protoc 命令的获得
源码在 https://github.com/google/protobuf ， 如果不想自己编译获得最新版本，则可以下载官方编译好的各个平台的，下载地址：https://github.com/google/protobuf/releases ，注意不是带语言后缀的文件，那是源码，如下图：
下载后的解压缩包含的内容如下（以mac下为例）
我们通过 which 命令可以查到 protoc 的安装目录， 覆盖它即可。
$ which protoc
/usr/local/bin/protoc
This package contains a precompiled Win32 binary version of the protocol buffer
compiler (protoc). This binary is intended for Windows users who want to
use Protocol Buffers in Java or Python but do not want to compile protoc
themselves. To install, simply place this binary somewhere in your PATH.
This binary was built using MinGW, but the output is the same regardless of
the C++ compiler used.
You will still need to download the source code package in order to obtain the
Java or Python runtime libraries. Get it from:
https://github.com/google/protobuf/releases/
命令参数
$ protoc -help
Usage: protoc [OPTION] PROTO_FILES
Parse PROTO_FILES and generate output based on the options given:
-IPATH, --proto_path=PATH Specify the directory in which to search for
imports. May be specified multiple times;
directories will be searched in order. If not
given, the current working directory is used.
--version Show version info and exit.
-h, --help Show this text and exit.
--encode=MESSAGE_TYPE Read a text-format message of the given type
from standard input and write it in binary
to standard output. The message type must
be defined in PROTO_FILES or their imports.
--decode=MESSAGE_TYPE Read a binary message of the given type from
standard input and write it in text format
to standard output. The message type must
be defined in PROTO_FILES or their imports.
--decode_raw Read an arbitrary protocol message from
standard input and write the raw tag/value
pairs in text format to standard output. No
PROTO_FILES should be given when using this
flag.
-oFILE, Writes a FileDescriptorSet (a protocol buffer,
--descriptor_set_out=FILE defined in descriptor.proto) containing all of
the input files to FILE.
--include_imports When using --descriptor_set_out, also include
all dependencies of the input files in the
set, so that the set is self-contained.
--include_source_info When using --descriptor_set_out, do not strip
SourceCodeInfo from the FileDescriptorProto.
This results in vastly larger descriptors that
include information about the original
location of each decl in the source file as
well as surrounding comments.
--dependency_out=FILE Write a dependency output file in the format
expected by make. This writes the transitive
set of input file paths to FILE
--error_format=FORMAT Set the format in which to print errors.
FORMAT may be 'gcc' (the default) or 'msvs'
(Microsoft Visual Studio format).
--print_free_field_numbers Print the free field numbers of the messages
defined in the given proto files. Groups share
the same field number space with the parent
message. Extension ranges are counted as
occupied fields numbers.
--plugin=EXECUTABLE Specifies a plugin executable to use.
Normally, protoc searches the PATH for
plugins, but you may specify additional
executables not in the path using this flag.
Additionally, EXECUTABLE may be of the form
NAME=PATH, in which case the given plugin name
is mapped to the given executable even if
the executable's own name differs.
--cpp_out=OUT_DIR Generate C++ header and source.
--csharp_out=OUT_DIR Generate C# source file.
--java_out=OUT_DIR Generate Java source file.
--javanano_out=OUT_DIR Generate Java Nano source file.
--js_out=OUT_DIR Generate JavaScript source.
--objc_out=OUT_DIR Generate Objective C header and source.
--python_out=OUT_DIR Generate Python source file.
--ruby_out=OUT_DIR Generate Ruby source file.
例子
Java 文件生成
$ protoc --java_out=./java/ ./proto/helloworld.proto
protoc 的命令格式为 protoc [OPTION] PROTO_FILES （最后是待编译的 proto文件）
--java_out 为输出java代码的目录，这里指定的是 ./java/ 目录。
随后我们指定了proto文件的位置 ./proto/helloworld.proto 。
执行上述命令，我们就 ./java/ 目录下就产生了对应的 java文件。
go 文件生成
下面这几种方式生成都可以：
$ protoc --go_out=./go/ ./proto/helloworld.proto
跟上面Java的生成完全一样，只不过这次是让生成 go 的代码。
$ protoc --go_out=./go/ -I proto ./proto/helloworld.proto
这次多了一个参数 -I ， -I=IMPORT_PATH can be used as a short form of --proto_path.
-IPATH, --proto_path=PATH Specify the directory in which to search for imports. May be specified multiple times; directories will be searched in order. If not given, the current working directory is used.
IMPORT_PATH specifies a directory in which to look for .proto files when resolving import directives. If omitted, the current directory is used. Multiple import directories can be specified by passing the --proto_path option multiple times; they will be searched in order.
简单来说，就是如果多个proto文件之间有互相依赖，生成某个proto文件时，需要import其他几个proto文件，这时候就要用-I来指定搜索目录。
如果没有指定 –I 参数，则在当前目录进行搜索。
上面两种方法产生的目录如下图， –I 参数起作用了后，生成目录少了一级：
javanano 文件生成
$ protoc --javanano_out=ignore_services=true:./javanano/ -I proto ./proto/garlic.proto
由于 javanano 是给 android 用的，没有服务器端代码，所以多了--javanano_out=ignore_services=true:DST_DIR 这个设置，其他完全一样。
参考： https://github.com/grpc/grpc-common/issues/156
更复杂的可以参考：
Android protobuf nano documentation
http://stackoverflow.com/questions/22247951/android-protobuf-nano-documentation
https://developers.google.com/protocol-buffers/docs/proto3#generating
为了更方便的使用gRPC，包括protoc的命令，针对不同语言有下面额外的方法：
http://www.grpc.io/posts/installation
| Language | Platform | Command |
| Node.js | Linux, Mac, Windows | npm install grpc |
| Python | Linux, Mac, Windows | pip install grpcio |
| Ruby | Linux, Mac, Windows | gem install grpc |
| PHP | Linux, Mac, Windows | pecl install grpc-beta |
| Go | Linux, Mac, Windows | go get google.golang.org/grpc |
| Objective-C | Mac | Runtime source fetched automatically from Github by Cocoapods |
| C# | Windows | Install gRPC NuGet package from your IDE (Visual Studio, Monodevelop, Xamarin Studio) |
| Java | Linux, Mac, Windows | Use our Maven and Gradle plugins that provide gRPC with statically linked boringssl |
| C++ | Linux, Mac, Windows | Currently requires manual build and install |
参考资料：