go-getter

go-getter is a library for Go (golang) for downloading files or directories from various sources using a URL as the primary form of input.

The power of this library is being flexible in being able to download from a number of different sources (file paths, Git, HTTP, Mercurial, etc.) using a single string as input. This removes the burden of knowing how to download from a variety of sources from the implementer.

The concept of a detector automatically turns invalid URLs into proper URLs. For example: "github.com/hashicorp/go-getter" would turn into a Git URL. Or "./foo" would turn into a file URL. These are extensible.

This library is used by Terraform for downloading modules, Otto for dependencies and Appfile imports, and Nomad for downloading binaries.

Installation and Usage

Package documentation can be found on GoDoc.

Installation can be done with a normal go get:

$ go get github.com/hashicorp/go-getter

URL Format

go-getter uses a single string URL as input to downlaod from a variety of protocols. go-getter has various "tricks" with this URL to do certain things. This section documents the URL format.

Supported Protocols and Detectors

Protocols are used to download files/directories using a specific mechanism. Example protocols are Git and HTTP.

Detectors are used to transform a valid or invalid URL into another URL if it matches a certain pattern. Example: "github.com/user/repo" is automatically transformed into a fully valid Git URL. This allows go-getter to be very user friendly.

go-getter out of the box supports the following protocols. Additional protocols can be augmented at runtime by implementing the Getter interface.

• Local files
• Git
• Mercurial
• HTTP
• Amazon S3

In addition to the above protocols, go-getter has what are called "detectors." These take a URL and attempt to automatically choose the best protocol for it, which might involve even changing the protocol. The following detection is built-in by default:

• File paths such as "./foo" are automatically changed to absolute file URLs.
• GitHub URLs, such as "github.com/mitchellh/vagrant" are automatically changed to Git protocol over HTTP.
• BitBucket URLs, such as "bitbucket.org/mitchellh/vagrant" are automatically changed to a Git or mercurial protocol using the BitBucket API.

Forced Protocol

In some cases, the protocol to use is ambiguous depending on the source URL. For example, "http://github.com/mitchellh/vagrant.git" could reference an HTTP URL or a Git URL. Forced protocol syntax is used to disambiguate this URL.

Forced protocol can be done by prefixing the URL with the protocol followed by double colons. For example: git::http://github.com/mitchellh/vagrant.git would download the given HTTP URL using the Git protocol.

Forced protocols will also override any detectors.

In the absense of a forced protocol, detectors may be run on the URL, transforming the protocol anyways. The above example would've used the Git protocol either way since the Git detector would've detected it was a GitHub URL.

Checksumming

For file downloads of any protocol, go-getter can automatically verify a checksum for you. Note that checksumming only works for downloading files, not directories, but checksumming will work for any protocol.

To checksum a file, append a checksum query parameter to the URL. The paramter value should be in the format of type:value, where type is "md5", "sha1", "sha256", or "sha512". The "value" should be the actual checksum value. go-getter will parse out this query parameter automatically and use it to verify the checksum. An example URL is shown below:

./foo.txt?checksum=md5:b7d96c89d09d9e204f5fedc4d5d55b21

The checksum query parameter is never sent to the backend protocol implementation. It is used at a higher level by go-getter itself.