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jlenv: Robust Julia management for your applications.

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How It Works

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At a high level, jlenv intercepts Julia commands using shim executables injected into your PATH, determines which Julia version has been specified by your application, and passes your commands along to the correct Julia installation.

Understanding PATH

When you run a command like julia, your operating system searches through a list of directories to find an executable file with that name. This list of directories lives in an environment variable called PATH, with each directory in the list separated by a colon:

/usr/local/bin:/usr/bin:/bin

Directories in PATH are searched from left to right, so a matching executable in a directory at the beginning of the list takes precedence over another one at the end. In this example, the /usr/local/bin directory will be searched first, then /usr/bin, then /bin.

Understanding Shims

jlenv works by inserting a directory of shims at the front of your PATH:

~/.jlenv/shims:/usr/local/bin:/usr/bin:/bin

Through a process called rehashing, jlenv maintains shims in that directory to match every Julia command across every installed version of Julia.

Shims are lightweight executables that simply pass your command along to jlenv. So with jlenv installed, when you run, say, julia, your operating system will do the following:

  • Search your PATH for an executable file named julia
  • Find the jlenv shim named julia at the beginning of your PATH
  • Run the shim named julia, which in turn passes the command along to jlenv

Choosing the Julia Version

When you execute a shim, jlenv determines which Julia version to use by reading it from the following sources, in this order:

  1. The JLENV_VERSION environment variable, if specified. You can use the jlenv shell command to set this environment variable in your current shell session.

  2. The first .julia-version file found by searching the directory of the script you are executing and each of its parent directories until reaching the root of your filesystem.

  3. The first .julia-version file found by searching the current working directory and each of its parent directories until reaching the root of your filesystem. You can modify the .julia-version file in the current working directory with the jlenv local command.

  4. The global ~/.jlenv/version file. You can modify this file using the jlenv global command. If the global version file is not present, jlenv assumes you want to use the “system” Julia—i.e. whatever version would be run if jlenv weren’t in your path.

Locating the Julia Installation

Once jlenv has determined which version of Julia your application has specified, it passes the command along to the corresponding Julia installation.

Each Julia version is installed into its own directory under ~/.jlenv/versions. For example, you might have these versions installed:

  • ~/.jlenv/versions/v0.6.0/
  • ~/.jlenv/versions/v0.6.0-rc3/
  • ~/.jlenv/versions/v0.5.0/

Version names to jlenv are simply the names of the directories in ~/.jlenv/versions.