Use the installation instructions to install a copy of Bazel on your machine.
All Bazel builds take place in a workspace,
a directory on your filesystem that contains source code for the software you
want to build, as well symbolic links to directories that contain the build
outputs (for example,
bazel-out). The location of the
workspace directory is not significant, but it must contain a file called
WORKSPACE in the top-level directory; an empty file is a valid workspace.
WORKSPACE file can be used to reference
external dependencies required to build the outputs.
One workspace can be shared among multiple projects if desired.
To know which targets can be built in your project, Bazel inspects
files. They are written in Bazel's build language which is syntactically
similar to Python. Usually they are just a sequence of declarations of rules.
Each rule specifies its inputs, outputs, and a way to compute the outputs from
The rule probably most familiar to people who have used
Makefiles before (as
it is the only rule available there) is the
genrule, which specifies how the output can
be generated by invoking a shell command.
genrule( name = "hello", outs = ["hello_world.txt"], cmd = "echo Hello World > $@", )
The shell command may contain Make variables.
Using the above
BUILD file, you can ask Bazel to generate the target.
$ bazel build :hello . INFO: Found 1 target... Target //:hello up-to-date: bazel-genfiles/hello_world.txt INFO: Elapsed time: 2.255s, Critical Path: 0.07s
We note two things. First, targets are normally referred to by their
label, which is specified by the
name attribute of the rule. (Referencing
them by the output file name is also possible, but this is not the preferred
way.) Second, Bazel puts the generated files into a separate directory (the
bazel-genfiles directory is actually a symbolic link) so as not to pollute
your source tree.
Rules may use the output of other rules as input, as in the following example. Again, the generated sources are referred to by their label.
genrule( name = "hello", outs = ["hello_world.txt"], cmd = "echo Hello World > $@", ) genrule( name = "double", srcs = [":hello"], outs = ["double_hello.txt"], cmd = "cat $< $< > $@", )