Executing bash commands from a CMake file

Question

I am having trouble understanding CMake. What I want to do is execute the following bash command during the build process:

date +"%F %T" > timestamp

This fetches the current date and writes it to a file. However, I cannot seem to reproduce this simple action using CMake commands.

Here are a few things that I've tried:

execute_process(COMMAND "date +'%F %T' > timestamp")

add_custom_command(OUTPUT timestamp COMMAND date +"%F %T")

file(WRITE timestamp date +"%F %T")

Neither seem to work. I almost wonder if they are even being executed at all.

I have a very limited knowledge of how CMake and its syntax, so I am probably doing things very wrong. I am hoping someone could point me in the right direction. Thanks!

Answer 1

I think my main issue was the lack of quotes around my command arguments. Also, thanks to @Mark Setchell I realized I should be using OUTPUT_VARIABLE in lieu of OUTPUT

At any rate, here is the answer I arrived at:

execute_process (
    COMMAND bash -c "date +'%F %T'"
    OUTPUT_VARIABLE outVar
)

This stores the output of the bash command into the variable outVar

file(WRITE "datestamp" "${outVar}")

And this writes the contents of outVar to a file called "datestamp".

Answer 2

Note -Using bash -c will also prep-end a new line to end of the variable which will cause make to complain depending on how you are using it

build.make: *** missing separator. Stop.

this should solve the above

execute_process(COMMAND  which grpc_cpp_plugin OUTPUT_VARIABLE GRPC_CPP_PLUGIN)
string(STRIP ${GRPC_CPP_PLUGIN} GRPC_CPP_PLUGIN)
message(STATUS "MY_VAR=${GRPC_CPP_PLUGIN}")