Before, we'd always use `cc = null`, and check for that. The problem is
this breaks for cross compilation to platforms that don't support a C
compiler.
It's a very subtle issue. One might think there is no problem because we
have `stdenvNoCC`, and presumably one would only build derivations that
use that. The problem is that one still wants to use tools at build-time
that are themselves built with a C compiler, and those are gotten via
"splicing". The runtime version of those deps will explode, but the
build time / `buildPackages` versions of those deps will be fine, and
splicing attempts to work this by using `builtins.tryEval` to filter out
any broken "higher priority" packages (runtime is the default and
highest priority) so that both `foo` and `foo.nativeDrv` works.
However, `tryEval` only catches certain evaluation failures (e.g.
exceptions), and not arbitrary failures (such as `cc.attr` when `cc` is
null). This means `tryEval` fails to let us use our build time deps, and
everything comes apart.
The right solution is, as usually, to get rid of splicing. Or, baring
that, to make it so `foo` never works and one has to explicitly do
`foo.*`. But that is a much larger change, and certaily one unsuitable
to be backported to stable.
Given that, we instead make an exception-throwing `cc` attribute, and
create a `hasCC` attribute for those derivations which wish to
condtionally use a C compiler: instead of doing `stdenv.cc or null ==
null` or something similar, one does `stdenv.hasCC`. This allows quering
without "tripping" the exception, while also allowing `tryEval` to work.
No platform without a C compiler is yet wired up by default. That will
be done in a following commit.
Rewrite the `stripHash` helper function with 2 differences:
* Paths starting with `--` will no longer produce an error.
* Use Bash string manipulation instead of shelling out to `grep` and
`cut`. This should be faster.
A bunch of stdenv-internal variables were deleted in
1601a7fcce, but these are needed in the
fixup phase, whereas the rest are just needed for the initial work
(findInputs, etc) before the user phases.
CC @matthewbauer
There were two issues:
* builtins.getEnv was called deep into the nixpkgs tree making it hard
to discover. This is solved by moving the call into
pkgs/top-level/impure.nix
* when the config was explicitly set by the user to false, it would
still try and load the environment variable. This meant that it was
not possible to guarantee the same outcome on two different systems.
Apparently this option trades compression time for size,
and explicitly does so without increasing resources needed in decomp.
Doesn't make tarball creation unbearable, so add it to options!
Before, we very carefully unapplied and reapplied `set -u` so the rest
of Nixpkgs could continue to not fail on undefined variables. Let's rip
off the band-aid.
Go beyond the obvious setup hooks now, with a bit of sed, with a skipped case:
- cc-wrapper's `dontlink`, because it already is handled.
Also, in nix files escaping was manually added.
EMP
A package's meta.license can either be a single license or a list. The
code to check config.whitelistedLicenses and config.blackListedLicenses
wasn't handling this, nor was the showLicense function.
This logic should be in the pkgs/top-level/static.nix. We don’t want
to pollute Nixpkgs with =if stdenv.static=. Also, "static" is not
descriptive. We have two types of static stdenvs, ‘makeStaticLibaries’
and ‘makeStaticBinaries’. We shouldn’t rely on a static boolean like
this.
These can be used to determine whether a ELF file with ELF header is an
executable or shared library.
We can't implement it in pure bash, as bash has problems with null
bytes.
That's very much consistent with the spirit of nix-shell --pure
BTW, nix 1.x shells will be always treated as pure;
in that version detection isn't possible.
https://github.com/NixOS/nix/commit/1bffd83e1a9c
Some SSL libs don't react to $SSL_CERT_FILE.
That actually makes sense to me, as we add this behavior
as nixpkgs-specific, so it seems "safer" to use $NIX_*.
We want initialPath to have lowest precedence.
In addition, unset _PATH and _HOST_PATH as they shouldn’t be needed
after final PATH and HOST_PATH are set.
Adds pkgsCross.wasm32 and pkgsCross.wasm64. Use it to build Nixpkgs
with a WebAssembly toolchain.
stdenv/cross: use static overlay on isWasm
isWasm doesn’t make sense dynamically linked.
This puts patches in all derivations even if it unspecified by the
derivation. By default it will be an empty list. This simplifies
overrides, as we can now assume that patches is a valid name so that
this works:
self: super: {
mypkg = super.pkg.overrideAttrs (o: {
patches = o.patches ++ [ ./my-very-own.patch ];
});
}
That is, you don’t need to provide a default "or []", make-derivation
provides one for you.
Unfortunately, this is a mass rebuild.
this adds libc++ to the LLVM cross, giving us access to the full
Nixpkgs set. This requires 4 stages of wrapped compilers:
- Clang with no libraries
- Clang with just compiler-rt
- Clang with Libc, and compiler-rt
- Clang with Libc++, Libc, and compiler-rt
New android ndk (18) now uses clang. We were going through the wrapper
that are provided. This lead to surprising errors when building.
Ideally we could use the llvm linker as well, but this leads to errors
as many packages don’t support the llvm linker.
This is needed to avoid confusing and repeated boilerplate for
`fooForTarget`. The vast majority of use-cases can still use
`buildPackages or `targetPackages`, which are now defined in terms of
these.
You can build (partially) with LLVM toolchain using the useLLVM flag.
This works like so:
nix-build -A hello --arg crossSystem '{ system =
"aarch64-unknown-linux-musl"; useLLVM = true }'
also don’t separate debug info in lldClang
It doesn’t work currently with that setup hook. Missing build-id?
Comments on conflicts:
- llvm: d6f401e1 vs. 469ecc70 - docs for 6 and 7 say the default is
to build all targets, so we should be fine
- some pypi hashes: they were equivalent, just base16 vs. base32
For a long time now, tracing has been broken in Nixpkgs. So when you
have an eval error you would get something like this:
error: while evaluating the attribute 'buildInputs' of the derivation 'hello-2.10' at /home/mbauer/nixpkgs/pkgs/stdenv/generic/make-derivation.nix:185:11:
while evaluating 'chooseDevOutputs' at /home/mbauer/nixpkgs/lib/attrsets.nix:474:22, called from undefined position:
while evaluating 'optionals' at /home/mbauer/nixpkgs/lib/lists.nix:257:5, called from /home/mbauer/nixpkgs/pkgs/stdenv/generic/make-derivation.nix:132:17:
This is coming from how Nix handles string context and how
make-derivation messes with the "name" attribute. This commit should
restore the old behavior so you get a nice line number like:
error: while evaluating the attribute 'buildInputs' of the derivation 'hello-2.10' at /home/mbauer/nixpkgs/pkgs/applications/misc/hello/default.nix:4:3:
while evaluating 'chooseDevOutputs' at /home/mbauer/nixpkgs/lib/attrsets.nix:474:22, called from undefined position:
while evaluating 'optionals' at /home/mbauer/nixpkgs/lib/lists.nix:257:5, called from /home/mbauer/nixpkgs/pkgs/stdenv/generic/make-derivation.nix:132:17:
NOTE: This will still be broken for cross compilation due to the
prefixes we are adding to name.
This behavior ended up breaking the handleEvalIssue functionality by hiding those packages. So something like this:
$ nix-env -iA nixpkgs.zoom-us
would silently fail, without telling the user how to fix it! Regardless, this "bug" should be handled in Nix - not Nixpkgs.
Fixes#38952.
We can't run the checkPhase when build != host, so we may as well make
the checkInputs native.
This signicantly improves the situation of Python packages when enabling
strictDeps.
* add generic x86_32 support
- Add support for i386-i586.
- Add `isx86_32` predicate that can replace most uses of `isi686`.
- `isi686` is reinterpreted to mean "exactly i686 arch, and not say i585 or i386".
- This branch was used to build working i586 kernel running on i586 hardware.
* revert `isi[345]86`, remove dead code
- Remove changes to dead code in `doubles.nix` and `for-meta.nix`.
- Remove `isi[345]86` predicates since other cpu families don't have specific model predicates.
* remove i386-linux since linux not supported on that cpu
Hydra's page showing evaluation errors is about a mile long, showing
buckets of user-friendly errors, like this:
in job ‘seyren.aarch64-linux’:
Package ‘oraclejre-8u191’ in /nix/store/fa9zzkbljkvdavwzirkrr5irg25ymbjl-source/pkgs/development/compilers/oraclejdk/jdk-linux-base.nix:71 has an unfree license (‘unfree’), refusing to evaluate.
a) For `nixos-rebuild` you can set
{ nixpkgs.config.allowUnfree = true; }
in configuration.nix to override this.
b) For `nix-env`, `nix-build`, `nix-shell` or any other Nix command you can add
{ allowUnfree = true; }
to ~/.config/nixpkgs/config.nix.
in job ‘jetbrains.webstorm.x86_64-linux’:
Package ‘webstorm-2018.3.1’ in /nix/store/fa9zzkbljkvdavwzirkrr5irg25ymbjl-source/pkgs/applications/editors/jetbrains/default.nix:230 has an unfree license (‘unfree’), refusing to evaluate.
a) For `nixos-rebuild` you can set
{ nixpkgs.config.allowUnfree = true; }
in configuration.nix to override this.
b) For `nix-env`, `nix-build`, `nix-shell` or any other Nix command you can add
{ allowUnfree = true; }
to ~/.config/nixpkgs/config.nix.
This makes it extremely hard to find actual issues in the output. This
patch set makes the output much more condensed in Hydra:
Failed to evaluate nifticlib-2.0.0: «unsupported»: is not supported on ‘x86_64-apple-darwin’
Failed to evaluate dmd-2.081.2: «unsupported»: is not supported on ‘aarch64-unknown-linux-gnu’
Failed to evaluate dmdBuild-2.081.2: «unsupported»: is not supported on ‘aarch64-unknown-linux-gnu’
Failed to evaluate ldc-1.11.0: «unsupported»: is not supported on ‘aarch64-unknown-linux-gnu’
Failed to evaluate ldcBuild-1.11.0: «unsupported»: is not supported on ‘aarch64-unknown-linux-gnu’
Failed to evaluate ldc-0.17.5: «unsupported»: is not supported on ‘aarch64-unknown-linux-gnu’
Failed to evaluate ldcBuild-0.17.5: «unsupported»: is not supported on ‘aarch64-unknown-linux-gnu’
Adds the static overlay that can be used to build Nixpkgs statically.
Can be used like:
nix build pkgsStatic.hello
Not all packages build, as some rely on dynamic linking.
crossOverlays only apply to the packages being built, not the build
packages. It is useful when you don’t care what is used to build your
packages, just what is being built. The idea relies heavily on the
cross compiling infrastructure. Using this implies that we need to
create a cross stdenv.
Some trivial builders use the name attr to choose the exec name
produced. For example nixos-install,
nixos-install = makeProg {
name = "nixos-install";
src = ./nixos-install.sh;
nix = config.nix.package.out;
path = makeBinPath [ nixos-enter ];
};
When cross compiling, this puts the prog in,
/bin/nixos-install-powerpc64le-unknown-linux-gnu
When separateDebugInfo = true & !stdenv.hostPlatform.isLinux, we
always gave an error. There is no reason to do this. Instead, just
don’t add separate debug info when we aren’t on Linux.
propagateNativeBuildInputs will end up going in the output derivation.
This case is allowed to end up in references because of that. Sorry
for the disruption!
Fixes#50865
Completely breaks darwin. Every package in the stdenv that has shebangs
in the output will end up with references to bootstrap-tools.
This reverts commit eb7c50a993.
Completely breaks darwin. Every package in the stdenv that has shebangs
in the output will end up with references to bootstrap-tools.
This reverts commit 2f2e635dd5.
This will make the list much easier to re-use, eg. for `nixosTests`
The drawback is that this approaches makes the
```
nix-build release.nix -A tests.opensmtpd.x86_64-linux
```
command about twice as slow (3s to 6s): it now has to evaluate `nixpkgs`
once for each architecture, instead of just having the hardcoded list of
tests that allowed to say “ok just evaluate for x86_64-linux”.
On the other hand, complete evaluation of `release.nix` should be much
faster because we no longer import `nixpkgs` for each test: testing with
the following command went from 30s to 18s, and that's just for a few
tests.
```
time nix-instantiate --eval --strict nixos/release.nix -A tests.nat
```
I initially wanted to test on the whole `release.nix`, but there are too
many broken tests and it takes too long to eval them all, especially
compared to the fact that the current implementation breaks some setup.
Given developers can just `nix-build nixos/tests/my-test.nix`, it sounds
like an overall win.
Fixes#49071
On ld.gold, we produce broken executables when linking with the Musl
libc. This appears to be a known bug when using ld.gold and Musl. This
thread describes the workaround as enabling PIE when using ld.gold and
Musl:
https://www.openwall.com/lists/musl/2015/05/01/5
By default we don’t enable PIE to avoid breaking things. But in the
Musl case we are breaking things by not enabling PIE. So this adds a
special case for defaultHardeningFlags which keeps the pie hardening
for everything. Any packages that break with PIE can add the pie flag
to disableHardeningFlags array (a no-op for now on anything but Musl).
When strictDeps = true, we don’t want native build inputs to end up in
the output. For instance gcc is a builtin native build input and
should only show up in an output if it is also listed in buildInputs.
/cc @ericson2314
In strictDeps=false, autoPatchshebangs should use
--build (corresponding to PATH) to lookup commands. This restores the
previous behavior of patchshebangs so that we don’t break stuff that
isn’t careful in the buildInputs vs. nativeBuildInputs distinction.
Unfortunately this won’t work under cross compilation.
Rationale
---------
Currently, tests are hard to discover. For instance, someone updating
`dovecot` might not notice that the interaction of `dovecot` with
`opensmtpd` is handled in the `opensmtpd.nix` test.
And even for someone updating `opensmtpd`, it requires manual work to go
check in `nixos/tests` whether there is actually a test, especially
given not so many packages in `nixpkgs` have tests and this is thus most
of the time useless.
Finally, for the reviewer, it is much easier to check that the “Tested
via one or more NixOS test(s)” has been checked if the file modified
already includes the list of relevant tests.
Implementation
--------------
Currently, this commit only adds the metadata in the package. Each
element of the `meta.tests` attribute is a derivation that, when it
builds successfully, means the test has passed (ie. following the same
convention as NixOS tests).
Future Work
-----------
In the future, the tools could be made aware of this `meta.tests`
attribute, and for instance a `--with-tests` could be added to
`nix-build` so that it also builds all the tests. Or a `--without-tests`
to build without all the tests. @Profpatsch described in his NixCon talk
such systems.
Another thing that would help in the future would be the possibility to
reasonably easily have cross-derivation nix tests without the whole
NixOS VM stack. @7c6f434c already proposed such a system.
This RFC currently handles none of these concerns. Only the addition of
`meta.tests` as metadata to be used by maintainers to remember to run
relevant tests.
Uses uname data to find what to set these variables:
- CMAKE_SYSTEM_NAME
- CMAKE_SYSTEM_PROCESSOR
- CMAKE_SYSTEM_VERSION
- CMAKE_HOST_SYSTEM_NAME
- CMAKE_HOST_SYSTEM_PROCESSOR
- CMAKE_HOST_SYSTEM_VERSION
/cc @Ericson2314
PR was https://github.com/NixOS/nixpkgs/pull/46857
This line broke MacOS cross compilation. paxctl cannot be built on
macOS. Maybe it can be fixed, but no reason to break things
unnecessarily.
Regardless, you definitely need to be more careful about backporting.
I think it’s fine to move fast and break things on master but
with release-18.09 we should be more careful. Something like more
automated testing for cross compilation would also be
helpful (hopefully even making it block).
(cherry picked from commit f9c4075873cb56464126f993d22a1a72f7cfac45)
The isELF function only checks whether ELF is contained within the first
4 bytes of the file, which is a bit fuzzy and will also return
successful if it's a text file starting with ELF, for example:
ELF headers
-----------
Some text here about ELF headers...
So instead, we're now doing a precise match on \x7fELF.
Signed-off-by: aszlig <aszlig@nix.build>
Acked-by: @Ericson2314
Closes: https://github.com/NixOS/nixpkgs/pull/47244
Since gcc.lib/lib64 is a symlink to 'lib', the use of
"lib*/libgcc_s.so*" triggered a warning (error) with
the latest coreutils. Essentially we were doing:
$ cp a/x b/x y/
And latest coreutils rejects such invocations.
Just copy from 'lib', lib64 is a link to it anyway.
* Nothing else in this file bothers looking at lib*
* AFAICT lib* only ever possibly matched lib64 anyway
02c09e0171 (NixOS/nixpkgs#44558) was reverted in
c981787db9 but, as it turns out, it fixed an issue
I didn't know about at the time: the values of `propagateDoc` options were
(and now again are) inconsistent with the underlying things those wrappers wrap
(see NixOS/nixpkgs#46119), which was (and now is) likely to produce more instances
of NixOS/nixpkgs#43547, if not now, then eventually as stdenv changes.
This patch (which is a simplified version of the original reverted patch) is the
simplest solution to this whole thing: it forces wrappers to directly inspect the
outputs of the things they are wrapping instead of making stdenv guess the correct
values.
The `unfree` and `unfreeRedistributable` licenses both have `free = false`,
which will trigger the first portion of logic. This removes dead code to
simplify the logic.
As a follow-up, I plan to add an attribute `redistributable = [true|false]`,
which can be used by Hydra to determine whether a given package with a given
license can be included in the channel.
This accidentally added some unwanted dependencies on the bootstrap
tools, and I don't have time to fix before I go on vacation, so I'm
backing it out until I have time to address it properly.
This reverts commit dc5c68a7bb.
If meta.outputsToInstall is set to include absent outputs, various
tools break including channel updates and nix-env.
grahamc@Morbo> nix-env -i -f . -A elf-header-real
installing 'elf-header'
error: this derivation has bad 'meta.outputsToInstall'
This patch verifies each value in meta.outputsToInstall is a valid
output. It validates this condition only if checkMeta is true.
grahamc@Morbo> nix-build . -A elf-header-real
error: Package ‘elf-header’ in /home/grahamc/projects/nixpkgs/pkgs/development/libraries/elf-header/default.nix:36 has invalid meta.outputsToInstall, refusing to evaluate.
The package elf-header has set meta.outputsToInstall to: bin
however elf-header only has the outputs: out
and is missing the following ouputs:
- bin
(use '--show-trace' to show detailed location information)
Note, now the nix-env experience is decidedly worse for users who have
checkMeta set to true:
grahamc@Morbo> nix-env -i -f . -A elf-header-real; echo $?
0
though since this is already an issue for unfree, broken, unsupported,
and insecure validity problems I'm not sure we should do something
different here.
a4630c65ca was incorrect in assuming $SHELL would be a path to the
bash derivation. In fact $SHELL will be a path to the bash executable.
Unfortunately this did not fix the original issue. So instead, we just
have to reuse initialPath can be added like PATH is.
Sorry for the inconvenience! I hadn’t thought through the effects of
the last commit.
/cc @copumpkin @ericson2314
To avoid breaking things, we need to make sure SHELL goes into
HOST_PATH. This reflects my changes to patch-shebangs to make it cross
compilation ready. When a script is patched from the Nix store it now
looks to HOST_PATH to get the targeted machine’s executables.
Unfortunately, this only works in native builds.
LTO is disabled during bootstrap to keep the bootstrap tools small and
avoid unnecessary LLVM rebuilds, but is enabled in the final stdenv
stage and should be usable by normal packages.
This also updates the bootstrap tool builder to LLVM 5, but not the ones
we actually use for bootstrap. I'll make that change in a subsequent commit
so as to provide traceable provenance of the bootstrap tools.
Intuitively, one cares mainly about the host platform: Platforms differ
in meaningful ways but compilation is morally a pure process and
probably doesn't care, or those difference are already abstracted away.
@Dezgeg also empirically confirmed that > 95% of checks are indeed of
the host platform.
Yet these attributes in the old cross infrastructure were defined to be
the build platform, for expediency. And this was never before changed.
(For native builds build and host coincide, so it isn't clear what the
intention was.)
Fixing this doesn't affect native builds, since again they coincide. It
also doesn't affect cross builds of anything in Nixpkgs, as these are no
longer used. It could affect external cross builds, but I deem that
unlikely as anyone thinking about cross would use more explicit
attributes for clarity, all the more so because the rarity of inspecting
the build platform.
Works similarly to `enableParallelBuilding`, but is set by default when
`enableParallelBuilding` is set. In my experience most packages that build
fine in parallel also check fine in parallel.
Derivations where drawing their `system` attribute from `hostPlatform`
instead of `buildPlatform`. Fix that, and add an explanatory commment.
Fixes#45993
This has been not touched in 6 years. Let's remove it to cause less
problems when adding new cross-compiling infrastructure.
This also simplify gcc significantly.
This reverts commit a809fdc8e1 and then
achieves the same result (not rebuilding texinfo three times)
but without dragging bootstrap tools into the closure.
I *want* cross-specific overrides to be verbose, so I rather not have
this shorthand. This makes the syntactic overhead more proportional to
the maintainence cost. Hopefully this pushes people towards fewer
conditionals and more abstractions.
On darwin llvmPackages is built using python-boot to avoid dependencies
in the stdenv, but we can't and shouldn't use that when building the
manpages since it depends on python packages.
* substitute(): --subst-var was silently coercing to "" if the variable does not exist.
* libffi: simplify using `checkInputs`
* pythonPackges.hypothesis, pythonPackages.pytest: simpify dependency cycle fix
* utillinux: 2.32 -> 2.32.1
https://lkml.org/lkml/2018/7/16/532
* busybox: 1.29.0 -> 1.29.1
* bind: 9.12.1-P2 -> 9.12.2
https://ftp.isc.org/isc/bind9/9.12.2/RELEASE-NOTES-bind-9.12.2.html
* curl: 7.60.0 -> 7.61.0
* gvfs: make tests run, but disable
* ilmbase: disable tests on i686. Spooky!
* mdds: fix tests
* git: disable checks as tests are run in installcheck
* ruby: disable tests
* libcommuni: disable checks as tests are run in installcheck
* librdf: make tests run, but disable
* neon, neon_0_29: make tests run, but disable
* pciutils: 3.6.0 -> 3.6.1
Semi-automatic update generated by https://github.com/ryantm/nixpkgs-update tools. This update was made based on information from https://repology.org/metapackage/pciutils/versions.
* mesa: more include fixes
mostly from void-linux (thanks!)
* npth: 1.5 -> 1.6
minor bump
* boost167: Add lockfree next_prior patch
* stdenv: cleanup darwin bootstrapping
Also gets rid of the full python and some of it's dependencies in the
stdenv build closure.
* Revert "pciutils: use standardized equivalent for canonicalize_file_name"
This reverts commit f8db20fb3a.
Patching should no longer be needed with 3.6.1.
* binutils-wrapper: Try to avoid adding unnecessary -L flags
(cherry picked from commit f3758258b8895508475caf83e92bfb236a27ceb9)
Signed-off-by: Domen Kožar <domen@dev.si>
* libffi: don't check on darwin
libffi usages in stdenv broken darwin. We need to disable doCheck for that case.
* "rm $out/share/icons/hicolor/icon-theme.cache" -> hicolor-icon-theme setup-hook
* python.pkgs.pytest: setupHook to prevent creation of .pytest-cache folder, fixes#40273
When `py.test` was run with a folder as argument, it would not only
search for tests in that folder, but also create a .pytest-cache folder.
Not only is this state we don't want, but it was also causing
collisions.
* parity-ui: fix after merge
* python.pkgs.pytest-flake8: disable test, fix build
* Revert "meson: 0.46.1 -> 0.47.0"
With meson 0.47.0 (or 0.47.1, or git)
things are very wrong re:rpath handling
resulting in at best missing libs but
even corrupt binaries :(.
When we run patchelf it masks the problem
by removing obviously busted paths.
Which is probably why this wasn't noticed immediately.
Unfortunately the binary already
has a long series of paths scribbled
in a space intended for a much smaller string;
in my testing it was something like
lengths were 67 with 300+ written to it.
I think we've reported the relevant issues upstream,
but unfortunately it appears our patches
are what introduces the overwrite/corruption
(by no longer being correct in what they assume)
This doesn't look so bad to fix but it's
not something I can spend more time on
at the moment.
--
Interestingly the overwritten string data
(because it is scribbled past the bounds)
remains in the binary and is why we're suddenly
seeing unexpected references in various builds
-- notably this is is the reason we're
seeing the "extra-utils" breakage
that entirely crippled NixOS on master
(and probably on staging before?).
Fixes#43650.
This reverts commit 305ac4dade.
(cherry picked from commit 273d68eff8f7b6cd4ebed3718e5078a0f43cb55d)
Signed-off-by: Domen Kožar <domen@dev.si>
`depsHostBuild` is not a thing, would never be a thing per the rules,
and isn't used anywhere. This is just my typo, hitherto unnoticed
because "host -> host" dependencies are by far the most obscure form.
HOST_PATH contains the path of the host package. This will include the
packages listed in buildInputs & depsHostHost. Use this to find
runtime commands that the host needs.
For instance to find the runtime version of perl,
$ PATH="$HOST_PATH" command -v perl
/nix/store/...-perl-5.28.0-aarch64-unknown-linux-android/bin/perl
This path should not be executed directly (it will break for cross
compilation). Only use it to find the location of executables that
will be run by your host system. Your build tools will, as always, be
available on the default PATH.
