The Julia REPL

Julia comes with a full-featured interactive command-line REPL (read-eval-print loop) built into the julia executable. In addition to allowing quick and easy evaluation of Julia statements, it has a searchable history, tab-completion, many helpful keybindings, and dedicated help and shell modes. The REPL can be started by simply calling julia with no arguments or double-clicking on the executable:

$ julia

               _
   _       _ _(_)_     |  Documentation: https://docs.julialang.org
  (_)     | (_) (_)    |
   _ _   _| |_  __ _   |  Type "?" for help, "]?" for Pkg help.
  | | | | | | |/ _` |  |
  | | |_| | | | (_| |  |  Version 1.5.2 (2020-09-23)
 _/ |\__'_|_|_|\__'_|  |  Official https://julialang.org/ release
|__/                   |


julia>

To exit the interactive session, type ^D – the control key together with the d key on a blank line – or type exit() followed by the return or enter key. The REPL greets you with a banner and a julia> prompt.

The different prompt modes

The Julian mode

The REPL has four main modes of operation. The first and most common is the Julian prompt. It is the default mode of operation; each new line initially starts with julia>. It is here that you can enter Julia expressions. Hitting return or enter after a complete expression has been entered will evaluate the entry and show the result of the last expression.

julia> string(1 + 2)
"3"

There are a number useful features unique to interactive work. In addition to showing the result, the REPL also binds the result to the variable ans. A trailing semicolon on the line can be used as a flag to suppress showing the result.

julia> string(3 * 4);

julia> ans
"12"

In Julia mode, the REPL supports something called prompt pasting. This activates when pasting text that starts with julia> into the REPL. In that case, only expressions starting with julia> are parsed, others are removed. This makes it is possible to paste a chunk of code that has been copied from a REPL session without having to scrub away prompts and outputs. This feature is enabled by default but can be disabled or enabled at will with REPL.enable_promptpaste(::Bool). If it is enabled, you can try it out by pasting the code block above this paragraph straight into the REPL. This feature does not work on the standard Windows command prompt due to its limitation at detecting when a paste occurs.

Objects are printed at the REPL using the show function with a specific IOContext. In particular, the :limit attribute is set to true. Other attributes can receive in certain show methods a default value if it's not already set, like :compact. It's possible, as an experimental feature, to specify the attributes used by the REPL via the Base.active_repl.options.iocontext dictionary (associating values to attributes). For example:

julia> rand(2, 2)
2×2 Array{Float64,2}:
 0.8833    0.329197
 0.719708  0.59114

julia> show(IOContext(stdout, :compact => false), "text/plain", rand(2, 2))
 0.43540323669187075  0.15759787870609387
 0.2540832269192739   0.4597637838786053
julia> Base.active_repl.options.iocontext[:compact] = false;

julia> rand(2, 2)
2×2 Array{Float64,2}:
 0.2083967319174056  0.13330606013126012
 0.6244375177790158  0.9777957560761545

In order to define automatically the values of this dictionary at startup time, one can use the atreplinit function in the ~/.julia/config/startup.jl file, for example:

atreplinit() do repl
    repl.options.iocontext[:compact] = false
end

Help mode

When the cursor is at the beginning of the line, the prompt can be changed to a help mode by typing ?. Julia will attempt to print help or documentation for anything entered in help mode:

julia> ? # upon typing ?, the prompt changes (in place) to: help?>

help?> string
search: string String Cstring Cwstring RevString randstring bytestring SubString

  string(xs...)

  Create a string from any values using the print function.

Macros, types and variables can also be queried:

help?> @time
  @time

  A macro to execute an expression, printing the time it took to execute, the number of allocations,
  and the total number of bytes its execution caused to be allocated, before returning the value of the
  expression.

  See also @timev, @timed, @elapsed, and @allocated.

help?> Int32
search: Int32 UInt32

  Int32 <: Signed

  32-bit signed integer type.

Help mode can be exited by pressing backspace at the beginning of the line.

Shell mode

Just as help mode is useful for quick access to documentation, another common task is to use the system shell to execute system commands. Just as ? entered help mode when at the beginning of the line, a semicolon (;) will enter the shell mode. And it can be exited by pressing backspace at the beginning of the line.

julia> ; # upon typing ;, the prompt changes (in place) to: shell>

shell> echo hello
hello

julia> ; # upon typing ;, the prompt changes (in place) to: shell>

shell> dir
ERROR: IOError: could not spawn `dir`: no such file or directory (ENOENT)
Stacktrace!
.......

However, you can get access to PowerShell like this:

julia> ; # upon typing ;, the prompt changes (in place) to: shell>

shell> powershell
Windows PowerShell
Copyright (C) Microsoft Corporation. All rights reserved.
PS C:\Users\elm>

... and to cmd.exe like that (see the dir command):

julia> ; # upon typing ;, the prompt changes (in place) to: shell>

shell> cmd
Microsoft Windows [version 10.0.17763.973]
(c) 2018 Microsoft Corporation. All rights reserved.
C:\Users\elm>dir
 Volume in drive C has no label
 Volume Serial Number is 1643-0CD7
  Directory of C:\Users\elm

29/01/2020  22:15    <DIR>          .
29/01/2020  22:15    <DIR>          ..
02/02/2020  08:06    <DIR>          .atom

Search modes

In all of the above modes, the executed lines get saved to a history file, which can be searched. To initiate an incremental search through the previous history, type ^R – the control key together with the r key. The prompt will change to (reverse-i-search)`':, and as you type the search query will appear in the quotes. The most recent result that matches the query will dynamically update to the right of the colon as more is typed. To find an older result using the same query, simply type ^R again.

Just as ^R is a reverse search, ^S is a forward search, with the prompt (i-search)`':. The two may be used in conjunction with each other to move through the previous or next matching results, respectively.

Key bindings

The Julia REPL makes great use of key bindings. Several control-key bindings were already introduced above (^D to exit, ^R and ^S for searching), but there are many more. In addition to the control-key, there are also meta-key bindings. These vary more by platform, but most terminals default to using alt- or option- held down with a key to send the meta-key (or can be configured to do so), or pressing Esc and then the key.

Customizing keybindings

Julia's REPL keybindings may be fully customized to a user's preferences by passing a dictionary to REPL.setup_interface. The keys of this dictionary may be characters or strings. The key '*' refers to the default action. Control plus character x bindings are indicated with "^x". Meta plus x can be written "\\M-x" or "\ex", and Control plus x can be written "\\C-x" or "^x". The values of the custom keymap must be nothing (indicating that the input should be ignored) or functions that accept the signature (PromptState, AbstractREPL, Char). The REPL.setup_interface function must be called before the REPL is initialized, by registering the operation with atreplinit . For example, to bind the up and down arrow keys to move through history without prefix search, one could put the following code in ~/.julia/config/startup.jl:

import REPL
import REPL.LineEdit

const mykeys = Dict{Any,Any}(
    # Up Arrow
    "\e[A" => (s,o...)->(LineEdit.edit_move_up(s) || LineEdit.history_prev(s, LineEdit.mode(s).hist)),
    # Down Arrow
    "\e[B" => (s,o...)->(LineEdit.edit_move_down(s) || LineEdit.history_next(s, LineEdit.mode(s).hist))
)

function customize_keys(repl)
    repl.interface = REPL.setup_interface(repl; extra_repl_keymap = mykeys)
end

atreplinit(customize_keys)

Users should refer to LineEdit.jl to discover the available actions on key input.

Tab completion

In both the Julian and help modes of the REPL, one can enter the first few characters of a function or type and then press the tab key to get a list all matches:

julia> stri[TAB]
stride     strides     string      strip

julia> Stri[TAB]
StridedArray    StridedMatrix    StridedVecOrMat  StridedVector    String

The tab key can also be used to substitute LaTeX math symbols with their Unicode equivalents, and get a list of LaTeX matches as well:

julia> \pi[TAB]
julia> π
π = 3.1415926535897...

julia> e\_1[TAB] = [1,0]
julia> e₁ = [1,0]
2-element Array{Int64,1}:
 1
 0

julia> e\^1[TAB] = [1 0]
julia> e¹ = [1 0]
1×2 Array{Int64,2}:
 1  0

julia> \sqrt[TAB]2     # √ is equivalent to the sqrt function
julia> √2
1.4142135623730951

julia> \hbar[TAB](h) = h / 2\pi[TAB]
julia> ħ(h) = h / 2π
ħ (generic function with 1 method)

julia> \h[TAB]
\hat              \hermitconjmatrix  \hkswarow          \hrectangle
\hatapprox        \hexagon           \hookleftarrow     \hrectangleblack
\hbar             \hexagonblack      \hookrightarrow    \hslash
\heartsuit        \hksearow          \house             \hspace

julia> α="\alpha[TAB]"   # LaTeX completion also works in strings
julia> α="α"

A full list of tab-completions can be found in the Unicode Input section of the manual.

Completion of paths works for strings and julia's shell mode:

julia> path="/[TAB]"
.dockerenv  .juliabox/   boot/        etc/         lib/         media/       opt/         root/        sbin/        sys/         usr/
.dockerinit bin/         dev/         home/        lib64/       mnt/         proc/        run/         srv/         tmp/         var/
shell> /[TAB]
.dockerenv  .juliabox/   boot/        etc/         lib/         media/       opt/         root/        sbin/        sys/         usr/
.dockerinit bin/         dev/         home/        lib64/       mnt/         proc/        run/         srv/         tmp/         var/

Tab completion can help with investigation of the available methods matching the input arguments:

julia> max([TAB] # All methods are displayed, not shown here due to size of the list

julia> max([1, 2], [TAB] # All methods where `Vector{Int}` matches as first argument
max(x, y) in Base at operators.jl:215
max(a, b, c, xs...) in Base at operators.jl:281

julia> max([1, 2], max(1, 2), [TAB] # All methods matching the arguments.
max(x, y) in Base at operators.jl:215
max(a, b, c, xs...) in Base at operators.jl:281

Keywords are also displayed in the suggested methods after ;, see below line where limit and keepempty are keyword arguments:

julia> split("1 1 1", [TAB]
split(str::AbstractString; limit, keepempty) in Base at strings/util.jl:302
split(str::T, splitter; limit, keepempty) where T<:AbstractString in Base at strings/util.jl:277

The completion of the methods uses type inference and can therefore see if the arguments match even if the arguments are output from functions. The function needs to be type stable for the completion to be able to remove non-matching methods.

Tab completion can also help completing fields:

julia> import UUIDs

julia> UUIDs.uuid[TAB]
uuid1        uuid4         uuid_version

Fields for output from functions can also be completed:

julia> split("","")[1].[TAB]
lastindex  offset  string

The completion of fields for output from functions uses type inference, and it can only suggest fields if the function is type stable.

Dictionary keys can also be tab completed:

julia> foo = Dict("qwer1"=>1, "qwer2"=>2, "asdf"=>3)
Dict{String,Int64} with 3 entries:
  "qwer2" => 2
  "asdf"  => 3
  "qwer1" => 1

julia> foo["q[TAB]

"qwer1" "qwer2"
julia> foo["qwer

Customizing Colors

The colors used by Julia and the REPL can be customized, as well. To change the color of the Julia prompt you can add something like the following to your ~/.julia/config/startup.jl file, which is to be placed inside your home directory:

function customize_colors(repl)
    repl.prompt_color = Base.text_colors[:cyan]
end

atreplinit(customize_colors)

The available color keys can be seen by typing Base.text_colors in the help mode of the REPL. In addition, the integers 0 to 255 can be used as color keys for terminals with 256 color support.

You can also change the colors for the help and shell prompts and input and answer text by setting the appropriate field of repl in the customize_colors function above (respectively, help_color, shell_color, input_color, and answer_color). For the latter two, be sure that the envcolors field is also set to false.

It is also possible to apply boldface formatting by using Base.text_colors[:bold] as a color. For instance, to print answers in boldface font, one can use the following as a ~/.julia/config/startup.jl:

function customize_colors(repl)
    repl.envcolors = false
    repl.answer_color = Base.text_colors[:bold]
end

atreplinit(customize_colors)

You can also customize the color used to render warning and informational messages by setting the appropriate environment variables. For instance, to render error, warning, and informational messages respectively in magenta, yellow, and cyan you can add the following to your ~/.julia/config/startup.jl file:

ENV["JULIA_ERROR_COLOR"] = :magenta
ENV["JULIA_WARN_COLOR"] = :yellow
ENV["JULIA_INFO_COLOR"] = :cyan

TerminalMenus

TerminalMenus is a submodule of the Julia REPL and enables small, low-profile interactive menus in the terminal.

Examples

import REPL
using REPL.TerminalMenus

options = ["apple", "orange", "grape", "strawberry",
            "blueberry", "peach", "lemon", "lime"]

RadioMenu

The RadioMenu allows the user to select one option from the list. The request function displays the interactive menu and returns the index of the selected choice. If a user presses 'q' or ctrl-c, request will return a -1.

# `pagesize` is the number of items to be displayed at a time.
#  The UI will scroll if the number of options is greater
#   than the `pagesize`
menu = RadioMenu(options, pagesize=4)

# `request` displays the menu and returns the index after the
#   user has selected a choice
choice = request("Choose your favorite fruit:", menu)

if choice != -1
    println("Your favorite fruit is ", options[choice], "!")
else
    println("Menu canceled.")
end

Output:

Choose your favorite fruit:
^  grape
   strawberry
 > blueberry
v  peach
Your favorite fruit is blueberry!

MultiSelectMenu

The MultiSelectMenu allows users to select many choices from a list.

# here we use the default `pagesize` 10
menu = MultiSelectMenu(options)

# `request` returns a `Set` of selected indices
# if the menu us canceled (ctrl-c or q), return an empty set
choices = request("Select the fruits you like:", menu)

if length(choices) > 0
    println("You like the following fruits:")
    for i in choices
        println("  - ", options[i])
    end
else
    println("Menu canceled.")
end

Output:

Select the fruits you like:
[press: d=done, a=all, n=none]
   [ ] apple
 > [X] orange
   [X] grape
   [ ] strawberry
   [ ] blueberry
   [X] peach
   [ ] lemon
   [ ] lime
You like the following fruits:
  - orange
  - grape
  - peach

Customization / Configuration

All interface customization is done through the keyword only TerminalMenus.config() function.

Arguments

• charset::Symbol=:na: ui characters to use (:ascii or :unicode); overridden by other arguments
• cursor::Char='>'|'→': character to use for cursor
• up_arrow::Char='^'|'↑': character to use for up arrow
• down_arrow::Char='v'|'↓': character to use for down arrow
• checked::String="[X]"|"✓": string to use for checked
• unchecked::String="[ ]"|"⬚"): string to use for unchecked
• scroll::Symbol=:na: If :wrap then wrap the cursor around top and bottom, if :nowrap do not wrap cursor
• supress_output::Bool=false: For testing. If true, menu will not be printed to console.
• ctrl_c_interrupt::Bool=true: If false, return empty on ^C, if true throw InterruptException() on ^C

Examples

julia> menu = MultiSelectMenu(options, pagesize=5);

julia> request(menu) # ASCII is used by default
[press: d=done, a=all, n=none]
   [ ] apple
   [X] orange
   [ ] grape
 > [X] strawberry
v  [ ] blueberry
Set([4, 2])

julia> TerminalMenus.config(charset=:unicode)

julia> request(menu)
[press: d=done, a=all, n=none]
   ⬚ apple
   ✓ orange
   ⬚ grape
 → ✓ strawberry
↓  ⬚ blueberry
Set([4, 2])

julia> TerminalMenus.config(checked="YEP!", unchecked="NOPE", cursor='⧐')

julia> request(menu)
[press: d=done, a=all, n=none]
   NOPE apple
   YEP! orange
   NOPE grape
 ⧐ YEP! strawberry
↓  NOPE blueberry
Set([4, 2])

References

atreplinit(f)