.jl File Extension

What kind of file is a .jl?

A .jl file contains Julia source code. It is editable text, not a native executable. A file may be a stand-alone script, one part of a module, or the entry point for a package. Julia evaluates a source file when you pass it to the julia command or call include("file.jl").¹²

There are no fixed magic bytes. Software identifies Julia code from the .jl suffix, project context, or source syntax. The suffix does not declare a character encoding, although current projects normally use UTF-8 so identifiers and mathematical Unicode characters remain portable.

IANA does not register a Julia-specific media type.³ Some tools use an unregistered text/x-julia, while Poly classifies .jl as readable text/plain.

How do I open a .jl on my computer?

Any text editor can open .jl files on Windows, macOS, or Linux. Visual Studio Code with the official Julia extension adds syntax highlighting, code navigation, inline results, plotting, debugging, and an integrated Julia REPL.⁴ Vim, Neovim, Emacs, and other programming editors also work.

Install Julia when you need to evaluate the file. From a terminal, run:¹

julia script.jl

For a package or reproducible project, activate the accompanying environment instead of running an isolated file with whatever packages happen to be installed. Opening a source file as text is safe, but evaluating unfamiliar Julia code can read files, start processes, access the network, or install dependencies. Review it first.

Poly support for .jl

Poly opens .jl files in a code-friendly viewer with Julia syntax highlighting, word wrap, and in-file search. It indexes readable source for exact and semantic search, so you can find a function by name or search for code by purpose.

The Poly agent can summarize a script, explain a method, and help trace likely behavior. Poly does not run the code, instantiate its packages, expand its runtime state, or reproduce a Julia language server. Keep related source, tests, Project.toml, and Manifest.toml files together when project context matters.

History of .jl

Jeff Bezanson, Stefan Karpinski, Viral B. Shah, and Alan Edelman publicly introduced Julia on February 14, 2012, after about two and a half years of development.⁵ The project aimed to combine an interactive, high-level language with performance suited to technical computing. .jl became the compact convention for Julia source files.

Julia 1.0 arrived on August 8, 2018.⁶ The extension stayed the same, so .jl alone does not tell you which Julia release or package versions the code expects. Check the project's compatibility declarations and manifest.

Working with .jl and related files

Julia projects use several files with distinct jobs:

Julia's code loader looks for src/PackageName.jl in a package and uses project and manifest files to resolve dependencies.² Keep .jl source and Project.toml in version control. Include Manifest.toml when the project needs a reproducible application environment, following the project's own policy. Use .ipynb for a cell-based narrative workflow, but prefer .jl for ordinary modules, scripts, diffs, and package source.