The GNU Makefile conditional control structures

In common programming language, using control structures such as if ... else if ... else ... is a very unusual thing, then how to use GNU Makefile in it?

• ifeq
• ifneq

For example: foo.sh

 1 #!/bin/bash
 2 
 3 ARCH=$(uname -p)
 4 if [[ $ARCH == "x86_64" ]]; then
 5     ARCH32="i686"
 6     ARCH64="x86_64"
 7 elif [[ $ARCH == "ppc64le" ]]; then
 8     ARCH32=""
 9     ARCH64="ppc64le"
10 else
11     ARCH32=$ARCH
12     ARCH64=""
13 fi
14 
15 if [[ -n $ARCH32 ]]; then
16     OUTPUT+=" ARCH32=$ARCH32"
17 fi
18 
19 if [[ -n $ARCH64 ]]; then
20     OUTPUT+=" ARCH64=$ARCH64"
21 fi
22 
23 echo $OUTPUT

The above foo.sh is achieved with a Makefile:

 1 ARCH = $(shell uname -p)
 2 
 3 ifeq ($(ARCH), x86_64)
 4        ARCH32 = i686
 5        ARCH64 = x86_64
 6 else ifeq ($(ARCH), ppc64le)
 7        ARCH32 =
 8        ARCH64 = ppc64le
 9 else
10        ARCH32 = $(ARCH)
11        ARCH64 =
12 endif
13 
14 ifneq ($(ARCH32), )
15     OUTPUT += ARCH32=$(ARCH32)
16 endif
17 
18 ifneq ($(ARCH64), )
19     OUTPUT += ARCH64=$(ARCH64)
20 endif
21 
22 all: foo
23 foo:
24     @echo $(OUTPUT)

Foo.sh Makefile and run results are as follows:

$ uname -p
x86_64
$ bash foo.sh
ARCH32=i686 ARCH64=x86_64
$ make -f Makefile
ARCH32=i686 ARCH64=x86_64

Thus,

• ifeq ($ (VAR),)  is equivalent to the bash [[-z $ VAR]]
• ifneq ($ (on VAR),) is equivalent to the bash [[-n $ VAR]]