get a default value when variable is unset in make

Question

(edit: question more accurate based on @Michael feedback)

In bash, I often use parameter expansion: the following commands print \"default value\" when

Answer 1

Just remove the colon. If you use :- in your substitution the default value will be used if the variable is null, an empty string or it does not exist, but just using - on its own will only substitute the default value if the variable has not been defined.

# var1=default
# var2=

# echo var2 is ${var2:-$var1}
var2 is something
# echo var3 is ${var3:-$var1}
var3 is something

# echo var2 is ${var2-$var1}
var2 is
# echo var3 is ${var3-$var1}
var3 is something

Answer 2

If you want to test if a variable has a non-empty value, you can use:

ifeq ($(VARNAME),)
        VARNAME="default value"
else
        do_something_else
endif

For checking if a variable has been defined or not, use ifdef.

Refer to Syntax of Conditionals in the manual for more.

Answer 3

I have a similar case where the result of filtering a shell command could be a single word or empty string. When empty, it should fallback to the default word. In the example below APPLE_LINUX will be 'apple' on macOS or 'linux' on other platforms. MSG will be set to the message for the appropriate platform. The example intentionality avoids using ifeq.

MACHINE         := $(shell $(COMPILE.cpp) -dumpmachine)
MACHINE_APPLE   := $(findstring apple,$(MACHINE))
APPLE_LINUX     := $(firstword $(MACHINE_APPLE) linux)
apple.MSG       := You are building on macOS
linux.MSG       := You are building on Linux or another OS
MSG             := $($(APPLE_LINUX).MSG)

Answer 4

If you want to use the expansion of a GNU make variable if it is non-empty and a default value if it is empty, but not set the variable, you can do something like this:

all:
        echo $(or $(VARNAME),default value)