variable name expansion not working inside target in GNU Makefile

问题

I am trying to create a directory inside target based on a rule but for some reason the variable is not getting expanded i.e., $(OUT_DIR) is blank and mkdir -p does not work. Here is the code

target_%:  /home/user/%/.file : file.c
    export OUT_DIR = /home/user/$* ; \
    mkdir -p $(OUT_DIR)     ;\
    .
    .
    .

After making the changes suggested by @Beta, here is how the code looks like

target_%:  /home/user/%/.file : file.c
    export OUT_DIR=/home/user/$* ; \
    mkdir -p $${OUT_DIR}     ;\
    cd $${OUT_DIR}           ; \
    ln -s /home/user/file1.c  file1.c.link ;\  

When I run the code, i get the Error

/bin/sh: line 3:        : command not found

on mkdir -p command. I have played by removing ;\ and it works till mkdir $${OUT_DIR} and cd $${OUT_DIR} but i see the soft link created in the directory where the makefile is rather than in the OUT_DIR where it should be created.

I am also not sure when to use \ vs ; \ vs not using at all. Any suggestion on that would be awesome

回答1:

Make variables and shell variables are not the same thing.

If you have something like $(OUT_DIR) in the makefile, Make will expand it, and since you haven't defined it in the makefile, it expands to nothing. Your rule then looks like:

target_%:  /home/user/%/.file : file.c
    export OUT_DIR = /home/user/$* ; \
    mkdir -p      ;\
    ...

If the command (export ...) you define OUT_DIR as a shell variable. The way to expand a shell variable in the shell is with '$', like this:

...$OUT_DIR ...

But '$' is a special character in Make, so to use it as you intend, you must escape is with another '$':

target_%:  /home/user/%/.file : file.c
    export OUT_DIR = /home/user/$* ; \
    mkdir -p $$OUT_DIR     ;\
    ...

来源：https://stackoverflow.com/questions/49411762/variable-name-expansion-not-working-inside-target-in-gnu-makefile