IEx (IEx v1.12.0-dev) View Source

Elixir's interactive shell.

Some of the functionalities described here will not be available depending on your terminal. In particular, if you get a message saying that the smart terminal could not be run, some of the features described here won't work.


IEx provides a bunch of helpers. They can be accessed by typing h() into the shell or as a documentation for the IEx.Helpers module.


To discover a module's public functions or other modules, type the module name followed by a dot, then press tab to trigger autocomplete. For example:


A module may export functions that are not meant to be used directly: these functions won't be autocompleted by IEx. IEx will not autocomplete functions annotated with @doc false, @impl true, or functions that aren't explicitly documented and where the function name is in the form of __foo__.

Autocomplete may not be available on some Windows shells. You may need to pass the --werl option when starting IEx, as in iex --werl for it to work. --werl may be permanently enabled by setting the IEX_WITH_WERL environment variable.


Coloring is enabled by default on most Unix terminals. They are also available on Windows consoles from Windows 10, although it must be explicitly enabled for the current user in the registry by running the following command:

reg add HKCU\Console /v VirtualTerminalLevel /t REG_DWORD /d 1

After running the command above, you must restart your current console.

Shell history

It is possible to get shell history by passing some options that enable it in the VM. This can be done on a per-need basis when starting IEx:

iex --erl "-kernel shell_history enabled"

If you would rather enable it on your system as a whole, you can use the ERL_AFLAGS environment variable and make sure that it is set accordingly on your terminal/shell configuration.

On Unix-like / Bash:

export ERL_AFLAGS="-kernel shell_history enabled"

On Windows:

set ERL_AFLAGS "-kernel shell_history enabled"

On Windows 10 / PowerShell:

$env:ERL_AFLAGS = "-kernel shell_history enabled"

Expressions in IEx

As an interactive shell, IEx evaluates expressions. This has some interesting consequences that are worth discussing.

The first one is that the code is truly evaluated and not compiled. This means that any benchmarking done in the shell is going to have skewed results. So never run any profiling nor benchmarks in the shell.

Second, IEx allows you to break an expression into many lines, since this is common in Elixir. For example:

iex(1)> "ab
...(1)> c"

In the example above, the shell will be expecting more input until it finds the closing quote. Sometimes it is not obvious which character the shell is expecting, and the user may find themselves trapped in the state of incomplete expression with no ability to terminate it other than by exiting the shell.

For such cases, there is a special break-trigger (#iex:break) that when encountered on a line by itself will force the shell to break out of any pending expression and return to its normal state:

iex(1)> ["ab
...(1)> c"
...(1)> "
...(1)> ]
...(1)> #iex:break
** (TokenMissingError) iex:1: incomplete expression

Pasting multiline expressions into IEx

IEx evaluates its input line by line in an eager fashion. If at the end of a line the code seen so far is a complete expression, IEx will evaluate it at that point.

iex(1)> [1, [2], 3]
[1, [2], 3]

To prevent this behaviour breaking valid code where the subsequent line begins with a pipe operator, (|>/2 ~>> <<~ ~> <~ <~> <|>), IEx automatically treats such lines as if they were prepended with IEx.Helpers.v/0, which returns the value of the previous expression, if available.

iex(1)> [1, [2], 3]
[1, [2], 3]
iex(2)> |> List.flatten()
[1, 2, 3]

The above is equivalent to:

iex(1)> [1, [2], 3]
[1, [2], 3]
iex(2)> v() |> List.flatten()
[1, 2, 3]

If there are no previous expressions in the history, the pipe operator will fail:

iex(1)> |> List.flatten()
** (RuntimeError) v(-1) is out of bounds

The BREAK menu

Inside IEx, hitting Ctrl+C will open up the BREAK menu. In this menu you can quit the shell, see process and ETS tables information and much more.

Exiting the shell

There are a few ways to quit the IEx shell:

  • via the BREAK menu (available via Ctrl+C) by typing q, pressing enter
  • by hitting Ctrl+C, Ctrl+C
  • by hitting Ctrl+\

If you are connected to remote shell, it remains alive after disconnection.

Prying and breakpoints

IEx also has the ability to set breakpoints on Elixir code and "pry" into running processes. This allows the developer to have an IEx session run inside a given function.

IEx.pry/0 can be used when you are able to modify the source code directly and recompile it:

def my_fun(arg1, arg2) do
  require IEx; IEx.pry()
  ... implementation ...

When the code is executed, it will ask you for permission to be introspected.

Alternatively, you can use IEx.break!/4 to setup a breakpoint on a given module, function and arity you have no control of. While IEx.break!/4 is more flexible, it does not contain information about imports and aliases from the source code.

The User switch command

Besides the BREAK menu, one can type Ctrl+G to get to the User switch command menu. When reached, you can type h to get more information.

In this menu, developers are able to start new shells and alternate between them. Let's give it a try:

User switch command
 --> s 'Elixir.IEx'
 --> c

The command above will start a new shell and connect to it. Create a new variable called hello and assign some value to it:

hello = :world

Now, let's roll back to the first shell:

User switch command
 --> c 1

Now, try to access the hello variable again:

** (UndefinedFunctionError) undefined function hello/0

The command above fails because we have switched shells. Since shells are isolated from each other, you can't access the variables defined in one shell from the other one.

The User switch command can also be used to terminate an existing session, for example when the evaluator gets stuck in an infinite loop or when you are stuck typing an expression:

User switch command
 --> i
 --> c

The User switch command menu also allows developers to connect to remote shells using the r command. A topic which we will discuss next.

Remote shells

IEx allows you to connect to another node in two fashions. First of all, we can only connect to a shell if we give names both to the current shell and the shell we want to connect to.

Let's give it a try. First start a new shell:

$ iex --sname foo

The string between the parentheses in the prompt is the name of your node. We can retrieve it by calling the node/0 function:

iex(foo@HOST)1> node()
iex(foo@HOST)2> Node.alive?()

For fun, let's define a simple module in this shell too:

iex(foo@HOST)3> defmodule Hello do
...(foo@HOST)3>   def world, do: "it works!"
...(foo@HOST)3> end

Now, let's start another shell, giving it a name as well:

$ iex --sname bar

If we try to dispatch to, it won't be available as it was defined only in the other shell:

** (UndefinedFunctionError) undefined function

However, we can connect to the other shell remotely. Open up the User switch command prompt (Ctrl+G) and type:

User switch command
 --> r 'foo@HOST' 'Elixir.IEx'
 --> c

Now we are connected into the remote node, as the prompt shows us, and we can access the information and modules defined over there:

"it works!"

In fact, connecting to remote shells is so common that we provide a shortcut via the command line as well:

$ iex --sname baz --remsh foo@HOST

Where "remsh" means "remote shell". In general, Elixir supports:

  • remsh from an Elixir node to an Elixir node
  • remsh from a plain Erlang node to an Elixir node (through the ^G menu)
  • remsh from an Elixir node to a plain Erlang node (and get an erl shell there)

Connecting an Elixir shell to a remote node without Elixir is not supported.

The .iex.exs file

When starting, IEx looks for a local .iex.exs file (located in the current working directory), then a global one (located at ~/.iex.exs) and loads the first one it finds (if any). Note the location of the .iex.exs files, both in the current directory and the global one, are taken relative to the user that started the application, not to the user that is connecting to the node in case of remote IEx connections.

The code in the loaded .iex.exs file is evaluated in the shell's context. For instance, any modules that are loaded or variables that are bound in the .iex.exs file will be available in the shell after it has booted.

Take the following .iex.exs file:

# Load another ".iex.exs" file

# Import some module from lib that may not yet have been defined

# Print something before the shell starts
IO.puts("hello world")

# Bind a variable that'll be accessible in the shell
value = 13

Running IEx in the directory where the above .iex.exs file is located results in:

$ iex
Erlang/OTP 21 [...]

hello world
Interactive Elixir - press Ctrl+C to exit (type h() ENTER for help)
iex(1)> value

It is possible to load another file by supplying the --dot-iex option to IEx. See iex --help.

Configuring the shell

There are a number of customization options provided by IEx. Take a look at the docs for the IEx.configure/1 function by typing h IEx.configure/1.

Those options can be configured in your project configuration file or globally by calling IEx.configure/1 from your ~/.iex.exs file. For example:

# .iex.exs
IEx.configure(inspect: [limit: 3])

Now run the shell:

$ iex
Erlang/OTP 21 [...]

Interactive Elixir - press Ctrl+C to exit (type h() ENTER for help)
iex(1)> [1, 2, 3, 4, 5]
[1, 2, 3, ...]

Link to this section Summary


Returns registered after_spawn callbacks.

Registers a function to be invoked after the IEx process is spawned.

Macro-based shortcut for IEx.break!/4.

Sets up a breakpoint in module, function and arity with the given number of stops.

Returns string escaped using the specified color.

Returns IEx configuration.

Configures IEx.

Returns the options used for inspecting.

Pries into the process environment.

Returns true if IEx was started, false otherwise.

Returns the IEx width for printing.

Link to this section Functions


after_spawn() :: [(... -> any())]

Returns registered after_spawn callbacks.


after_spawn((... -> any())) :: :ok

Registers a function to be invoked after the IEx process is spawned.

Link to this macro

break!(ast, stops \\ 1)

View Source (macro) (since 1.5.0)

Macro-based shortcut for IEx.break!/4.

Link to this function

break!(module, function, arity, stops \\ 1)

View Source (since 1.5.0)


break!(module(), atom(), arity(), non_neg_integer()) ::

Sets up a breakpoint in module, function and arity with the given number of stops.

This function will instrument the given module and load a new version in memory with breakpoints at the given function and arity. If the module is recompiled, all breakpoints are lost.

When a breakpoint is reached, IEx will ask if you want to pry the given function and arity. In other words, this works similar to IEx.pry/0 as the running process becomes the evaluator of IEx commands and is temporarily changed to have a custom group leader. However, differently from IEx.pry/0, aliases and imports from the source code won't be available in the shell.

IEx helpers includes many conveniences related to breakpoints. Below they are listed with the full module, such as IEx.Helpers.breaks/0, but remember it can be called directly as breaks() inside IEx. They are:

By default, the number of stops in a breakpoint is 1. Any follow-up call won't stop the code execution unless another breakpoint is set.

Alternatively, the number of stops can be increased by passing the stops argument. IEx.Helpers.reset_break/1 and IEx.Helpers.reset_break/3 can be used to reset the number back to zero. Note the module remains "instrumented" even after all stops on all breakpoints are consumed. You can remove the instrumentation in a given module by calling IEx.Helpers.remove_breaks/1 and on all modules by calling IEx.Helpers.remove_breaks/0.

To exit a breakpoint, the developer can either invoke continue(), which will block the shell until the next breakpoint is found or the process terminates, or invoke respawn(), which starts a new IEx shell, freeing up the pried one.


The examples below will use break!, assuming that you are setting a breakpoint directly from your IEx shell. But you can set up a break from anywhere by using the fully qualified name IEx.break!.

The following sets up a breakpoint on URI.decode_query/2:

break! URI, :decode_query, 2

This call will setup a breakpoint that stops once. To set a breakpoint that will stop 10 times:

break! URI, :decode_query, 2, 10

IEx.break!/2 is a convenience macro that allows breakpoints to be given in the format:

break! URI.decode_query/2

Or to set a breakpoint that will stop 10 times:

break! URI.decode_query/2, 10

This function returns the breakpoint ID and will raise if there is an error setting up the breakpoint.

Patterns and guards

IEx.break!/2 allows patterns to be given, triggering the breakpoint only in some occasions. For example, to trigger the breakpoint only when the first argument is the "foo=bar" string:

break! URI.decode_query("foo=bar", _)

Or to trigger it whenever the second argument is a map with more than one element:

break! URI.decode_query(_, map) when map_size(map) > 0

Only a single break point can be set per function. So if you call IEx.break! multiple times with different patterns, only the last pattern is kept.

Note that, while patterns may be given to macros, remember that macros receive ASTs as arguments, and not values. For example, if you try to break on a macro with the following pattern:

break! MyModule.some_macro(pid) when pid == self()

This breakpoint will never be reached, because a macro never receives a PID. Even if you call the macro as MyModule.some_macro(self()), the macro will receive the AST representing the self() call, and not the PID itself.

Breaks and mix test

To use IEx.break!/4 during tests, you need to run mix inside the iex command and pass the --trace to mix test to avoid running into timeouts:

iex -S mix test --trace
iex -S mix test path/to/file:line --trace


color(atom(), iodata()) :: iodata()

Returns string escaped using the specified color.

ANSI escapes in string are not processed in any way.


configuration() :: keyword()

Returns IEx configuration.


configure(keyword()) :: :ok

Configures IEx.

The supported options are:

  • :colors
  • :inspect
  • :width
  • :history_size
  • :default_prompt
  • :continuation_prompt
  • :alive_prompt
  • :alive_continuation_prompt
  • :parser

They are discussed individually in the sections below.


A keyword list that encapsulates all color settings used by the shell. See documentation for the IO.ANSI module for the list of supported colors and attributes.

List of supported keys in the keyword list:

  • :enabled - boolean value that allows for switching the coloring on and off
  • :eval_result - color for an expression's resulting value
  • :eval_info - ... various informational messages
  • :eval_error - ... error messages
  • :eval_interrupt - ... interrupt messages
  • :stack_info - ... the stacktrace color
  • :blame_diff - ... when blaming source with no match
  • :ls_directory - ... for directory entries (ls helper)
  • :ls_device - ... device entries (ls helper)

When printing documentation, IEx will convert the Markdown documentation to ANSI as well. Colors for this can be configured via:

  • :doc_code - the attributes for code blocks (cyan, bright)
  • :doc_inline_code - inline code (cyan)
  • :doc_headings - h1 and h2 (yellow, bright)
  • :doc_title - the overall heading for the output (reverse, yellow, bright)
  • :doc_bold - (bright)
  • :doc_underline - (underline)

IEx will also color inspected expressions using the :syntax_colors option. Such can be disabled with:

IEx.configure(colors: [syntax_colors: false])

You can also configure the syntax colors, however, as desired:

IEx.configure(colors: [syntax_colors: [atom: :red]])

Configuration for most built-in data types are supported: :atom, :string, :binary, :list, :number, :boolean, :nil, and others. The default is:

[number: :magenta, atom: :cyan, string: :green,
 boolean: :magenta, nil: :magenta]


A keyword list containing inspect options used by the shell when printing results of expression evaluation. Default to pretty formatting with a limit of 50 entries.

To show all entries, configure the limit to :infinity:

IEx.configure(inspect: [limit: :infinity])

See Inspect.Opts for the full list of options.


An integer indicating the maximum number of columns to use in output. The default value is 80 columns. The actual output width is the minimum of this number and result of :io.columns. This way you can configure IEx to be your largest screen size and it should always take up the full width of your current terminal screen.

History size

Number of expressions and their results to keep in the history. The value is an integer. When it is negative, the history is unlimited.


This is an option determining the prompt displayed to the user when awaiting input.

The value is a keyword list with two possible keys representing prompt types:

  • :default_prompt - used when Node.alive?/0 returns false

  • :continuation_prompt - used when Node.alive?/0 returns false and more input is expected

  • :alive_prompt - used when Node.alive?/0 returns true

  • :alive_continuation_prompt - used when Node.alive?/0 returns true and more input is expected

The following values in the prompt string will be replaced appropriately:

  • %counter - the index of the history
  • %prefix - a prefix given by IEx.Server
  • %node - the name of the local node


This is an option determining the parser to use for IEx.

The parser is a "mfargs", which is a tuple with three elements: the module name, the function name, and extra arguments to be appended. The parser receives at least three arguments, the current input as a string, the parsing options as a keyword list, and the buffer as a string. It must return {:ok, expr, buffer} or {:incomplete, buffer}.

If the parser raises, the buffer is reset to an empty string.


inspect_opts() :: keyword()

Returns the options used for inspecting.

Pries into the process environment.

This is useful for debugging a particular chunk of code when executed by a particular process. The process becomes the evaluator of IEx commands and is temporarily changed to have a custom group leader. Those values are reverted by calling IEx.Helpers.respawn/0, which starts a new IEx shell, freeing up the pried one.

When a process is pried, all code runs inside IEx and has access to all imports and aliases from the original code. However, the code is evaluated and therefore cannot access private functions of the module being pried. Module functions still need to be accessed via

Alternatively, you can use IEx.break!/4 to setup a breakpoint on a given module, function and arity you have no control of. While IEx.break!/4 is more flexible, it does not contain information about imports and aliases from the source code.


Let's suppose you want to investigate what is happening with some particular function. By invoking IEx.pry/0 from the function, IEx will allow you to access its binding (variables), verify its lexical information and access the process information. Let's see an example:

import Enum, only: [map: 2]

defmodule Adder do
  def add(a, b) do
    c = a + b
    require IEx; IEx.pry()

When invoking Adder.add(1, 2), you will receive a message in your shell to pry the given environment. By allowing it, the shell will be reset and you gain access to all variables and the lexical scope from above:

pry(1)> map([a, b, c], &IO.inspect(&1))

Keep in mind that IEx.pry/0 runs in the caller process, blocking the caller during the evaluation cycle. The caller process can be freed by calling respawn/0, which starts a new IEx evaluation cycle, letting this one go:

pry(2)> respawn()

Interactive Elixir - press Ctrl+C to exit (type h() ENTER for help)

Setting variables or importing modules in IEx does not affect the caller's environment. However, sending and receiving messages will change the process state.

Pry and macros

When setting up Pry inside a code defined by macros, such as:

defmacro __using__(_) do
  quote do
    def add(a, b) do
      c = a + b
      require IEx; IEx.pry()

The variables defined inside quote won't be available during prying due to the hygiene mechanism in quoted expressions. The hygiene mechanism changes the variable names in quoted expressions so they don't collide with variables defined by the users of the macros. Therefore the original names are not available.

Pry and mix test

To use IEx.pry/0 during tests, you need to run mix inside the iex command and pass the --trace to mix test to avoid running into timeouts:

iex -S mix test --trace
iex -S mix test path/to/file:line --trace


started?() :: boolean()

Returns true if IEx was started, false otherwise.


width() :: pos_integer()

Returns the IEx width for printing.

Used by helpers and it has a default maximum cap of 80 chars.