CMake: Get version from multiline text file - cmake

Question

I have a file version.txt
VERSION_MAJOR 1
VERSION_MINOR 1
VERSION_PATCH 3
I want to use cmake to add a definition for major, minor and patch.
I've tries using
file(STRING "version.txt" myvar)
but this just puts the whole file in myvar.
How do I get the numbers?

Answer 1

Your use of file is incorrect, you want to use READ in order to read the contents of the file into a variable.
file(READ "version.txt" ver)
Once you have the file's contents into a variable, you can then use REGEX MATCH with a capture group, and access the capture group using CMAKE_MATCH_N
REGEX MATCH:
Create a regular expression with a capture group which will capture the numbers following "VERSION_MAJOR":
string(REGEX MATCH "VERSION_MAJOR ([0-9]*)" _ ${ver})
Note that if the regex matches the input variable, the entire match will be stored in the output variable. However, we don't want the entire match (as that includes the string "VERSION_MAJOR"), so I've just used a variable name _ as the output variable, which, by convention, tells the user I am not interested in this variable
CMAKE_MATCH_N:
If the match is successful, then the capture groups are available in CMAKE_MATCH_N. In this instance there is only one capture group, so we want to use CMAKE_MATCH_1
set(ver_major ${CMAKE_MATCH_1})
At this point ver_major contains just the major version no.
You can then repeat this for the other version components.
Full example below:
cmake_minimum_required(VERSION 3.5)
file(READ "version.txt" ver)
string(REGEX MATCH "VERSION_MAJOR ([0-9]*)" _ ${ver})
set(ver_major ${CMAKE_MATCH_1})
string(REGEX MATCH "VERSION_MINOR ([0-9]*)" _ ${ver})
set(ver_minor ${CMAKE_MATCH_1})
string(REGEX MATCH "VERSION_PATCH ([0-9]*)" _ ${ver})
set(ver_patch ${CMAKE_MATCH_1})
message("version: ${ver_major}.${ver_minor}.${ver_patch}")
Output:
$ cmake .
version: 1.1.3
-- Configuring done
-- Generating done
-- Build files have been written to: /tmp
For production code you would obviously want to make the cmake script more robust by checking whether the match was successful, and emitting an error if not.