How to assign the output of a command to a Makefile variable

Question

I need to execute some make rules conditionally  only if the Python installed is greater than a certain version  say 2 5    I thought I could do something like executing   python -c  import sys  print int sys version info  gt    2 5      and then using the output   1  if ok   0  otherwise  in a ifeq make statement   In a simple bash shell script it s just   MY VAR  python -c  import sys  print int sys version info  gt    2 5       but that doesn t work in a Makefile   Any suggestions  I could use any other sensible workaround to achieve this

User · Answer

With GNU Make  you can use shell and eval to store  run  and assign output from arbitrary command line invocations  The difference between the example below and those which use    is the    assignment happens once  when it is encountered  and for all  Recursively expanded variables set with   are a bit more  lazy   references to other variables remain until the variable itself is referenced  and the subsequent recursive expansion takes place each time the variable is referenced  which is desirable for making  consistent  callable  snippets   See the manual on setting variables for more info     Generate a random number    This is not run initially  GENERATE ID     shell od -vAn -N2 -tu2  lt   dev urandom     Generate a random number  and assign it to MY ID   This is not run initially  SET ID     eval MY ID   GENERATE ID      You can use  PHONY to tell make that we aren t building a target output file  PHONY  mytarget mytarget    This is empty when we begin      echo   MY ID    This recursively expands SET ID  which calls the shell command and sets MY ID       SET ID    This will now be a random number      echo   MY ID    Recursively expand SET ID again  which calls the shell command  again  and sets MY ID  again        SET ID    This will now be a different random number      echo   MY ID

User · Answer

In the below example  I have stored the Makefile folder path to LOCAL PKG DIR and then use LOCAL PKG DIR variable in targets   Makefile   LOCAL PKG DIR      shell eval pwd    PHONY  print print       echo   LOCAL PKG DIR    Terminal output     make print  home amrit folder

User · Answer

Wrapping the assignment in an eval is working for me     dependency on  PHONY prevents Make from    thinking there s  nothing to be done  set opts   PHONY     eval DOCKER OPTS   -v   shell mktemp -d -p  scratch   output

User · Answer

Beware of recipes like this  target      MY ID   GENERATE ID       echo  MY ID    It does two things wrong   The first line in the recipe is executed in a separate shell instance from the second line   The variable is lost in the meantime   Second thing wrong is that the   is not escaped   target      MY ID   GENERATE ID         echo   MY ID    Both problems have been fixed and the variable is useable   The backslash combines both lines to run in one single shell  hence the setting of the variable and the reading of the variable afterwords  works   I realize the original post said how to get the results of a shell command into a MAKE variable  and this answer shows how to get it into a shell variable   But other readers may benefit   One final improvement  if the consumer expects an  environment variable  to be set  then you have to export it   my shell script     echo  MY ID   would need this in the makefile  target      export MY ID   GENERATE ID           my shell script    Hope that helps someone   In general  one should avoid doing any real work outside of recipes  because if someone use the makefile with  --dry-run  option  to only SEE what it will do  it won t have any undesirable side effects   Every   shell  call is evaluated at compile time and some real work could accidentally be done   Better to leave the real work  like generating ids  to the inside of the recipes when possible

User · Answer

Use the Make shell builtin like in MY VAR   shell echo whatever   me Zack   make MY VAR IS whatever     me Zack    cat Makefile  MY VAR      shell echo whatever   all       echo MY VAR IS   MY VAR

User · Answer

Here s a bit more complicated example with piping and variable assignment inside recipe   getpodname        Getting pod name      eval    minikube docker-env           eval PODNAME   shell sh -c  kubectl get pods   grep profile-posts-api   grep Running    awk   print   1         echo   PODNAME

User · Answer

I m writing an answer to increase visibility to the actual syntax that solves the problem  Unfortunately  what someone might see as trivial can become a very significant headache to someone looking for a simple answer to a reasonable question   Put the following into the file  Makefile    MY VAR      shell python -c  import sys  print int sys version info  gt    2 5      all       echo MY VAR IS   MY VAR    The behavior you would like to see is the following  assuming you have recent python installed    make MY VAR IS 1   If you copy and paste the above text into the Makefile  will you get this  Probably not  You will probably get an error like what is reported here   makefile 4      missing separator  Stop  Why  Because although I personally used a genuine tab  Stack Overflow  attempting to be helpful  converts my tab into a number of spaces  You  frustrated internet citizen  now copy this  thinking that you now have the same text that I used  The make command  now reads the spaces and finds that the  all  command is incorrectly formatted  So copy the above text  paste it  and then convert the whitespace before   echo  to a tab  and this example should  at last  hopefully  work for you

[shell] How to assign the output of a command to a Makefile variable

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