Let us take a closer look at the invocation of add_custom_target:

add_custom_target(unpack-eigen
  ALL
  COMMAND
    ${CMAKE_COMMAND} -E tar xzf ${CMAKE_CURRENT_SOURCE_DIR}/eigen-eigen-5a0156e40feb.tar.gz
  COMMAND
    ${CMAKE_COMMAND} -E rename eigen-eigen-5a0156e40feb eigen-3.3.4
  WORKING_DIRECTORY
    ${CMAKE_CURRENT_BINARY_DIR}
  COMMENT
    "Unpacking Eigen3 in ${CMAKE_CURRENT_BINARY_DIR}/eigen-3.3.4"
  )

We are introducing a target called unpack-eigen into our build system. The target will always be executed since we passed the ALL argument. The COMMAND argument lets you specify what commands to execute. In this example, we wish to extract the archive and rename the extracted directory to eigen-3.3.4. This is achieved with these two commands:

1. ${CMAKE_COMMAND} -E tar xzf ${CMAKE_CURRENT_SOURCE_DIR}/eigen-eigen-5a0156e40feb.tar.gz
   
2. ${CMAKE_COMMAND} -E rename eigen-eigen-5a0156e40feb eigen-3.3.4

Notice how we are calling the CMake command itself, with the -E flag, to execute the actual work. For many common operations, CMake implements an interface common to all the operating systems it runs on. This allows the build system generation to be largely independent of the specific platform. The next argument in the add_custom_target command is the working directory, which in our example corresponds to the build directory: CMAKE_CURRENT_BINARY_DIR. The last argument, COMMENT,  is used to specify what message CMake should print out when executing the custom target.