= Simple =
Add the following line to your Makefile

{{{
HGVERSION:= $(shell hg parents --template 'hgid: {node|short}')
}}}
and

{{{
-DHGVERSION="\"${HGVERSION}\""
}}}
to your CPPFLAGS. Now one can use the define HGVERSION inside the code.

== Rebuild files using HGVERSION ==
Make rebuilds targets only if one of the dependencies is newer. That means even if the version has changed, due to a change somewhere, files using HGVERSION will not be rebuilt automatically. To overcome this, add a new dependency in the Makefile to update these files:

{{{
version.o: hgstamp
.PHONY: update_hgstamp
hgstamp: update_hgstamp
        [ -f $@ ] || touch $@
        echo $(HGVERSION) | cmp -s $@ - || echo $(HGVERSION) > $@
}}}
You will see in this example `version.o` has the extra dependency on the file `hgstamp`.

How this works:
 1. The rule for the `hgstamp` target specifies the prerequisite `update_hgstamp`.
 1. The `update_hgstamp` target is marked as a ''phony target'' (see [[http://www.gnu.org/software/make/manual/html_node/Phony-Targets.html|make manual]]) which forces the recipe for `hgstamp` to be executed every time.
 1. '''''However''''' `make` only forces `version.o` to be remade when the modification time of the `hgstamp` ''file'' changes and ''this'' only happens when the `hgstamp` recipe ''actually'' updates the content of the `hgstamp` file--which only happens if and only if the version has ''really'' changed.

See https://bitbucket.org/follower/hgversion-handling for an example of a correctly functioning Makefile.

{{{#!wiki caution
It is ''essential'' that the `hgstamp` target is '''''not''''' marked as phony.

If `hgstamp` is marked as phony then the `hgstamp` file will be considered out of date every time `make` is run--which will cause `version.o` and everything that depends on `version.o` to be rebuilt ''every time''. (Revisions 14-17 of this page used `.PHONY` for the wrong target causing rebuilds every time.)
}}}

= Autotools =
If you are using automake add the following lines to your Makefile.am:

{{{
HGVERSION:= $(shell hg -R $(top_srcdir) parents --template 'hgid: {node|short}' 2> /dev/null || grep node $(top_srcdir)/.hg_archival.txt 2> /dev/null || true )
AM_CPPFLAGS = -DHGVERSION="\"${HGVERSION}\""

version.o: hgstamp
.PHONY: update_hgstamp
hgstamp: update_hgstamp
        [ -f $@ ] || touch $@
        echo $(HGVERSION) | cmp -s $@ - || echo $(HGVERSION) > $@
}}}
The definition of HGVERSION is more robust here. It tries to get the version in the following order

 * hg parents, the -R option is added to prevent hg from searching in lower directories in case we don't have a repository
 * grabbing it from .hg_archival.txt, in case we are using a tarball made by hg archive
 * undefined

= LaTeX =
Just add the following lines to your Makefile

{{{
HGID:=$(shell hg parents -R . --template "{node|short}" | sed 's/.*/\\renewcommand{\\hgid}{&}/')
.PHONY: update_hgid
hgid.tex: update_hgid
        [ -f $@ ] || touch $@
        echo '$(HGID)' | cmp -s $@ - || echo '$(HGID)' > $@
}}}
and this lines to your main tex file:

{{{
\newcommand{\hgid}{null}
\input{hgid}
}}}
Now one can use the command \hgid to get the version everywhere.

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CategoryHowTo