Loops
- [x] While Loops
- [x] If Conditionals
- [ ] Match Statements
- [x] Functions
- [x] Optionally-typed Function Parameters
- [x] Script Execution
- [x] Handling arguments w/ @args Array
- [x] Mutables
- [x] Aliases
- [x] Strings (**$variable**)
- [x] Multiple Variable Assignments
- [ ] Optionally-typed Variable Assignments
- [x] Grapheme-based String Slicing
- [x] String Methods (**$join(array, ', ')**)
- [x] **$join(array)**
- [x] **$len(string)**
- [x] **$len_bytes(string)**
- [x] Arrays (**@array**)
- [x] Array Expressions (**[]**)
- [x] Array Slicing
- [x] Array Methods (**@split(var, ' ')**)
- [x] **@split(string)**
- [x] **@len(array)**
- [ ] HashMaps
- [x] Line Editor (Provided by [Liner](https://github.com/MovingtoMars/liner))
- [x] Multiline Comments and Commands
- [ ] Multi-line Editing
- [x] Tab Completion
- [x] Auto-suggestions
- [x] History suggestions
- [x] vi and emacs keybindings (`set -o (vi|emacs)`)
- [ ] Syntax Highlighting
- [x] Implicit cd
- [x] Signal Handling
- [x] **&&** and **||** Conditionals
- [x] Redirecting Stdout / Stderr
- [ ] Redirecting Stdout & Stderr
- [x] Piping Builtins
- [x] Background Jobs
- [ ] Piping Functions
- [ ] Redirecting Functions
- [ ] Background Job Control
- [ ] XDG App Dirs
- [ ] Plugins Support
- [ ] Builtins
- [ ] Prompt
- [ ] Syntax
## Shell Syntax
### Implicit Directory Changing
Like the [Friendly Interactive Shell](https://fishshell.com/), Ion also supports implicitly executing the cd command when given
a path, so long as that path begins with either `.`/`/`/`~`, or ends with a `/`. This will thereby invoke
the internal built-in cd command with that path as the argument.
```ion
~/Documents # cd ~/Documents
.. # cd ..
.config # cd .config
examples/ # cd examples/
```
### Defining Variables
The `let` keyword is utilized to create local variables within the shell. The `export` keyword performs
a similar action, only setting the variable globally as an environment variable for the operating system.
```ion
let git_branch = $(git rev-parse --abbrev-ref HEAD ^> /dev/null)
```
It is also possible to assign multiple variables at once, or swap variables.
```ion
let a b = 1 2
let a b = [1 2]
let a b = [$b $a]
```
If the command is executed without any arguments, it will simply list all available variables.
### Using Variables
Variables may be called with the **$** sigil, where the value that follows may be a local or global value.
They may also be optionally defined using a braced syntax, which is useful in the event that you need the value
integrated alongside other characters that do not terminate the variable parsing.
```ion
let A = one
let B = two
echo $A:$B
echo ${A}s and ${B}s
```
### Substrings from Variables
Ion natively supports splitting supplied strings by graphemes using the same slicing syntax for arrays:
```ion
$ let string = "one two three"
$ echo $string[0]
o
$ echo $string[..3]
one
$ echo $string[4..7]
two
$ echo $string[8..]
three
```
### Dropping Variables
To drop a value from the shell, the `drop` keyword may be used:
```ion
drop git_branch
```
### Variable Arithmetic
The `let` command also supports basic arithmetic.
```ion
let a = 1
echo $a
let a += 4
echo $a
let a *= 10
echo $a
let a /= 2
echo $a
let a -= 5
echo $a
```
### Export
The `export` command works similarly to the `let` command, but instead of defining a local variable, it defines a
global variable that other processes can access.
```ion
export PATH = "~/.cargo/bin:${PATH}"
```
### Export Arithmetic
The `export` command also supports basic arithmetic.
```ion
export a = 1
echo $a
export a += 4
echo $a
export a *= 10
echo $a
export a /= 2
echo $a
export a -= 5
echo $a
```
### Aliases
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 = 'exa'
```
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
```
### Brace Expansion
Brace expansions are used to create permutations of a given input. In addition to simple permutations, Ion supports
brace ranges and nested branches.
```ion
echo abc{3..1}def{1..3,a..c}
echo ghi{one{a,b,c},two{d,e,f}}
```
### Defining Arrays
Arrays can be create with the let keyword when the supplied expression evaluates to a vector of values:
#### Array Syntax
The basic syntax for creating an array of values is to wrap the values inbetween **[]** characters. The syntax within
will be evaluated into a flat-mapped vector, and the result can therefor be stored as an array.
```ion
let array = [ one two 'three four' ]
```
One particular use case for arrays is setting command arguments
```ion
let lsflags = [ -l -a ]
ls @lsflags
```
#### Braces Create Arrays
Brace expansions actually create a vector of values under the hood, and thus they can be used to create an array.
```ion
let braced_array = {down,up}vote
```
#### Array-based Command Substitution
Whereas the standard command substitution syntax will create a single string from the output, this variant will create
a whitespace-delimited vector of values from the output of the command.
```ion
let word_split_process = @(echo one two three)
```
### Using Arrays
Arrays may be called with the **@** sigil, which works identical to the variable syntax:
```ion
echo @braced_array
echo @{braced_array}
```
Arrays may also be sliced when an index or index range is supplied:
#### Slice by Index
Slicing by an index will take a string from an array:
```ion
let array = [ 1 2 3 ]
echo @array[0]
echo @array[1]
echo @array[2]
echo [ 1 2 3 ][0]
echo [ 1 2 3 ][1]
echo [ 1 2 3 ][2]
echo @(echo 1 2 3)[0]
echo @(echo 1 2 3)[1]
echo @(echo 1 2 3)[2]
```
#### Slice by Range
Slicing by range will take a subsection of an array as a new array:
```ion
let array = [ 1 2 3 4 5 ]
echo @array[0..1]
echo @array[0...1]
echo @array[..3]
echo @array[3..]
echo @array[..]
```
### Methods
There are two types of methods -- string-based and array-based methods. The type that a method returns is denoted
by the sigil that is used to invoke the method. Currently, there are only two supported methods: **$join()** and
**@split**.
```ion
let results = [ 1 2 3 4 5]
echo $join(results) @join # Both of these effectively do the same thing
echo $join(results, ', ') # You may provide a custom pattern instead
let line = "one two three four five"
echo @split(line) # Splits a line by whitespace
let row = "one,two,three,four,five"
echo @split(row, ',') # Splits by commas
```
### Substring Slicing on String Methods
```ion
echo $join(array)[3..6]
```
### Array Slicing on Array Methods
```ion
let cpu_model = $(grep "model name" /proc/cpuinfo | head -1)
echo @split(cpu_model)[3..5]
```
### Commands
Commands may be written line by line or altogether on the same line with semicolons separating them.
```ion
command arg1 arg2 arg3
command arg1 arg2 arg3
command arg1 arg2 arg3; command arg1 arg2 arg3; command arg1 arg2 arg3
```
### Piping & Redirecting Standard Output
The pipe (`|`) and redirect (`>`) operators are used for manipulating the standard output.
```ion
command arg1 | other_command | another_command arg2
command arg1 > file
```
### Piping & Redirecting Standard Error
The `^|` and `^>` operators are used for manipulating the standard error.
```ion
command arg1 ^| other_command
command arg1 ^> file
```
### Piping & Redirecting Both Standard Output & Standard Error
The `&|` and `&>` operators are used for manipulating both the standard output and error.
```ion
command arg1 &| other_command # Not supported yet
command arg1 &> file
```
### Conditional Operators
The Ion shell supports the `&&` and `||` operators in the same manner as the Bash shell. The `&&` operator
executes the following command if the previous command exited with a successful exit status. The `||`
operator performs the reverse -- executing if the previous command exited in failure.
```ion
test -e .git && echo Git directory exists || echo Git directory does not exist
```
### If Conditions
It is also possible to perform more advanced conditional expressions using the `if`, `else if`, and `else` keywords.
```ion
let a = 5;
if test $a -lt 5
echo "a < 5"
else if test $a -eq 5
echo "a == 5"
else
echo "a > 5"
end
```
### While Loops
While loops will evaluate a supplied expression for each iteration and execute all the contained statements if it
evaluates to a successful exit status.
```ion
let a = 1
while test $a -lt 100
echo $a
let a += 1
end
```
### For Loops
For loops, on the other hand, will take a variable followed by a list of values or a range expression, and
iterate through all contained statements until all values have been exhausted. If the variable is `_`, it
will be ignored. Take note that quoting rules are reversed for for loops, and values from string-based command
substitutions are split by lines.
```ion
# Obtaining Values From a Subshell
for a in $(seq 1 10)
echo $a
end
# Values Provided Directly
for a in 1 2 3 4 5
echo $a
end
# Exclusive Range
for a in 1..11
echo $a
end
# Inclusive Range
for a in 1...10
echo $a
end
# Ignore Value
for _ in 1..10
do_something
end
# Brace Ranges
for a in {1..10}
echo $a
end
# Globbing
for a in *
echo $a
end
```
### Command Substitution
Command substitution allows the user to execute commands within a subshell, and have the data written to standard
output used as the substitution for the expansion. There are two methods of performing command substitution: string and
array-based command substitution. String-based command substitutions are the standard, and they are created by wrapping
the external command between **$(** and **)**. Array-based command substitution is denoted by wrapping the command
between **@(** and **)**. The first merely captures the result as a single string, precisely as it was written, while
the second splits the data recieved into words delimited by whitespaces.
Try comparing the following:
```ion
for i in $(echo 1 2 3)
echo $i
end
```
```ion
for i in @(echo 1 2 3)
echo $i
end
```
### Slicing String-Based Command Substitutions
You may slice the string returned to obtain its substring:
```ion
echo $(echo one two three)[..3]
```
### Slicing Array-Based Command Substitutions
You may slice the array returned to obtained a specific set of elements:
```ion
echo @(grep "model name" /proc/cpuinfo | head -1)[3..5]
```
### Functions
Functions in the Ion shell are defined with a name along with a set of variables. The function
will check if the correct number of arguments were supplied and execute if all arguments
were given.
```ion
fn fib n
if test $n -le 1
echo $n
else
let output = 1
let previous = 1
for _ in 2..$n
let temp = $output
let output += $previous
let previous = $temp
end
echo $output
end
end
for i in 1..20
fib $i
end
```
### Executing Scripts with Array Arguments
Arguments supplied to a script are stored in the `@args` array.
#### Command executed
```ion
script.ion one two three
```
#### Script Contents
```ion
for argument in @args
echo $argument
end
```
#### Output
```
script.ion
one
two
three
```