CMake; download file with specific hash. Probably better to use ExternalProject module to do this instead of doing it manually

CMakeLists.txt

file(WRITE ${CMAKE_CURRENT_BINARY_DIR}/download.cmake
  "set(downloaded_file \"${CMAKE_CURRENT_BINARY_DIR}/pg44800.txt\")\n"
  "set(url \"https://www.gutenberg.org/cache/epub/44800/pg44800.txt\")\n"
  "set(expected_hash \"5a837b40a0f90211ff2883651f33a865\")\n"

  "if (EXISTS \${downloaded_file})\n"
  "  file(MD5 \${downloaded_file} filehash)\n"
  "  message(\"\${downloaded_file} exists with hash: \" \${filehash})\n"
  "  if (\${expected_hash} STREQUAL \${filehash})\n"
  "    message(\"Hash matches expected hash.\")\n"
  "  else()\n"
  "    message(\"Hash does not match expected hash.\")\n"
  "    file(DOWNLOAD \${url} \${downloaded_file} SHOW_PROGRESS)\n"
  "  endif()\n"
  "else()\n"
  "  message(\"\${downloaded_file} missing.\")\n"
  "  file(DOWNLOAD \${url} \${downloaded_file} SHOW_PROGRESS)\n"
  "endif()\n"
)

add_custom_command(OUTPUT "${CMAKE_CURRENT_BINARY_DIR}/pg44800.txt"
  COMMAND ${CMAKE_COMMAND} -P "${CMAKE_CURRENT_BINARY_DIR}/download.cmake"
  WORKING_DIRECTORY "${CMAKE_CURRENT_BINARY_DIR}"
  DEPENDS "${CMAKE_CURRENT_BINARY_DIR}/download.cmake"
  COMMENT "Downloading test text file."
  VERBATIM
)

add_custom_target(file_download ALL DEPENDS "${CMAKE_CURRENT_BINARY_DIR}/pg44800.txt")

The CMake "FILE" DOWNLOAD command will accept the additional "EXPECTED_HASH" argument in recent versions of CMake. The example above is obsolete.

See: https://cmake.org/cmake/help/latest/command/file.html