Interacting With Julia

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
               _
   _       _ _(_)_     |  A fresh approach to technical computing
  (_)     | (_) (_)    |  Documentation: http://docs.julialang.org
   _ _   _| |_  __ _   |  Type "help()" to list help topics
  | | | | | | |/ _` |  |
  | | |_| | | | (_| |  |  Version 0.3.0-prerelease+2834 (2014-04-30 03:13 UTC)
 _/ |\__'_|_|_|\__'_|  |  Commit 64f437b (0 days old master)
|__/                   |  x86_64-apple-darwin13.1.0

julia>

To exit the interactive session, type ^D — the control key together with the d key on a blank line — or type quit() 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"

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
Base.string(xs...)

   Create a string from any values using the "print" function.

In addition to function names, complete function calls may be entered to see which method is called for the given argument(s). Macros, types and variables can also be queried:

help> string(1)
string(x::Union(Int16,Int128,Int8,Int32,Int64)) at string.jl:1553

help> @printf
Base.@printf([io::IOStream], "%Fmt", args...)

   Print arg(s) using C "printf()" style format specification
   string. Optionally, an IOStream may be passed as the first argument
   to redirect output.

help> String
DataType   : String
  supertype: Any
  subtypes : {DirectIndexString,GenericString,RepString,RevString{T<:String},RopeString,SubString{T<:String},UTF16String,UTF8String}

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

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).

Program control
^D Exit (when buffer is empty)
^C Interrupt or cancel
Return/Enter, ^J New line, executing if it is complete
meta-Return/Enter Insert new line without executing it
? or ; Enter help or shell mode (when at start of a line)
^R, ^S Incremental history search, described above
Cursor movement
Right arrow, ^F Move right one character
Left arrow, ^B Move left one character
Home, ^A Move to beginning of line
End, ^E Move to end of line
^P Change to the previous or next history entry
^N Change to the next history entry
Up arrow Move up one line (or to the previous history entry)
Down arrow Move down one line (or to the next history entry)
Page-up Change to the previous history entry that matches the text before the cursor
Page-down Change to the next history entry that matches the text before the cursor
meta-F Move right one word
meta-B Move left one word
Editing
Backspace, ^H Delete the previous character
Delete, ^D Forward delete one character (when buffer has text)
meta-Backspace Delete the previous word
meta-D Forward delete the next word
^W Delete previous text up to the nearest whitespace
^K “Kill” to end of line, placing the text in a buffer
^Y “Yank” insert the text from the kill buffer
^T Transpose the characters about the cursor
Delete, ^D Forward delete one character (when buffer has text)