How to check return value from the shell directive

Question

In my Makefile, I need to test if the current directory is an SVN repo or not and if it is not I want to indicate an error using the $(error) directive in Makefile.

So I plan to use the return value of $(shell svn info .) but I'm not sure how to get this value from within the Makefile.

Note: I'm not trying to get the return value in a recipe, but rather in the middle of the Makefile.

Right now I'm doing something like this, which works just because stdout is blank when it is an error:

SVN_INFO := $(shell svn info . 2> /dev/null)
ifeq ($(SVN_INFO),)
    $(error "Not an SVN repo...")
endif

I'd still like to find out if it is possible to get the return value instead within the Makefile.

Solution 1

This worked fine for me - based on @eriktous' answer with a minor modification of redirecting stdout as well to skip the output from svn info on a valid svn repo.

SVN_INFO := $(shell svn info . 1>&2 2> /dev/null; echo $$?)
ifneq ($(SVN_INFO),0)
    $(error "Not an SVN repo...")
endif

Solution 2

How about using $? to echo the exit status of the last command?

SVN_INFO := $(shell svn info . 2> /dev/null; echo $$?)
ifeq ($(SVN_INFO),1)
    $(error "Not an SVN repo...")
endif

Solution 3

If you want to preserve the original output then you need to do some tricks. If you are lucky enough to have GNU Make 4.2 (released on 2016-05-22) or later at your disposal you can use the .SHELLSTATUS variable as follows.

var := $(shell echo "blabla" ; false)

ifneq ($(.SHELLSTATUS),0)
  $(error shell command failed! output was $(var))
endif

all:
    @echo Never reached but output would have been $(var)

Alternatively you could use a temporary file or play with Make's eval to store the string and/or the exit code into a Make variable. The example below gets this done but I would certainly like to see a better implementation than this embarrassingly complicated version.

ret := $(shell echo "blabla"; false; echo " $$?")
rc := $(lastword $(ret))
# Remove the last word by calculating <word count - 1> and
# using it as the second parameter of wordlist.
string:=$(wordlist 1,$(shell echo $$(($(words $(ret))-1))),$(ret))

ifneq ($(rc),0)
  $(error shell command failed with $(rc)! output was "$(string)")
endif

all:
    @echo Never reached but output would have been \"$(string)\"

Solution 4

Maybe something like this?

IS_SVN_CHECKED_OUT := $(shell svn info . 1>/dev/null 2>&1 && echo "yes" || echo "no")
ifne ($(IS_SVN_CHECKED_OUT),yes)
    $(error "The current directory must be checked out from SVN.")
endif

Solution 5

I use .NOTPARALLEL and a make function:

.NOTPARALLEL:   

# This function works almost exactly like the builtin shell command, except it
# stops everything with an error if the shell command given as its argument
# returns non-zero when executed.  The other difference is that the output
# is passed through the strip make function (the shell function strips only
# the last trailing newline).  In practice this doesn't matter much since
# the output is usually collapsed by the surroundeing make context to the
# same result produced by strip.
SHELL_CHECKED =                                                      \
  $(strip                                                            \
    $(if $(shell (($1) 1>/tmp/SC_so) || echo nonempty),              \
      $(error shell command '$1' failed.  Its stderr should be above \
              somewhere.  Its stdout is in '/tmp/SC_so'),            \
      $(shell cat /tmp/SC_so)))

View more solutions

Comments

[Tuxdude]

Thanks @eriktous - Cool trick - didn't think of that... I had to do some minor modifications for it to work - I've posted my answer based on your input :)

[Tuxdude]

Thanks Roland - I got the general idea for my final solution from both yours and eriktous's :)

[jeremfg]

Is there a way to do that, while keeping the output instead of directing it to '/dev/null'? What if I wanted to call $(error $(SVN_INFO)), but only when the return code was "1"?

[jww]

I think @eriktous probably should have received the credit for the answer.

[jww]

When I try to use $(.SHELLSTATUS) in GNUmake on Solaris it is empty. $(info "status: $(.SHELLSTATUS)") prints nothing.

[stefanct]

@jww: have you checked the version? 4.2 is relatively new (to stable distributions) and might not have propagated to wherever you got yours :) make -v I don't know other reasons than that for your problem.

[obskyr]

Can't wait for Make 4.2 to make it into production in popular distros!

[akhan]

@jeremfg You can skip the pipe to /dev/null and the code should still work as shown.

[listerreg]

shouldn't it be 2>/dev/null 1>&2

[Daniel Kamil Kozar]

@jeremfg : The trick I use is to have a construct like $(shell svn info . 2> /dev/null || echo fail and then an ifeq ($(SVN_INFO), fail) for handling the error. This guarantees that you get the standard output if the command executed successfully, and "fail" if it failed. It's not pretty, and it obviously won't work for commands that can legitimately return the string "fail", but it works for all my usages.

[jeremfg]

@DanielKamilKozar I don't follow how this help. Now you're not only hiding the return code, but the error message as well.

[Daniel Kamil Kozar]

@jeremfg : Like I said, it's far away from the perfect solution, but if you're stuck on an older version of GNU Make and can't use .SHELLSTATUS, I don't think there are better solutions except perhaps writing more elaborate scripts in the $(shell invocation to handle the error (for example, by writing the message to a file in /tmp, which isn't a lot prettier either). My example works for calling pkg-config well enough, which is pretty much the only thing I call from my makefiles via $(shell anyway.

[jeremfg]

@akhan Won't the variable SVN_INFO contain everything that was sent to the error stream if I remove the null redirection? How can I then do the following condition against "1" alone?

[jeremfg]

@DanielKamilKozar I agree. I was just trying to see how your proposal was better than this answer. I'd rather return the error code than a hard-coded string. It already provides more information, even if it's not everything.

[akhan]

@jeremfg The assignment will only capture stdout, which is the output of echo $$?. However, you should pipe stdout of svn info to /dev/null as done in the accepted answer. SVN_INFO := $(shell svn info . >/dev/null; echo $$?)

[ash]

@listerreg depends on what the user wants. 2>/dev/null 1>&2 is going to discard all output, while 1>&2 2>/dev/null is going to send the standard output to the original error output (usually the terminal) while discarding the standard error output.