Question

I am running the following inside the makefile:-

@START=$(shell date +%s) && \
echo $${START} &&\
sleep 10s && \
END=$(shell date +%s) && \
echo "$${END} $${START}" &&\
DIFF_SUB=$$(($$END - $$START)) && \
echo IT TOOK $${DIFF_SUB} SECONDS

and it outputs to the following:-

1309950228
1309950228 1309950228
IT TOOK 0 SECONDS

Answer 1

The reason you are getting the same result for each $(shell date +%s) is that each is being executed and substituted at the same time – that is when make is running the command.

After make does its substitutions, this is the command the shell sees:

@START=1309950228 && \
echo ${START} &&\
sleep 10s && \
END=1309950228 && \
echo "${END} ${START}" &&\
DIFF_SUB=$(($END - $START)) && \
echo IT TOOK ${DIFF_SUB} SECONDS

If you want the date command to be run twice with a 10 second time difference between them, you will need the shell to perform the command substitution, not make:

@START=$$(date +%s) && \
echo $${START} &&\
sleep 10s && \
END=$$(date +%s) && \
echo "$${END} $${START}" &&\
DIFF_SUB=$$(($$END - $$START)) && \
echo IT TOOK $${DIFF_SUB} SECONDS

Note the double $$ and the removal of the shell make command.