execute_process(COMMAND <cmd1> [args1...]]
[COMMAND <cmd2> [args2...] [...]]
[WORKING_DIRECTORY <directory>]
[TIMEOUT <seconds>]
[RESULT_VARIABLE <variable>]
[OUTPUT_VARIABLE <variable>]
[ERROR_VARIABLE <variable>]
[INPUT_FILE <file>]
[OUTPUT_FILE <file>]
[ERROR_FILE <file>]
[OUTPUT_QUIET]
[ERROR_QUIET]
[OUTPUT_STRIP_TRAILING_WHITESPACE]
[ERROR_STRIP_TRAILING_WHITESPACE])
执行进程
这条命令可以执行系统命令，将输出保存到cmake变量或文件中。比如你想通过git命令读取版本号，在代码中使用。又比如你想列出某些文件的名称在代码中使用。
后面还会给出一个简单的例子供大家参考。
Runs the given sequence of one or more commands with the standard output of each process piped to the standard input of the next. A single standard error pipe is used for all processes.
运行一个或多个给定的命令序列，每一个进程的标准输出流向（管道）下一个进程的标准输入。所有的流程使用一个单独的标准错误管道。
Options:
COMMAND
A child process command line.
CMake executes the child process using operating system APIs directly. All arguments are passed VERBATIM to the child process. No intermediate shell is used, so shell operators such as > are treated as normal arguments. (Use the INPUT_*, OUTPUT_*, and ERROR_* options to redirect stdin, stdout, and stderr.)
命令
子进程命令行。CMake使用操作系统的APIs直接执行子进程。所有的参数逐字传递。没有中间脚本被使用，所以像>（输出重定向）这样的脚本操作符被当作普通参数处理。（使用INPUT_、OUTPUT_、ERROR_那些选项去重定向 stdin、stdout、stderr。）
WORKING_DIRECTORY
The named directory will be set as the current working directory of the child processes.
工作路径
指定的目录将被设置为子进程的当前工作目录。
TIMEOUT
The child processes will be terminated if they do not finish in the specified number of seconds (fractions are allowed).
超时
子进程如果在指定的秒数内未结束将被中断（允许使用分数）。
RESULT_VARIABLE
The variable will be set to contain the result of running the processes. This will be an integer return code from the last child or a string describing an error condition.
结果变量
变量被设置为包含子进程的运行结果。返回码将是一个来自于最后一个子进程的整数或者一个错误描述字符串。
OUTPUT_VARIABLE, ERROR_VARIABLE
The variable named will be set with the contents of the standard output and standard error pipes, respectively. If the same variable is named for both pipes their output will be merged in the order produced.
输出变量，错误变量
命名的变量将被分别设置为标准输出和标准错误管道的内容。如果为2个管道命名了相同的名字，他们的输出将按照产生顺序被合并。
INPUT_FILE, OUTPUT_FILE, ERROR_FILE
The file named will be attached to the standard input of the first process, standard output of the last process, or standard error of all processes, respectively. If the same file is named for both output and error then it will be used for both.
输入文件，输出文件，错误文件
命名的文件将分别与第一个子进程的标准输入，最后一个子进程的标准输出，所有进程的标准输出相关联。如果为输出和错误指定了相同的文件名，2个都将会被关联。
OUTPUT_QUIET, ERROR_QUIET
The standard output or standard error results will be quietly ignored.
输出忽略，错误忽略
标准输出和标准错误的结果将被静默地忽略。
If more than one OUTPUT_* or ERROR_* option is given for the same pipe the precedence is not specified. If no OUTPUT_* or ERROR_* options are given the output will be shared with the corresponding pipes of the CMake process itself.
如果为同一个管道指定了多个错误和输出选项，优先级是未知的。如果未指定输出和错误选项，输出将和cmake进程共享管道。
The execute_process() command is a newer more powerful version of exec_program(), but the old command has been kept for compatibility. Both commands run while CMake is processing the project prior to build system generation. Use add_custom_target() and add_custom_command() to create custom commands that run at build time.
execute_process() 命令是 exec_program()的新的更强大的版本，但是旧命令仍被兼容。这两个命令运行在cmake处理项目时，构建系统生成器之前。使用add_custom_target()和add_custom_command()创建在构建时运行的自定义命令。
下面的例子经本人测试，如果指定了OUTPUT_FILE，OUTPUT_VARIABLE将无效。
cmake_minimum_required(VERSION 3.0)
execute_process(COMMAND touch aaa.jpg
COMMAND touch bbb.png
COMMAND ls
COMMAND grep -E "png|jpg"
OUTPUT_FILE pics)