What does the "@" symbol mean in Bazel?

Question

I'm studying Bazel building system at present. I always see the @ symbol in Bazel script, but I cannot find any documentation about it. I searched it on the website of Bazel but the result seems useless. @ in Bazel. For example:

filegroup(
name = "toolchain_fg",
srcs = [
    ":cc-compiler-amd64",
    "@x86_64_unknown_linux_gnu_gcc_730//:compiler_components",
    ],
)

Could anybody explain the @ symbol here for me?

Answer 1

This is to reference a remote repository.

From the doc, depending on other Bazel projects

local_repository(
    name = "coworkers_project",
    path = "/path/to/coworkers-project",
)

If your coworker has a target //foo:bar, your project can refer to it as @coworkers_project//foo:bar.

See also the design doc of remote repository and bind example in workspace rules.

Answer 2

In Bazel, targets are referred by labels.

Bazel labels have the following format:

@repoName//packageName:target

For example, in the following packages found in myRepo:

 myRepo
    ├── WORKSPACE
    ├── package1
    │  └── BUILD
    │  └── src
    └── package2
       ├── BUILD
       └── src

a target called myTarget in package1/BUILD can be labeled as as @myRepo//package1:myTarget globally. If referenced from the same repo, for example from package2/BUILD, then the @myRepo prefix can be omitted and you can use //package1:myTarget. If referenced from the same package, for example in another target from package1/BUILD, then the package name can be omitted and you can use :myTarget. The colon can also be omitted if it does not create confusion with a name. Such short form labels should not be confused with the names. Labels start with '//' or ':'. But names never do. For example, the name of the first package is package1 but its label is //package1.

Reference: https://docs.bazel.build/versions/master/build-ref.html