created_at2015-12-16 18:49:47.749344
updated_at2017-07-15 01:45:41.234678
descriptionThe Ion Shell
Michael Murphy (mmstick)



Ion Shell

Build Status MIT licensed LOC

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), on which it is developed and tested. 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 its borrowing of ideas from Bash, Fish, and Oil; whilst also offering some unique ideas of its 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 its 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.


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.

  • Expansions
    • String Expansions
    • Array Expansions
    • Glob Expansions
    • Brace Expansions
      • Ranges
      • Permutations
      • Nested Braces
    • Process Expansions
      • String-based Command Substitution ($())
      • Array-based Command Substitution (@())
    • Arithmetic Expansions
  • Flow Control
    • For Loops
    • Foreach Loops
    • While Loops
    • If Conditionals
    • Match Statements
  • Functions
    • Optionally-typed Function Parameters
  • Script Execution
    • Handling arguments w/ @args Array
  • Mutables
    • Aliases
    • Strings ($variable)
      • Multiple Variable Assignments
      • Optionally-typed Variable Assignments
      • Grapheme-based String Slicing
      • String Methods ($join(array, ', '))
        • $join(array)
        • $len(string)
        • $len_bytes(string)
    • Arrays (@array)
      • Array Expressions ([])
      • Array Slicing
      • Array Methods (@split(var, ' '))
        • @split(string)
        • @len(array)
    • HashMaps
  • Line Editor (Provided by Liner)
    • Multiline Comments and Commands
    • Multi-line Editing
    • Tab Completion
    • Auto-suggestions
    • History suggestions
    • vi and emacs keybindings (set -o (vi|emacs))
    • Syntax Highlighting
  • Implicit cd
  • Signal Handling
  • && and || Conditionals
  • Redirecting Stdout / Stderr
  • Redirecting Stdout & Stderr
  • Piping Builtins
  • 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, 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.

~/Documents # cd ~/Documents
..          # cd ..
.config     # cd .config
examples/   # cd examples/

Job Control

Disowning Processes

Ion features a disown command which supports the following flags:

  • -r: Remove all running jobs from the background process list.
  • -h: Specifies that each job supplied will not receive the SIGHUP signal when the shell receives a SIGHUP.
  • -a: If no job IDs were supplied, remove all jobs from the background process list.

Unlike Bash, job arguments are their specified job IDs.

Foreground & Background Tasks

This area is still a work in progress. When a foreground task is stopped with the Ctrl+Z signal, that process will be added to the background process list as a stopped job. When a supplied command ends with the & operator, this will specify to run the task the background as a running job. To resume a stopped job, executing the bg <job_id> command will send a SIGCONT to the specified job ID, hence resuming the job. The fg command will similarly do the same, but also set that task as the foreground process.

Exiting the Shell

The exit command will exit the shell, sending a SIGTERM to any background tasks that are still active.

Suspending the Shell

While the shell ignores SIGTSTP signals, you can forcefully suspend the shell by executing the suspend command, which forcefully stops the shell via a SIGSTOP signal.

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.

let git_branch = $(git rev-parse --abbrev-ref HEAD ^> /dev/null)

It is also possible to assign multiple variables at once, or swap variables.

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.

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:

$ let string = "one two three"
$ echo $string[0]
$ echo $string[..3]
$ echo $string[4..7]
$ echo $string[8..]

Dropping Variables

To drop a value from the shell, the drop keyword may be used:

drop git_branch

Variable Arithmetic

The let command also supports basic arithmetic.

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


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.

export PATH = ~/.cargo/bin:${PATH}

Export Arithmetic

The export command also supports basic arithmetic.

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


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 its specific arguments, and to rename a command to something more familiar.

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.

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.

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 in between [] characters. The syntax within will be evaluated into a flat-mapped vector, and the result can therefore be stored as an array.

let array = [ one two 'three four' ]

One particular use case for arrays is setting command arguments

let lsflags = [ -l -a ]
ls @lsflags

Copying Arrays

Do note, however, that arrays must always be explicitly created with [].

let array = [ 1 2 3 ]
let copy_of_array = [ @array ]
let string = @array

When an array variable is passed by itself, it will coerced into a string.

Braces Can Create Arrays

Brace expansions actually create a vector of values under the hood, and thus they can be used to create an array.

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.

let word_split_process = [ @(echo one two three) ]

Using Arrays

Arrays may be called with the @ sigil, which works identical to the variable syntax:

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:

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:

let array = [ 1 2 3 4 5 ]
echo @array[0..1]
echo @array[0...1]
echo @array[..3]
echo @array[3..]
echo @array[..]


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.

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

echo $join(array)[3..6]

Array Slicing on Array Methods

let cpu_model = $(grep "model name" /proc/cpuinfo | head -1)
echo @split(cpu_model)[3..5]


Commands may be written line by line or altogether on the same line with semicolons separating them.

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.

command arg1 | other_command | another_command arg2
command arg1 > file

Piping & Redirecting Standard Error

The ^| and ^> operators are used for manipulating the standard error.

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.

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.

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.

let a = 5;
if test $a -lt 5
    echo "a < 5"
else if test $a -eq 5
    echo "a == 5"
    echo "a > 5"

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.

let a = 1
while test $a -lt 100
    echo $a
    let a += 1

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.

# Obtaining Values From a Subshell
for a in $(seq 1 10)
    echo $a

# Values Provided Directly
for a in 1 2 3 4 5
    echo $a

# Exclusive Range
for a in 1..11
    echo $a

# Inclusive Range
for a in 1...10
    echo $a

# Ignore Value
for _ in 1..10

# Brace Ranges
for a in {1..10}
    echo $a

# Globbing
for a in *
    echo $a

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:

for i in $(echo 1 2 3)
    echo $i
for i in @(echo 1 2 3)
    echo $i

Slicing String-Based Command Substitutions

You may slice the string returned to obtain its substring:

echo $(echo one two three)[..3]

Slicing Array-Based Command Substitutions

You may slice the array returned to obtained a specific set of elements:

echo @(grep "model name" /proc/cpuinfo | head -1)[3..5]


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.

fn fib n
    if test $n -le 1
        echo $n
        let output = 1
        let previous = 1
        for _ in 2..$n
            let temp = $output
            let output += $previous
            let previous = $temp
        echo $output

for i in 1..20
    fib $i

Executing Scripts with Array Arguments

Arguments supplied to a script are stored in the @args array.

Command executed

script.ion one two three

Script Contents

for argument in @args
    echo $argument


Commit count: 2182

cargo fmt