Standalone CMake script to cut off file contents by delimiters - cmake

Question

I have a project where one repeatable task to do involves manipulating files' contents.
Until now I used a Python script for it, but recently I discovered I can use standalone CMake scripts ("standalone" here means they can be invoked outside of configure/build/test/etc. workflow). As my project already uses CMake for project management I concluded I can save others' problem of installing a Python interpreter (welcome Windows users!) and use CMake project-wide.
Part of my script needs to read a file and cut off everything that appears before "[START-HERE]" and after "[END-HERE]" lines. I am stuck with that part and don't know how to implement it. How can it be done?

Answer 1

You could combine file(READ) with if(MATCHES) to accompilish this. The former is used to read the file, the latter allows you to check for the occurance of a regular expression and to extract a capturing group:
foo.cmake
#[===[
Params:
INPUT_FILE : the path to the file to read
#]===]
file(READ "${INPUT_FILE}" FILE_CONTENTS)
if (FILE_CONTENTS MATCHES "(^|[\r\n])\\[START-HERE\\][\r\n]+(.*)[\r\n]+\\[END-HERE\\]")
# todo: use extracted match stored in CMAKE_MATCH_2 for your own logic
message("Content: '${CMAKE_MATCH_2}'")
else()
message(FATAL_ERROR "[START-HERE]...[END-HERE] doesn't occur in the input file '${INPUT_FILE}'")
endif()
foo.txt
Definetly not
[START-HERE]
working
[END-HERE]
Try again!
Output:
> cmake -D INPUT_FILE=foo.txt -P foo.cmake
Content: 'working'

Answer 2

For the part where you are stuck, here's one approach using the string, file, and math commands:
file(READ data.txt file_str)
string(FIND "${file_str}" "[START-HERE]" start_offset)
# message("${start_offset}")
math(EXPR start_offset "${start_offset}+12")
# message("${start_offset}")
string(FIND "${file_str}" "[END-HERE]" end_offset)
math(EXPR substr_len "${end_offset}-${start_offset}")
# message("${substr_len}")
string(SUBSTRING "${file_str}" "${start_offset}" "${substr_len}" trimmed_str)
# message("${trimmed_str}")
You could also probably do it by using the file(STRINGS) command, which reads lines of a file into an array, and then use the list(FIND) command. The approach shown above has the advantage of working if your delimiters are not on their own lines.
As #fabian shows in their answer post, you can also do this using a regular expression with if(MATCHES) like this:
file(READ "${INPUT_FILE}" FILE_CONTENTS)
if (FILE_CONTENTS MATCHES "(^|[\r\n])\\[START-HERE\\][\r\n]+(.*)[\r\n]+\\[END-HERE\\]")
# todo: use extracted match stored in CMAKE_MATCH_2 for your own logic
message("Content: '${CMAKE_MATCH_2}'")
else()
message(FATAL_ERROR "[START-HERE]...[END-HERE] doesn't occur in the input file '${INPUT_FILE}'")
endif()