makefile中的for循环中出现意外的Opearator

Question

for dir in " b/. c/. obj/. src/. "; do \
        if [ $dir != "src/." ] && [ $dir != "obj/." ]; then \
            make -C $dir; \
        fi; \
    done 

此处出现的错误是/bin/sh: 2: [: b/.: unexpected operator我为什么会收到此错误。 B /。是makefile所在目录的子目录。