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HOW_IT_WORKS.md

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How It Works

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

Understanding PATH

When you run all the variety of Go commands using go, 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

goenv works by inserting a directory of shims at the end of your PATH, so if you have go in /usr/bin it will be found first:

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

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

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

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

Choosing the Go Version

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

  1. The GOENV_VERSION environment variable (if specified). You can use the goenv shell command to set this environment variable in your current shell session.

  2. The application-specific .go-version file in the current directory (if present). You can modify the current directory's .go-version file with the goenv local command.

  3. The first .go-version file found (if any) by searching each parent directory, until reaching the root of your filesystem.

  4. The global ~/.goenv/version file. You can modify this file using the goenv global command. If the global version file is not present, goenv assumes you want to use the "system" Go. (In other words, whatever version would run if goenv isn't present in PATH.)

NOTE: You can activate multiple versions at the same time, including multiple versions of Go simultaneously or per project.

Locating the Go Installation

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

Each Go version is installed into its own directory under ~/.goenv/versions.

For example, you might have these versions installed:

  • ~/.goenv/versions/1.6.1/
  • ~/.goenv/versions/1.6.2/

As far as goenv is concerned, version names are simply the directories in ~/.goenv/versions.