DH(1) Debhelper DH(1)
NAME
dh - debhelper command sequencer
SYNOPSIS
dh sequence [--with addon[,addon ...]] [--list] [debhelperoptions]
DESCRIPTION
dh runs a sequence of debhelper commands. The supported sequences
correspond to the targets of a debian/rules file: build-arch, build-
indep, build, clean, install-indep, install-arch, install, binary-arch,
binary-indep, and binary.
OVERRIDE AND HOOK TARGETS
A debian/rules file using dh can override the command that is run at
any step in a sequence, by defining an override target. It is also
possible to inject a command before or after any step without affecting
the step itself.
Injecting commands before or after a step
Note: This feature requires debhelper 12.8 or later plus the package
must use compatibility mode 10 or later.
To inject commands before dh_command, add a target named
execute_before_dh_command to the rules files. Similarly, if you want
to inject commands after dh_command, add the target
execute_after_dh_command. Both targets can be used for the same
dh_command and also even if the command is overridden (as described in
"Overriding a command" below).
When these targets are defined, dh will call the targets respectively
before or after it would invoke dh_command (or its override target).
Overriding a command
To override dh_command, add a target named override_dh_command to the
rules file. When it would normally run dh_command, dh will instead call
that target. The override target can then run the command with
additional options, or run entirely different commands instead. See
examples below.
Architecture dependent/independent override and hook targets
The override and hook targets can also be defined to run only when
building architecture dependent or architecture independent packages.
Use targets with names like override_dh_command-arch and
execute_afterdh_command-indep.
This feature is available since debhelper 8.9.7 (for override targets)
and 12.8 (for hook targets).
Completely empty targets
As a special optimization, dh will skip a target if it is completely
empty. This is mostly useful for override targets, where the command
will simply be skipped without the overhead of invoking a dummy target.
Note that the target has to be completely empty for this to work:
# Skip dh_bar - the good and optimized way
# Some rationale for skipping dh_bar goes here
override_dh_bar:
# Skip dh_foo - the slow way
override_dh_foo:
# Some rationale for skipping dh_foo goes here
# (these comments causes a dummy target to be run)
Verifying targets are picked up by dh
If you want to confirm that dh has seen an override or a hook target,
you can use the following command as an example:
$ dh binary --no-act | grep dh_install | head -n5
dh_installdirs
dh_install
debian/rules execute_after_dh_install
dh_installdocs
dh_installchangelogs
The debian/rules execute_after_dh_install in the output, which signals
that dh registered a execute_after_dh_install target and would run it
directly after dh_install(1).
Note that "Completely empty targets" will be omitted in the listing
above. This makes it a bit harder to spot as you are looking for the
omission of a command name. But otherwise, the principle remains the
same.
Caveats with hook targets and makefile conditionals
If you choose to wrap a hook target in makefile conditionals, please be
aware that dh computes all the hook targets a head of time and caches
the result for that run. Furthermore, the conditionals will be invoked
again when dh calls the hook target later and will assume the answer
did not change.
The parsing and caching often happens before dh knows whether it will
build arch:any (-a) or/and arch:all (-i) packages, which can produce
confusing results - especially when dh_listpackages(1) is part of the
conditional.
Most of the problems can be avoided by making the hook target
unconditional and then have the "body" be partially or completely
conditional. As an example:
# SIMPLE: It is well-defined what happens. The hook target
# is always considered. The "maybe run this" bit is
# conditional but dh_foo is definitely skipped.
#
# Note: The conditional is evaluated "twice" where its
# influences what happens. Once when dh check which hook
# targets exist and once when the override_dh_foo hook target
# is run. If *either* times return false, "maybe run this"
# is skipped.
override_dh_foo:
ifneq (...)
maybe run this
endif
# SIMPLE: This is also well-defined. The hook target is always
# run and dh_bar is skipped. The "maybe run this" bit is
# conditional as one might expect.
#
# Note: The conditional is still evaluated multiple times (in
# different process each time). However, only the evaluation
# that happens when the hook target is run influences what
# happens.
override_dh_bar:
: # Dummy command to force the target to always be run
ifneq (...)
maybe run this
endif
# COMPLICATED: This case can be non-trivial and have sharp edges.
# Use at your own peril if dh_listpackages in the conditional.
#
# Here, either dh_baz is run normally OR "maybe run this" is run
# instead.
#
# And it gets even more complicated to reason about if dh needs to
# recurse into debian/rules because you have an "explicit"
# standard target (e.g. a "build-arch:" target separate from "%:").
ifneq (...)
override_dh_baz:
maybe run this
endif
These recipes are also relevant for conditional dependency targets,
which are often seen in a variant of the following example:
COND_TASKS =
ifneq (...)
COND_TASKS += maybe-run-this
endif
...
maybe-run-this:
...
# SIMPLE: It is well-defined what happens. Either the
# $(COND_TASKS) are skipped or run.
#
# Note: The conditional is evaluated "twice" where its
# influences what happens. Once when dh check which hook
# targets exist and once when the override_dh_foo hook target
# is run. If *either* times return false, $(COND_TASKS)
# is skipped.
override_dh_foo: $(COND_TASKS)
# SIMPLE: This is also well-defined. The hook target is always
# run and dh_bar is skipped. The $(COND_TASKS) bit is
# conditional as one might expect.
#
# Note: The conditional is still evaluated multiple times (in
# different process each time). However, only the evaluation
# that happens when the hook target is run influences what
# happens.
override_dh_bar: $(COND_TASKS)
: # Dummy command to force the target to always be run
# COMPLICATED: This case can be non-trivial and have sharp edges.
# Use at your own peril if dh_listpackages in the conditional.
#
ifneq (...)
override_dh_baz: $(COND_TASKS)
endif
When in doubt, pick the relevant SIMPLE case in the examples above that
match your need.
OPTIONS
--with addon[,addon ...]
Add the debhelper commands specified by the given addon to
appropriate places in the sequence of commands that is run. This
option can be repeated more than once, or multiple addons can be
listed, separated by commas. This is used when there is a third-
party package that provides debhelper commands. See the PROGRAMMING
file for documentation about the sequence addon interface.
A Build-Depends relation on the package dh-sequence-addon implies a
--with addon. This avoids the need for an explicit --with in
debian/rules that only duplicates what is already declared via the
build dependencies in debian/control. The relation can (since
12.5) be made optional via e.g. build-profiles. This enables you
to easily disable an addon that is only useful with certain
profiles (e.g. to facilitate bootstrapping).
Since debhelper 12.5, addons can also be activated in indep-only
mode (via Build-Depends-Indep) or arch-only mode (via Build-
Depends-Arch). Such addons are only active in the particular
sequence (e.g. binary-indep) which simplifies dependency management
for cross-builds.
Please note that addons activated via Build-Depends-Indep or Build-
Depends-Arch are subject to additional limitations to ensure the
result is deterministic even when the addon is unavailable (e.g.
during clean). This implies that some addons are incompatible with
these restrictions and can only be used via Build-Depends (or
manually via debian/rules). Currently, such addons can only add
commands to sequences.
--without addon
The inverse of --with, disables using the given addon. This option
can be repeated more than once, or multiple addons to disable can
be listed, separated by commas.
--list, -l
List all available addons.
When called only with this option, dh can be called from any
directory (i.e. it does not need access to files from a source
package).
--no-act
Prints commands that would run for a given sequence, but does not
run them.
Note that dh normally skips running commands that it knows will do
nothing. With --no-act, the full list of commands in a sequence is
printed.
Other options passed to dh are passed on to each command it runs. This
can be used to set an option like -v or -X or -N, as well as for more
specialised options.
EXAMPLES
To see what commands are included in a sequence, without actually doing
anything:
dh binary-arch --no-act
This is a very simple rules file, for packages where the default
sequences of commands work with no additional options.
#!/usr/bin/make -f
%:
dh $@
Often you'll want to pass an option to a specific debhelper command.
The easy way to do with is by adding an override target for that
command.
#!/usr/bin/make -f
%:
dh $@
override_dh_strip:
dh_strip -Xfoo
override_dh_auto_configure:
dh_auto_configure -- --with-foo --disable-bar
Sometimes the automated dh_auto_configure(1) and dh_auto_build(1) can't
guess what to do for a strange package. Here's how to avoid running
either and instead run your own commands.
#!/usr/bin/make -f
%:
dh $@
override_dh_auto_configure:
./mondoconfig
override_dh_auto_build:
make universe-explode-in-delight
Another common case is wanting to do something manually before or after
a particular debhelper command is run.
#!/usr/bin/make -f
%:
dh $@
# Example assumes debhelper/12.8 and compat 10+
execute_after_dh_fixperms:
chmod 4755 debian/foo/usr/bin/foo
If you are on an older debhelper or compatibility level, the above
example would have to be written as.
#!/usr/bin/make -f
%:
dh $@
# Older debhelper versions or using compat 9 or lower.
override_dh_fixperms:
dh_fixperms
chmod 4755 debian/foo/usr/bin/foo
Python tools are not run by dh by default, due to the continual change
in that area. Here is how to use dh_python2.
#!/usr/bin/make -f
%:
dh $@ --with python2
Here is how to force use of Perl's Module::Build build system, which
can be necessary if debhelper wrongly detects that the package uses
MakeMaker.
#!/usr/bin/make -f
%:
dh $@ --buildsystem=perl_build
Here is an example of overriding where the dh_auto_* commands find the
package's source, for a package where the source is located in a
subdirectory.
#!/usr/bin/make -f
%:
dh $@ --sourcedirectory=src
And here is an example of how to tell the dh_auto_* commands to build
in a subdirectory, which will be removed on clean.
#!/usr/bin/make -f
%:
dh $@ --builddirectory=build
If your package can be built in parallel, please either use compat 10
or pass --parallel to dh. Then dpkg-buildpackage -j will work.
#!/usr/bin/make -f
%:
dh $@ --parallel
If your package cannot be built reliably while using multiple threads,
please pass --no-parallel to dh (or the relevant dh_auto_* command):
#!/usr/bin/make -f
%:
dh $@ --no-parallel
Here is a way to prevent dh from running several commands that you
don't want it to run, by defining empty override targets for each
command.
#!/usr/bin/make -f
%:
dh $@
# Commands not to run:
override_dh_auto_test override_dh_compress override_dh_fixperms:
A long build process for a separate documentation package can be
separated out using architecture independent overrides. These will be
skipped when running build-arch and binary-arch sequences.
#!/usr/bin/make -f
%:
dh $@
override_dh_auto_build-indep:
$(MAKE) -C docs
# No tests needed for docs
override_dh_auto_test-indep:
override_dh_auto_install-indep:
$(MAKE) -C docs install
Adding to the example above, suppose you need to chmod a file, but only
when building the architecture dependent package, as it's not present
when building only documentation.
# Example assumes debhelper/12.8 and compat 10+
execute_after_dh_fixperms-arch:
chmod 4755 debian/foo/usr/bin/foo
INTERNALS
If you're curious about dh's internals, here's how it works under the
hood.
In compat 10 (or later), dh creates a stamp file
debian/debhelper-build-stamp after the build step(s) are complete to
avoid re-running them. It is possible to avoid the stamp file by
passing --without=build-stamp to dh. This makes "no clean" builds
behave more like what some people expect at the expense of possibly
running the build and test twice (the second time as root or under
fakeroot(1)).
Inside an override target, dh_* commands will create a log file
debian/package.debhelper.log to keep track of which packages the
command(s) have been run for. These log files are then removed once
the override target is complete.
In compat 9 or earlier, each debhelper command will record when it's
successfully run in debian/package.debhelper.log. (Which dh_clean
deletes.) So dh can tell which commands have already been run, for
which packages, and skip running those commands again.
Each time dh is run (in compat 9 or earlier), it examines the log, and
finds the last logged command that is in the specified sequence. It
then continues with the next command in the sequence.
A sequence can also run dependent targets in debian/rules. For
example, the "binary" sequence runs the "install" target.
dh uses the DH_INTERNAL_OPTIONS environment variable to pass
information through to debhelper commands that are run inside override
targets. The contents (and indeed, existence) of this environment
variable, as the name might suggest, is subject to change at any time.
Commands in the build-indep, install-indep and binary-indep sequences
are passed the -i option to ensure they only work on architecture
independent packages, and commands in the build-arch, install-arch and
binary-arch sequences are passed the -a option to ensure they only work
on architecture dependent packages.
SEE ALSO
debhelper(7)
This program is a part of debhelper.
AUTHOR
Joey Hess <joeyh@debian.org>
13.2 2020-07-05 DH(1)