.NET Core 2.1 RC1
was released this week.
This is the first supported version of the .NET Core CLI which includes a feature called “.NET Core Global Tools”.
This feature provides a simple way to create and share cross-platform console tools.
In this post, I’ll go over some of the basics, and then walk though what is going on under the hood.
You will need to download .NET Core 2.1 to use this to try this on your own.

Tip: For a real-world example of a global tool, see https://github.com/natemcmaster/dotnet-serve/.

Basic design

A .NET Core global tool is a special NuGet package that contains a console application.
When installing a tool, .NET Core CLI will download and make your console
tool available as a new command.

Users install tools by executing “dotnet tool install”:

Once installed, the command “awesome-tool” can now be used.

Creating your own package

To simplify getting started, I’ve created a project templates.

Install the templates package

Create a new project

Once you have this project, you can create your tool package like this:

This creates a file named awesome-tool.1.0.0.nupkg
You can install your package like this:

When you are ready to share with world, upload the package to https://nuget.org.

Note: in 2.1 RC1, use –source-feed instead of –add-source. This was renamed in https://github.com/dotnet/cli/pull/9164

Additional commands

dotnet tool has other commands you can invoke. For example,

Under the hood

The NuGet package that contains the tool is produced by running dotnet publish
and putting everything in publish output into the NuGet package, with a few manifest files.

When dotnet tool install –global executes, it…

uses dotnet restore with special parameters to fetch the package.
extracts the package into $HOME/.dotnet/.store/(package id)/(version)

Generates an executable in $HOME/.dotnet/tools

The executable generated is a small console app (written in C++) which automatically knows how to find your .NET Core .dll file and launch it.

When dotnet tool install –tool-path $installDir executes, it does the same thing, but installs into $installDir, not $HOME/.dotnet/tools

Package authoring and SDK

Authoring a global tool requires the .NET Core SDK.
This SDK provides a few simple settings to control the naming and contents of a .NET Core global tool package.

The required minimum project for a global tool looks like this.

Additional properties can be set to control the generated tool.

ToolCommandName – the command name users will invoke. It defaults to the name of the .dll file produced (i.e. assembly name)

This does not need to start with dotnet-. It can be anything without spaces.
If it begins with dotnet-, it can be used as a subcommand on dotnet, provided that command doesn’t already exist. Example: dotnet-serve can be invoked as either dotnet serve or dotnet-serve.

PackageId – the ID of the NuGet package (defaults to the .csproj file name)

The package ID can be different than the command name and assembly name

PackageVersion – the version of the NuGet package (defaults to 1.0.0)

AssemblyName – can be used to change the file name of the .dll file

Example of using these properties:

Deep-dive: package requirements

There are some very specific requirements for global tools. The SDK takes care of most of these for you
when you specify PackAsTool=true.

Publish output into pack

As mentioned above, the tools package must contain all your apps dependencies.
This can be collected into one place by using dotnet publish.
By default, dotnet pack only contains the output of dotnet build.
This output does not normally contain third-party assemblies, static files, and the DotnetToolSettings.xml file,
which is why you need to specify PackAsTool. This instructs the SDK to gather all publish output, not just assembly you compile.

DotnetToolSettings.xml

This file is generated by the SDK when you set PackAsTool to true, and normally, you do not need to create it yourself. The file must exist in the package. If not, dotnet will fail to install your tool with

The settings file in the tool’s NuGet package is invalid: Settings file ‘DotnetToolSettings.xml’ was not found in the package.

The schema for this file looks like this:

$name is the same value as ToolCommandName in your .csproj. It is the command users will type on the command line to invoke your tool

$file is the main file name for your .NET Core console app, such as pineapple.dll

$runner is the name of the command used to invoke your file. As for 2.1, the only allowed value is dotnet.

When you install, dotnet creates an “alias” (sort of) which maps the command $name to invoke $runner $file. It would sort of like using bash to set alias pineapple=dotnet pineapple.dll.

Currently, there are many restrictions on how this file can be used.

The file must exist in the NuGet package under tools/$targetframework/any/. Example: tools/netcoreapp2.1/any/DotnetToolSettings.xml.
You may only specify one DotnetToolSettings.xml file per package.
You may only specify one <Command> per DotnetToolSettings.xml file.
The only allowed value for Runner is “dotnet”.
The value for EntryPoint must be a .dll file that sits next to DotnetToolSettings.xml in the package.

Error NU1212 and package type

Installation may fail with this error

This error message is not very clear (see https://github.com/NuGet/Home/issues/6630 for improvement).
What this means is that dotnet-tool-install is currently restricted to only installing a .NET Core package that has specific metadata. That metadata can be defined in your nuspec file and must be set as follows:

Dependencies

You must redistribute any of your dependencies in your tools package. Dependencies defined in the <dependencies> metadata of your NuGet package are not
honored by dotnet-tool-install.

Common errors

Here are some common errors encountered when using global tools.

PATH and command not after installing

Global tools are actually “user global”, and are installed to %USERPROFILE%.dotnettools (Windows) or $HOME/.dotnet/tools (macOS/Linux). The .NET Core CLI tool makes a best effort to help you ensure this is in your PATH environment variable. However, this
can easily be broken. For instance:

if you have set the DOTNET_SKIP_FIRST_TIME_EXPERIENCE environment variable to speed up intial runs of .NET Core, then your PATH may not be updated on first use

macOS: if you install the CLI via .tar.gz and not .pkg, you’ll be missing the /etc/paths.d/dotnet-cli-tool file that configures PATH.

Linux: you will need to edit your shell environment file. e.g. ~/.bash_profile or ~/.zshrc

Workarounds

(May require restarting your shell.)

Windows:

Linux/macOS:

Tools are user-specific, not machine global

The .NET Core CLI installs global tools to $HOME/.dotnet/tools (Linux/macOS) or %USERPROFILE%.dotnettools (Windows).
This means you cannot install a global tool for the entire machine using dotnet tool install –global.
Installed tools are only available to the user who installed them.

Installing the .NET Core CLI into a non-default location

If you download the .NET Core CLI as a .zip/.tar.gz and extract it to a non default location, then you may encounter an error after installing and launching a tool:

Windows: A fatal error occurred, the required library hostfxr.dll could not be found

Linux: A fatal error occurred, the required library libhostfxr.so could not be found

macOS: A fatal error occurred, the required library libhostfxr.dylib could not be found

The error will also contain additional details such as:

The reason this happens is that the generated shim created by dotnet tool install only searchs
for .NET Core in the default install locations. You can override the default location by setting
the DOTNET_ROOT environment variable.

See https://github.com/dotnet/cli/issues/9114 for more details.

Wrapping up

This is an awesome feature. Super happy the .NET Core CLI team created it. Can’t wait to see what people make.