I've got a Makefile that runs a tool that takes a little while; it allows me to replace the command used:

make TOOL=alternative-tool

I'd like to skip that step in the Makefile, so I'm looking for a command that exits with status 0, and that has negligible side-effects.

Obviously, I could just use true, but that's kinda confusing:

make TOOL=true

That reads as if I want to run the tool, but I don't.

Is there a default-installed executable that isn't /bin/true that exits with status 0, and that has a catchy and easy to type name?

  • Makefiles support comments, gnu.org/software/make/manual/html_node/Makefile-Contents.html . Put a comment before the line and document your code. That reduces confusion and explains why you used a certain approach. – Freiheit Apr 20 at 18:33
  • 1
    What line? The TOOL=true will be on the command line when invoking make, probably documented in a README nearby. The actual line that invokes the tool is somewhere within a 7000-line (third party) makefile. – Roger Lipscombe Apr 20 at 18:45
up vote 42 down vote accepted

Even though you've asked for something that is "not /bin/true", replacing true with the full name /bin/true to show that it is the true program rather than some other meaning of "true" is probably the best solution.

Your concern is that the true in

make TOOL=true

appears to be something other than a command name. But if you write

make TOOL=/bin/true

then that is unambiguously a command. Somebody might misread TOOL=true to mean that some other tool somewhere is intended, but no such misreading of TOOL=/bin/true is likely.


I am unsure as to when :, which is a shell builtin but not an external command, will work. Henning Makholm has reported that it appears to work. But I think it does not work in all situations, and you found that it did not work for you.

As for a shell alias, you cannot use that because alias expansion is not performed in the arguments you pass to a command, nor do makefiles make any use of previously defined shell aliases. Even if make runs your commands in a new shell, that shell will not have the alias (and would not use it even if it did have it, because it would be a noninteractive shell, where alias expansion is not automatically enabled).

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    Using : seems to work fine for me. make always spawns a shell to run each command after it's done its own substitutions. – Henning Makholm Apr 19 at 12:49
  • @HenningMakholm Thanks, I've edited. You may want to post an answer. Lok Lam Cheng had commented on the question to suggest :, but it should be an answer (especially if correct) and that comment doesn't explain why one should expect it to work. In a directory with hello.c and no makefile, where make hello compiles hello.c to hello, I found make CC=: hello behaved according to what you are saying. – Eliah Kagan Apr 19 at 13:04
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    Unfortunately, for this particular Makefile (it uses erlang.mk), using : doesn't work. Being explicit about /bin/true is probably the best way to do it. – Roger Lipscombe Apr 19 at 14:22

While I agree that using the full path to true would be the best solution, I'd like to note what's probably by far the most common way to avoid real command execution: sticking an echo in front of it. So:

make TOOL=echo
  • I think this is sometimes useful when it's important one be reminded that one suppressed whatever action TOOL would otherwise do. Without -s or equivalent, make emits redundant information, because both make and the echo command run by make separately emit output. For example, when I run make CC=echo hello in a directory that has hello.c and no makefile, I get echo hello.c -o hello as the first line of output and hello.c -o hello as the second. But some builds use (or the user may add) -s, and then it's helpful. make -s CC=echo hello gives just hello.c -o hello. – Eliah Kagan Apr 19 at 15:23
  • (which might still be useful if the commands involve shell expansions, so you can get a clearer idea of what's being run, if interested) – muru Apr 19 at 15:57

You can always make your own command that does nothing but return a zero exit status. Some options:

  • an appropriately named symbolic link to /bin/true

  • an empty shell script

  • a nearly empty C program:

    int main() {
        return 0;
    }
    
  • A good name for such a program would be noop or no-op. – arp Apr 20 at 22:15

Create a symbolic link in the working directory to /bin/true named something like skip. Thus...

skip -> /bin/true

Then the make line can be ...

make TOOL=./skip

Here's a way to do it with slightly more characters:

function T() { /bin/true; }

After you've done it once you can re-execute it just by using the name T.

false; T; echo $?
  • 3
    Could you please explain how this function can be invoked from a Make variable value (to be of use in the situation in question)? – David Foerster Apr 20 at 22:17

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