If-equals statement in makefile as a one-liner

Question

The following works as intended:

branch := $(shell git rev-parse --abbrev-ref HEAD)

ifeq ($(branch), master)
    ami_regions = us-west-2
endif

show_regions:
    echo $(ami_regions)

How could this ifeq statement be written as a one-liner, where the variable is set if the condition passes, otherwise sets the variable as another value or simply leaves it unset?

Answer 1

That should be possible. Try from chapter 8.4 one conditional in combination with string search from chapter 8.2 of the manual:

branch := $(shell git rev-parse --abbrev-ref HEAD)

ami_regions = $(if $(findstring master,$(branch)),us-west-2,)

show_regions:
    echo $(ami_regions)

The function $if will return the value from the then branch if $findstring returns anything. $findstring will return master if $(branch) contains the string master. The downside is that this will also match something like master-tests.

To make this match exactly master you will have to rely on bash: $(shell if [ master = $(branch) ]; then echo "true"; fi ). But then you also have to take care that $(branch) does not contain shell code with side effects (usually not).

Update

Renaud Pacalet and MadScientist in the comments have given improvements to the use of $findstring: Use $filter or $patsubst. As the string you are searching isn't a pattern, using simple $subst instead of $patsubst is also possible.

$(if $(patsubst master,,$(branch)),,us-west-2)

Notice that us-west-2 moved to the else branch, as $patsubst/subst replace the found string with the empty string leading to a false condition if the substitution is successful.

$(if $(filter master,$(branch)),us-west-2,)

$filter returns words from the input that match the supplied patter. Because the pattern is a single word, in this case it only returns exact matches.

So basically both alternatives give similar results. Differences arise if $branch contains spaces: Spaces between removed words will be returned by $patsubst, while $filter only returns matches. Git doesn't allow branches with spaces, this case should not occur.