GNU make executes weird shell

Jakub Klinkovský

11/13/22, 8:35 PM

The shell function in GNU make behaves weird on Ubuntu. Consider this simple Makefile:

TEST1 := $(shell command -v sh)
TEST2 := $(shell /bin/sh -c 'command -v sh')
all:
        $(info TEST1 = $(TEST1))
        $(info TEST2 = $(TEST2))

Running make prints

make: command: Command not found
TEST1 = 
TEST2 = /usr/bin/sh

Weird. Why does the default shell not know the command built-in? And what is the default shell invoked by GNU make actually? According to the GNU make manual:

The program used as the shell is taken from the variable SHELL. If this variable is not set in your makefile, the program /bin/sh is used as the shell.

There is no SHELL variable in the Makefile above, so the shell should be /bin/sh. However, running /bin/sh explicitly from the default shell actually behaves as expected, so there must be some difference.

So what is the real problem? It seems to be specific to GNU make packaged in Ubuntu, since the aforementioned Makefile behaves as expected on other systems like Arch Linux (which symlinks /bin/sh to bash rather than dash, though), i.e. it prints TEST1 = /usr/bin/sh.

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makefile

make

steeldriver

11/13/22, 9:41 PM

Hmm... interesting. Based on `strace` output it seems not to use a shell at all (instead passing the command + args to `execve` directly), either when make variable `SHELL` is undefined **or** set to `/bin/sh`. It seems to be possible to trick it into using `sh` on systems where /bin and /usr/bin are symlinked by setting `SHELL=/usr/bin/sh`.

Jakub Klinkovský

11/14/22, 8:00 AM

Arch Linux also symlinks `/bin` to `/usr/bin` and it works just fine, with and without `SHELL := /bin/sh`. So what's the difference between using `SHELL := /bin/sh` and `SHELL := /usr/bin/sh` on Ubuntu?

Mei

8/23/23, 4:33 AM

In my Ubuntu 20.04.4 LTS system, `/bin/sh` is symlinked to *bash* and `/usr/bin/sh` does not exist. At one time, I think `/bin/sh` may have been symlinked to *dash* and `/usr/bin/sh` may have been *bash*. I'd have to confirm that.

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