schmichael / go-getter

Package for downloading things from a string URL using a variety of protocols.

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

go-getter also has a command you can use to test URL strings:

$ go install github.com/hashicorp/go-getter/cmd/go-getter
...

$ go-getter github.com/foo/bar ./foo
...

The command is useful for verifying URL structures.

URL Format

go-getter uses a single string URL as input to download 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.

Protocol-Specific Options

Each protocol can support protocol-specific options to configure that protocol. For example, the git protocol supports specifying a ref query parameter that tells it what ref to checkout for that Git repository.

The options are specified as query parameters on the URL (or URL-like string) given to go-getter. Using the Git example above, the URL below is a valid input to go-getter:

github.com/hashicorp/go-getter?ref=abcd1234

The protocol-specific options are documented below the URL format section. But because they are part of the URL, we point it out here so you know they exist.

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.

Unarchiving

go-getter will automatically unarchive files into a file or directory based on the extension of the file being requested (over any protocol). This works for both file and directory downloads.

go-getter looks for an archive query parameter to specify the format of the archive. If this isn't specified, go-getter will use the extension of the path to see if it appears archived. Unarchiving can be explicitly disabled by setting the archive query parameter to false.

The following archive formats are supported:

• tar.gz and tgz
• tar.bz2 and tbz2
• zip
• gz
• bz2

For example, an example URL is shown below:

./foo.zip

This will automatically be inferred to be a ZIP file and will be extracted. You can also be explicit about the archive type:

./some/other/path?archive=zip

And finally, you can disable archiving completely:

./some/path?archive=false

You can combine unarchiving with the other features of go-getter such as checksumming. The special archive query parameter will be removed from the URL before going to the final protocol downloader.

Protocol-Specific Options

This section documents the protocol-specific options that can be specified for go-getter. These options should be appended to the input as normal query parameters. Depending on the usage of go-getter, applications may provide alternate ways of inputting options. For example, Nomad provides a nice options block for specifying options rather than in the URL.

General (All Protocols)

The options below are available to all protocols:

• archive - The archive format to use to unarchive this file, or "" (empty string) to disable unarchiving. For more details, see the complete section on archive support above.

• checksum - Checksum to verify the downloaded file or archive. See the entire section on checksumming above for format and more details.

Local Files (file)

None

Git (git)

• ref - The Git ref to checkout. This is a ref, so it can point to a commit SHA, a branch name, etc. If it is a named ref such as a branch name, go-getter will update it to the latest on each get.

Mercurial (hg)

• rev - The Mercurial revision to checkout.

HTTP (http)

None

S3 (s3)

S3 takes various access configurations in the URL. Note that it will also read these from standard AWS environment variables if they're set. If the query parameters are present, these take priority.

• aws_access_key_id - AWS access key.
• aws_access_key_secret - AWS access key secret.
• aws_access_token - AWS access token if this is being used.

Using IAM Instance Profiles with S3

If you use go-getter and want to use an EC2 IAM Instance Profile to avoid using credentials, then just omit these and the profile, if available will be used automatically.

S3 Bucket Examples

S3 has several addressing schemes used to reference your bucket. These are listed here: http://docs.aws.amazon.com/AmazonS3/latest/dev/UsingBucket.html#access-bucket-intro

Some examples for these addressing schemes:

• s3::https://s3.amazonaws.com/bucket/foo
• s3::https://s3-eu-west-1.amazonaws.com/bucket/foo
• bucket.s3.amazonaws.com/foo
• bucket.s3-eu-west-1.amazonaws.com/foo/bar