Skip to content

Conditionals and Functions

Scope: just enough ifeq, wildcard, and $(shell ...) for real Makefiles — not the whole function catalog.

Conditionals

Sometimes a Makefile needs to vary by platform or by a flag. ifeq compares two values when the Makefile is read:

UNAME_S = $(shell uname -s)

ifeq ($(UNAME_S),Linux)
    PLATFORM = linux
else
    PLATFORM = other
endif

show:
    @echo "platform: $(PLATFORM)"

$ make show
platform: linux

Notes:

  • The ifeq line and its branches are not recipe lines — no tabs. They're Makefile syntax, evaluated while reading.
  • Indenting the branches (as above) is cosmetic but conventional.
  • Siblings: ifneq (not equal), ifdef (variable is defined), ifndef.

Conditionals are for decisions Make can make before running anything. For decisions during a recipe, use the shell's own if:

check:
    if [ -f hello ]; then echo "built"; fi

Functions

Make's built-in functions transform text. The handful worth knowing:

FILES = $(wildcard *.c)

$(wildcard PATTERN) expands to the matching files — empty if none match. This is how Makefiles collect sources without naming each one:

SRCS = $(wildcard *.c)
OBJS = $(SRCS:.c=.o)

The second line is a substitution reference: every .c becomes .o. ($(patsubst %.c,%.o,$(SRCS)) is the general form.)

show:
    @echo "c files here: $(if $(FILES),$(FILES),(none))"

$(if CONDITION,THEN,ELSE) picks a value — the real output from testing:

$ make show
platform: linux
c files here: a.c b.c

And $(shell ...) runs a shell command and captures its output, as the DATE and UNAME_S examples show.

When to stop

If your Makefile is growing nested conditionals and string surgery, that's a sign the logic belongs in a script the Makefile calls, not in Make itself. Make is a dependency tracker with a macro language, not a programming language.

Check: make -n on a conditional Makefile shows which branch's recipes would run — a quick way to confirm the condition evaluated the way you expected.