File function in makefile takes args prefixed by '@' symbol

Question

This code excerpt is from Chapter 8.6 of GNU makefile manual.

What does @$@.in for file function arg in a makefile mean? and why are shell commands like rm prefixed by '@' symbol

program: $(OBJECTS)
     $(file >$@.in,$^)
     $(CMD) $(CMDFLAGS) @$@.in
     @rm $@.in

File function syntax is

$(file op filename[,text])

Answer 1

There are three unrelated uses of @ here.

In $@, the character @ is the name of an automatic variable that can be used in a rule. The value of that variable is the target that the rule is building.

When @ is used at the very beginning of a recipe (command) line, just after the tab character, it causes the command not to be printed when it's about to be executed.

The character @ elsewhere isn't special.

Thus, in your example, to build program:

1. The file function is invoked. It writes the dependencies of the target ($^ automatic variable) to the file program.in.

2. Whatever command is stored in the variable CMD is executed, with the parameters stored in the variable CMDFLAGS, plus the extra parameter @program.in. What this does depends on what CMD is.

3. The command rm program.in is executed, without printing it first.

A few commands treat a parameter starting with @ as indicating a file from which to read more parameters. This is a DOS convention which came about because DOS had a stringent limit on the command line length and no way to interpolate the output of a command into a command line. It is uncommon in the Unix world since Unix doesn't have these limitations. The effect of the recipe is thus likely the same as

$(CMD) $(CMDFLAGS) $(OBJECTS)

Answer 2

@ prefix in line suppress echoing of the line.

By default, make prints each line of Makefile before it is executed. When lines starts with @, those lines won't be printed.

Without @:

$ cat > Makefile
hello:                                                                          
    echo hello world
$ make hello
echo hello world
hello world

With @:

$ cat > Makefile
hello:                                                                          
    @echo hello world
$ make hello
hello world

A note that only @ at starting of line cause suppressing echoing. >$@.in or @$@.in are just normal strings, and $@ in it will be expanded:

$ cat hello
hello:
    @echo @$@.in
$make hello
@hello.in