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In a makefile I need to check, whether a program rocq exists in the system. If it exists assign one value to the variable, otherwise assign another value.

I created the following code:

OUTPUT=$(shell which rocq 2>/dev/null)

ifneq ($(OUTPUT), "")
    COQC=rocq c
else
    COQC=coqc
endif

COQC := $(COQC)" -R . Mendelson"
all:
    @echo [$(COQC)]
    @echo [$(OUTPUT)]

I get the following output:

[rocq c -R . Mendelson]
[]

Surprisingly, $(OUTPUT) is empty, but the yes-branch was executed. COQC was assigned to "rocq c".

What am I doing wrong?

Improve this question
4
  • 2
    "" is not the same as empty; so ifneq ($(OUTPUT), ) will yield the correct result; or perhaps less cryptic :OUTPUT="$(shell which ...)". Commented May 28 at 19:04
  • In case that was not clear enough: make does not recognize quote-delimited strings. It interprets the "" in your ifneq statement as two literal characters. Remove them, leaving literally nothing to represent nothing. Commented May 29 at 0:12
  • Use ifneq ($(VAR),) or ifneq "$(VAR)" "" forms. Do not combine them. Commented Jun 5 at 19:16
  • ifdef OUTPUT can be used to do something when the variable is non-empty. Commented Jul 1 at 15:20
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