protoc 命令的獲得
源碼在 https://github.com/google/protobuf ， 如果不想自己編譯獲得最新版本，則可以下載官方編譯好的各個平台的，下載地址：https://github.com/google/protobuf/releases ，注意不是帶語言后綴的文件，那是源碼，如下圖：
下載后的解壓縮包含的內容如下（以mac下為例）
我們通過 which 命令可以查到 protoc 的安裝目錄， 覆蓋它即可。
$ 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/
命令參數
$ 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 的命令格式為 protoc [OPTION] PROTO_FILES （最后是待編譯的 proto文件）
--java_out 為輸出java代碼的目錄，這里指定的是 ./java/ 目錄。
隨后我們指定了proto文件的位置 ./proto/helloworld.proto 。
執行上述命令，我們就 ./java/ 目錄下就產生了對應的 java文件。
go 文件生成
下面這幾種方式生成都可以：
$ protoc --go_out=./go/ ./proto/helloworld.proto
跟上面Java的生成完全一樣，只不過這次是讓生成 go 的代碼。
$ protoc --go_out=./go/ -I proto ./proto/helloworld.proto
這次多了一個參數 -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.
簡單來說，就是如果多個proto文件之間有互相依賴，生成某個proto文件時，需要import其他幾個proto文件，這時候就要用-I來指定搜索目錄。
如果沒有指定 –I 參數，則在當前目錄進行搜索。
上面兩種方法產生的目錄如下圖， –I 參數起作用了后，生成目錄少了一級：
javanano 文件生成
$ protoc --javanano_out=ignore_services=true:./javanano/ -I proto ./proto/garlic.proto
由於 javanano 是給 android 用的，沒有服務器端代碼，所以多了--javanano_out=ignore_services=true:DST_DIR 這個設置，其他完全一樣。
參考： https://github.com/grpc/grpc-common/issues/156
更復雜的可以參考：
Android protobuf nano documentation
http://stackoverflow.com/questions/22247951/android-protobuf-nano-documentation
https://developers.google.com/protocol-buffers/docs/proto3#generating
為了更方便的使用gRPC，包括protoc的命令，針對不同語言有下面額外的方法：
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 |
參考資料：