Question

How can I use shell parameter extension in a Makefile?
I need to get the path to the leveldb database.

What I tried so far is in my Makefile:

$(shell db=$(locate leveldb/db.h); export LEVELDB_PATH=${db%%/include/leveldb/db.h})
LEVELDB_LIBS=-L$(LEVELDB_PATH) -I$(LEVELDB_PATH)/include -lleveldb

But LEVELDB_PATH is empty.

Thanks for help.

Answer 1

The shell cannot export stuff into your Makefile. Try something like this instead.

db := $(shell locate leveldb/db.h)
LEVELDB_PATH:=$(patsubst %/include/leveldb/db.h,%,$(db))

… or, to spare the temp variable,

LEVELDB_PATH:=$(shell locate leveldb/db.h | sed 's%/include/leveldb/db.h$$%%')

Edit: Fix to use $(patsubst) instead of $(subst), and double the dollar sign in the sed script.

Generally speaking, whatever goes on inside $(shell ...) happens in a subprocess, which (as ever) cannot modify its parent. make sees the output of the function when it finishes, but not what happened during the execution of the shell commands.