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Learn How to Use go mod download Command in Go Programming Language


How to Download Go Mod




Go mod is a tool that helps you manage dependencies in your Go projects. It allows you to specify the modules and versions that your code requires in a go.mod file, and then automatically downloads them to your local module cache. This way, you can ensure that your code builds and runs consistently across different environments.




how to download go mod


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In this article, you will learn how to use go mod download to fetch the modules specified in your go.mod file. You will also learn how to download specific modules or versions, and how to troubleshoot common errors with go mod download.


Prerequisites




Before you can use go mod download, you need to have the following:


  • A Go installation of version 1.11 or later. You can check your Go version by running go version.



  • A module path for your project. This is usually the repository location from which your code can be downloaded by Go tools, such as example.com/mymodule. You can set your module path by running go mod init in your project directory.



  • A go.mod file that defines your module's properties and dependencies. You can create a go.mod file by running go mod init, or edit it manually. For more information on go.mod files, see go.mod file reference.



Downloading Dependencies with Go Mod




To download all the dependencies listed in your go.mod file, you can use the following command:


$ go mod download


This command will fetch the modules from their sources (such as version control repositories or proxy servers) and store them in your local module cache. The default location for the module cache is $GOPATH/pkg/mod, where $GOPATH is your Go workspace directory.


Understanding the Output of Go Mod DownloadWhen you run go mod download, you will see some output that looks like this:


go: downloading example.com/mymodule v1.2.3 go: downloading github.com/some/dependency v0.1.0 example.com/mymodule@v1.2.3 h1:ZmZmZmZmZmZmZmZmZmZmZmZm example.com/mymodule@v1.2.3/go.mod h1:YmYmYmYmYmYmYmYmYmYmYmYm github.com/some/dependency@v0.1.0 h1:XnXnXnXnXnXnXnXnXnXnXnXn github.com/some/dependency@v0.1.0/go.mod h1:WlWlWlWlWlWlWlWlWlWlWlWl


This output shows the following information for each module:


  • The module path, such as example.com/mymodule or github.com/some/dependency.



  • The module version, such as v1.2.3 or v0.1.0. This is the version that matches the requirement in your go.mod file, or the latest available version if no requirement is specified.



  • The module checksum, such as h1:ZmZmZmZmZmZmZmZmZmZmZmZm or h1:XnXnXnXnXnXnXnXnXnXnXnXn. This is a cryptographic hash of the module content, used to verify its integrity and authenticity.



  • The module go.mod checksum, such as h1:YmYmYmYmYmYmYmYmYmYmYmYm or h1:WlWlWlWlWlWlWlWlWlWlWlWl. This is a cryptographic hash of the module's go.mod file, which defines its properties and dependencies.



Using Go Mod Download with Flags




You can also use some optional flags with the go mod download command to modify its behavior or output. Here are some of the most useful flags:


  • -x: This flag prints the commands that go mod download executes, such as git fetch or go mod tidy.



  • -json: This flag prints the output in JSON format, which can be useful for parsing or processing by other tools.



  • -dir: This flag specifies a different directory for the module cache, instead of the default $GOPATH/pkg/mod.



For example, you can run the following command to download the modules in JSON format and store them in a custom directory:


How to install go modules in GoLang


How to use go mod init command to create a go.mod file


How to manage dependencies with go mod tidy and go mod edit


How to upgrade or downgrade go modules with go get command


How to use go mod vendor to create a vendor directory


How to use go mod verify to check the integrity of go modules


How to use go mod why to find out why a module is needed


How to use go mod graph to visualize the dependency graph of go modules


How to use go mod download to download the source code of go modules


How to use semantic versioning for go modules


How to use pseudo-versions for go modules that are not tagged


How to use major version suffixes for go modules with breaking changes


How to use replace directive to replace a module with another version or a local directory


How to use exclude directive to exclude a specific version of a module from the build


How to use retract directive to mark a module version as deprecated or insecure


How to use minimal version selection algorithm for resolving module versions


How to use lazy module loading for faster builds and smaller binaries


How to use workspaces for working on multiple modules at once


How to use go work init command to create a go.work file


How to use go work edit command to add or remove modules from a workspace


How to use go work use command to switch between workspaces


How to use go work sync command to synchronize the workspace with the main module


How to use module proxies for faster and more reliable module downloads


How to use GOPROXY environment variable to configure the module proxy servers


How to use GOPRIVATE environment variable to bypass the proxy for private modules


How to use GONOPROXY and GONOSUMDB environment variables for finer control over proxy settings


How to use GOVCS environment variable to control the version control system used for fetching modules


How to serve modules directly from a proxy server using the GOPROXY protocol


How to communicate with proxies using HTTP requests and responses


How to find a repository for a module path using version control systems


How to map versions to commits and vice versa using version control systems


How to map branches and commits to versions using version control systems


How to locate module directories within a repository using version control systems


How to handle special cases for LICENSE files in module repositories


How to control version control tools with GOVCS environment variable


How to create module zip files for serving modules from a proxy server or storing them in the module cache


How to follow file path and size constraints for module zip files


How to authenticate modules using HTTPS or SSH protocols


How to pass credentials to private proxies or private repositories using netrc or ssh-agent mechanisms


How to protect privacy of module paths and versions using checksum database or local verification methods


How to use the module cache for storing and retrieving module files locally


How to use go.sum files for verifying the cryptographic checksums of module files


How to use checksum database for verifying the checksums of public modules online


How to use GOSUMDB environment variable for configuring the checksum database server


How to use GONOSUMDB environment variable for disabling the checksum database verification for some modules


How do I download Go Modules Reference documentation as PDF?


$ go mod download -json -dir /tmp/modcache


Downloading Specific Modules with Go Mod




Sometimes, you may want to download specific modules or versions, instead of all the modules listed in your go.mod file. You can do this by passing one or more arguments to the go mod download command. The arguments can be module paths, versions, or version queries.


Downloading Modules by Path




To download a module by its module path, you can simply pass the path as an argument to the go mod download command. For example, to download the module rsc.io/quote, you can run:


$ go mod download rsc.io/quote


This will download the latest available version of the module, unless you have a specific requirement for it in your go.mod file.


Downloading Modules by Version




To download a module by its version, you can append the version to the module path with an @ sign. For example, to download version v1.5.2 of the module rsc .io/quote, you can run:


$ go mod download rsc.io/quote@v1.5.2


This will download the exact version of the module, regardless of the requirement in your go.mod file. You can also use the special version latest to download the latest available version of the module.


Downloading Modules by Version Query




To download a module by a version query, you can use a syntax similar to the one used by the go get command. A version query can be a prefix, a suffix, or a range of versions. For example, to download the latest minor version of the module rsc.io/quote in the v1 series, you can run:


$ go mod download rsc.io/quote@v1


This will download the highest version with the prefix v1, such as v1.5.2. You can also use the suffix @patch to download the latest patch version of the module, such as v1.5.3. For more information on version queries, see go get documentation.


Troubleshooting Common Errors with Go Mod Download




Sometimes, you may encounter some errors when using go mod download. Here are some of the most common ones and how to fix them:


Module Not Found Error




This error occurs when the module path is invalid or the module is not available from any source. For example, if you try to download a non-existent module, you will see something like this:


$ go mod download example.com/nomodule go: example.com/nomodule@latest: unrecognized import path "example.com/nomodule": reading https://example.com/nomodule?go-get=1: 404 Not Found


To fix this error, you need to check that the module path is correct and that the module exists and can be downloaded by Go tools. You can use the Go Module Index to search for available modules and their sources.


Checksum Mismatch Error




This error occurs when the checksum of the downloaded module does not match the one in the go.sum file or the checksum database. For example, if you try to download a module that has been tampered with, you will see something like this:


$ go mod download rsc.io/quote@v1.5.2 verifying rsc.io/quote@v1.5.2: checksum mismatch downloaded: h1:YmYmYmYmYmYmYmYmYmYmYmYm go.sum: h1:ZmZmZmZmZmZmZmZmZmZmZmZm SECURITY ERROR This download does NOT match an earlier download recorded in go.sum. The bits may have been replaced on the origin server, or an attacker may have intercepted the download attempt. For more information, see 'go help module-auth'.


To fix this error, you need to verify that the source of the module is trustworthy and that the network connection is secure. You can also try to delete the go.sum file and run go mod tidy to regenerate it with the correct checksums.


Network Error




This error occurs when there is a problem with the network connection or the proxy server. For example, if you try to download a module from a blocked site, you will see something like this:


$ go mod download golang.org/x/net go: golang.org/x/net@v0.0.0-20210614182718-04defd469f4e: Get "https://proxy.golang.org/golang.org/x/net/@v/v0 .0.0-20210614182718-04defd469f4e.mod": dial tcp: lookup proxy.golang.org on 8.8.8.8:53: no such host


To fix this error, you need to check that your network connection is working and that you have access to the module source or the proxy server. You can also try to set a different proxy server by using the GOPROXY environment variable or the go env -w command.


Conclusion




In this article, you learned how to use go mod download to fetch the modules specified in your go.mod file. You also learned how to download specific modules or versions, and how to troubleshoot common errors with go mod download.


Go mod is a powerful tool that simplifies dependency management in your Go projects. By using go mod download, you can ensure that your code builds and runs consistently across different environments, and that your dependencies are secure and verified.


If you want to learn more about go mod and other Go tools, you can check out the following resources:


  • Go Modules Reference



  • Go Command Documentation



  • Go Blog: Using Go Modules



Happy coding!


FAQs




  • What is the difference between go mod download and go get?



go mod download downloads the modules specified in your go.mod file, while go get downloads and installs packages or modules from their sources. go get also updates your go.mod file with the new requirements, while go mod download does not.


  • How can I list the modules downloaded by go mod download?



You can use the go list -m all command to list all the modules downloaded by go mod download, along with their versions and paths. You can also use the -f flag to customize the output format.


  • How can I delete the modules downloaded by go mod download?



You can use the go clean -modcache command to delete all the modules downloaded by go mod download. This will remove all the files in your module cache directory, which is usually $GOPATH/pkg/mod.


  • How can I verify the checksums of the modules downloaded by go mod download?



You can use the Go Checksum Database, which is a public service that stores the checksums of publicly available modules. You can query the database by using the sum.golang.org module proxy, or by using the golang.org/dl/go1.16.5 command-line tool.


  • How can I download modules from private repositories with go mod download?



You can use the GOPRIVATE environment variable, which tells Go tools to treat certain module paths as private and not use the public proxy or checksum database. You can also use the GONOSUMDB environment variable, which tells Go tools to skip the checksum verification for certain module paths. The article is complete. I hope you find it useful and informative. Here is the custom message you requested:




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