View source for GNU Make

==Important [http://www.gnu.org/software/autoconf/manual/make/Special-Targets.html#Special-Targets Special Targets]==
===.DELETE_ON_ERROR===
* Generally, if make fails to build a target, and the target corresponds to a file, you want that file removed.
** Why might this happen? Ctrl-c being pressed during a build. Multiple statements in a build recipe. Memory allocation failure during build. Any number of things.
* By adding .DELETE_ON_ERROR to the Makefile, this becomes the default behavior.
** <tt>kill -9</tt>'ing <tt>make</tt> will prevent this from happening! :/
* [[gcc]] and some other tools will clean up on error, for some cases. Use <tt>.DELETE_ON_ERROR</tt> to be safe.

===.PHONY:===
* GNU Make manual section 4.6, "[http://www.gnu.org/software/autoconf/manual/make/Phony-Targets.html#Phony-Targets Phony Targets]"
* <tt>clean</tt>, <tt>all</tt>, <tt>test</tt> targets ought almost always be <tt>.PHONY</tt>

===.DEFAULT:===
* GNU Make manual section 10.6, "[http://www.gnu.org/software/autoconf/manual/make/Last-Resort.html Defining Last Resort Rules]"

==Make and [[subversion]]==
* After running <tt>make</tt> in a pristine tree, the output of <tt>svn status</tt> ought be empty. Use <tt>svn:ignore</tt> properties to ignore your generated files.
** Most importantly, <tt>svn status</tt> should not show any entries in the 'M'odified state. This indicates either generated files being checked into source control, or the build process modifying source in-place (the same thing, really)
** If <tt>svn:ignore</tt> is properly used, the following rule suffices as a project-independent <tt>clean</tt> target, assuming the presence of [[xmlstarlet]]:<pre>svn --xml --no-ignore status | xmlstarlet sel -t -m //entry -i "wc-status[@item='ignored']" -v @path -n | xargs rm -rf</pre>

==[[bash]] interactions==
* [http://www.gnu.org/s/hello/manual/make/Choosing-the-Shell.html The shell to use] can be specified via $(SHELL). By default, <tt>/bin/sh</tt> is used
** Unlike most variables, it cannot be inherited from the environment (except on Windows, where it is)
** It is not exported by default to sub-Makes, but [http://www.gnu.org/s/hello/manual/make/Variables_002fRecursion.html#Variables_002fRecursion it can be]
* Arguments to the shell can be specified via <tt>.SHELLFLAGS</tt>
** By default, it's <tt>-c</tt>, or <tt>-ec</tt> in POSIX mode
* Remember that errors in the body of a shell loop don't set the overall return value:
<pre>target:
        while true ; do false ; done</pre>
This recipe cannot be successfully completed! You can use<pre>target:
        set -e ; while true ; do false ; done</pre>or <pre>target:
        while true ; do false || exit ; done</pre>
The former is probably nicer, since you can then use ; in the place of &&, but it deviates from expected gmake semantics. In general, you don't want to be doing this kind of thing anyway; it's useful for cases like running unit testing on a set of inputs, where outputs won't be generated.