toybox/www/faq.html

<html><head><title>toybox FAQ</title>
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<h1>Frequently Asked Questions</h1>

<h2>General Questions</h2>

<ul>
<li><h2><a href="#why_toybox">Why toybox? (What was wrong with busybox?)</a></h2></li>
<li><h2><a href="#capitalize">Do you capitalize toybox?</a></h2></li>
<li><h2><a href="#support_horizon">Why a 7 year support horizon?</a></h2></li>
<li><h2><a href="#releases">Why time based releases?</a></h2></li>
<li><h2><a href="#code">Where do I start understanding the toybox source code?</a></h2></li>
<li><h2><a href="#when">When were historical toybox versions released?</a></h2></li>
<li><h2><a href="#bugs">Where do I report bugs?</a></h2></li>
<li><h2><a href="#b_links">What are those /b/number links in the git log?</a></h2></li>
<li><h2><a href="#opensource">What is the relationship between toybox and android?</a></h2></li>
<li><h2><a href="#backporting">Will you backport fixes to old versions?</a></h2></li>
<li><h2><a href="#dotslash">What's this ./ on the front of commands in your examples?</a></h2></li>
</ul>

<h2>Using toybox</h2>

<ul>
<!-- get binaries -->
<li><h2><a href="#install">How do I install toybox?</h2></li>
<li><h2><a href="#standalone">How do I make individual/standalone toybox command binaries?</h2></li>
<li><h2><a href="#hermetic">How do I build toybox on a system with a broken $PATH?</a></h2></li>
<li><h2><a href="#cross">How do I cross compile toybox?</h2></li>
<li><h2><a href="#targets">What architectures does toybox support?</li>
<li><h2><a href="#system">What part of Linux/Android does toybox provide?</h2></li>
<li><h2><a href="#mkroot">How do I build a working Linux system with toybox?</a></h2></li>
</ul>

<h2>Specific commands</h2>

<ul>
<li><h2><a href="#cttyhack">Why don't you have cttyhack?</h2></li>
</ul>

<hr /><h2><a name="why_toybox" />Q: "Why is there toybox? What was wrong with busybox?"</h2>

<p>A: Toybox started back in 2006 when I (Rob Landley)
<a href=https://lwn.net/Articles/202106/>handed off BusyBox maintainership</a>
and <a href=http://landley.net/notes-2006.html#28-09-2006>started over from
scratch</a> on a new codebase after a
<a href=http://lists.busybox.net/pipermail/busybox/2006-September/058617.html>protracted licensing argument</a> took all the fun out of working on BusyBox.</p>

<p>Toybox was just a personal project until it got
<a href=http://landley.net/notes-2011.html#13-11-2011>relaunched</a>
in November 2011 with a new goal to make Android
<a href=http://landley.net/aboriginal/about.html#selfhost>self-hosting</a>.
This involved me relicensing my own
code, which made people who had never used or participated in the project
<a href=https://lwn.net/Articles/478308/>loudly angry</a>. The switch came
after a lot of thinking <a href=http://landley.net/talks/ohio-2013.txt>about
licenses</a> and <a href=http://landley.net/notes-2011.html#21-03-2011>the
transition to smartphones</a>, which led to a
<a href=https://www.youtube.com/watch?v=SGmtP5Lg_t0>2013 talk</a> laying
out a
<a href=http://landley.net/talks/celf-2013.txt>strategy</a>
to make Android self-hosting using toybox. This helped
<a href=https://code.google.com/p/android/issues/detail?id=76861>bring
it to Android's attention</a>, and they
<a href=https://lwn.net/Articles/629362/>merged it</a> into Android M.</p>

<p>The unfixable problem with busybox was licensing: BusyBox predates Android
by almost a decade, but Android still doesn't ship with it because GPLv3 came
out around the same time Android did and caused many people to throw
out the GPLv2 baby with the GPLv3 bathwater.
Android <a href=https://source.android.com/source/licenses.html>explicitly
discourages</a> use of GPL and LGPL licenses in its products, and has gradually
reimplemented historical GPL components (such as its bluetooth stack) under the
Apache license. Apple's
<a href=http://meta.ath0.com/2012/02/05/apples-great-gpl-purge/>less subtle</a> response was to freeze xcode at the last GPLv2 releases
(GCC 4.2.1 with binutils 2.17) for over 5 years while sponsoring the
development of new projects (clang/llvm/lld) to replace them,
implementing a
<a href=https://www.osnews.com/story/24572/apple-ditches-samba-in-favour-of-homegrown-replacement/>new SMB server</a> from scratch to
<a href=https://archive.org/details/copyleftconf2020-allison>replace samba</a>,
switching <a href=https://www.theverge.com/2019/6/4/18651872/apple-macos-catalina-zsh-bash-shell-replacement-features>bash with zsh</a>, and so on.
Toybox itself exists because somebody in a legacy position
just wouldn't shut up about GPLv3, otherwise I would probably
still happily be maintaining BusyBox. (For more on how I wound
up working on busybox in the first place,
<a href=http://landley.net/aboriginal/history.html>see here</a>.)</p>

<hr /><h2><a name="capitalize" />Q: Do you capitalize toybox?</h2>

<p>A: Only at the start of a sentence. The command name is all lower case so
it seems silly to capitalize the project name, but not capitalizing the
start of sentences is awkward, so... compromise. (It is _not_ "ToyBox".)</p>

<hr /><h2><a name="support_horizon">Q: Why a 7 year support horizon?</a></h2>

<p>A: Our <a href=http://lists.busybox.net/pipermail/busybox/2006-September/058440.html>longstanding rule of thumb</a> is to try to run and build on
hardware and distributions released up to 7 years ago, and feel ok dropping
support for stuff older than that. (This is a little longer than Ubuntu's
Long Term Support, but not by much.)</p>

<p>My original theory was "4 to 5 of the 18-month cycles of moore's law should cover
the vast majority of the installed base of PC hardware", loosely based on some
research I did <a href=http://www.catb.org/esr/halloween/halloween9.html#id2867629>back in 2003</a>
and <a href=http://catb.org/esr/writings/world-domination/world-domination-201.html#id248066>updated in 2006</a>
which said that low end systems were 2 iterations of moore's
law below the high end systems, and that another 2-3 iterations should cover
the useful lifetime of most systems no longer being sold but still in use and
potentially being upgraded to new software releases.</p>

<p>That analysis missed <a href=http://landley.net/notes-2011.html#26-06-2011>industry
changes</a> in the 1990's that stretched the gap
from low end to high end from 2 cycles to 4 cycles, and ignored
<a href=https://landley.net/notes-2010.html#09-10-2010>the switch</a> from PC to smartphone cutting off the R&D air supply of the
laptop market. Meanwhile the Moore's Law <a href=https://en.wikipedia.org/wiki/Logistic_function>s-curve</a> started bending back down (as they
<a href=https://en.wikipedia.org/wiki/Diffusion_of_innovations>always do</a>)
back in 2000, and these days is pretty flat: the drive for faster clock
speeds <a href=http://www.anandtech.com/show/613>stumbled</a>
and <a href=http://www.pcworld.com/article/118603/article.html>died</a>, with
the subsequent drive to go "wide" maxing out for most applications
around 4x SMP with maybe 2 megabyte caches. These days the switch from exponential to
linear growth in hardware capabilities is
<a href=https://www.cnet.com/news/end-of-moores-law-its-not-just-about-physics/>common knowledge</a> and
<a href=http://www.acm.org/articles/people-of-acm/2016/david-patterson>widely
accepted</a>.</p>

<p>But the 7 year rule of thumb stuck around anyway: if a kernel or libc
feature is less than 7 years old, I try to have a build-time configure test
for it to let the functionality cleanly drop out. I also keep old Ubuntu
images around in VMs to perform the occasional defconfig build there to
see what breaks. (I'm not perfect about this, but I accept bug reports.)</p>

<hr /><h2><a name="releases" />Q: Why time based releases?</h2>
<p>A: Toybox targets quarterly releases (a similar schedule to the Linux
kernel) because Martin Michlmayr's excellent
<a href=http://www.youtube.com/watch?v=IKsQsxubuAA>talk on the
subject</a> was convincing. This is actually two questions, "why have
releases" and "why schedule them".</p>

<p>Releases provide synchronization points where the developers certify
"it worked for me". Each release is a known version with predictable behavior,
and right or wrong at least everyone should be seeing
similar results so might be able to google an unexpected outcome.
Releases focus end-user testing on specific versions
where issues can be reproduced, diagnosed, and fixed.
Releases also force the developers to do periodic tidying, packaging,
documentation review, finish up partially implemented features languishing
in their private trees, and give regular checkpoints to measure progress.</p>

<p>Changes accumulate over time: different feature sets, data formats,
control knobs... Toybox's switch from "ls -q" to "ls -b" as the default output
format was not-a-bug-it's-a "design improvement", but the
difference is academic if the change breaks somebody's script.
Releases give you the option to schedule upgrades as maintenance, not to rock
the boat just now, and use a known working release version until later.</p>

<p>The counter-argument is that "continuous integration"
can be made robust with sufficient automated testing. But like the
<a href=https://web.archive.org/web/20131123071427/http://www.shirky.com/weblog/2013/11/healthcare-gov-and-the-gulf-between-planning-and-reality/>waterfall method</a>, this places insufficent
emphasis on end-user feedback and learning from real world experience.
Developer testing is either testing that the code does what the developers
expect given known inputs running in an established environment, or it's
regression testing against bugs previously found in the field. No plan
survives contact with the enemy, and technology always breaks once it
leaves the lab and encounters real world data and use cases in new
runtime and build environments.</p>

<p>The best way to give new users a reasonable first experience is to point
them at specific stable versions where development quiesced and
extra testing occurred. There will still be teething troubles, but multiple
people experiencing the _same_ teething troubles can potentially
help each other out.</p>

<p>Releases on a schedule are better than releases "when it's ready" for
the same reason a regularly scheduled bus beats one that leaves when it's
"full enough": the schedule lets its users make plans. Even if the bus leaves
empty you know when the next one arrives so missing this one isn't a disaster.
and starting the engine to leave doesn't provoke a last-minute rush of nearby
not-quite-ready passengers racing to catch it causing further delay and
repeated start/stop cycles as it ALMOST leaves.
(The video in the first paragraph goes into much greater detail.)</p>

<hr /><h2><a name="code" />Q: Where do I start understanding the source code?</h2>

<p>A: Toybox is written in C. There are longer writeups of the
<a href=design.html>design ideas</a> and a <a href=code.html>code walkthrough</a>,
and the <a href=about.html>about page</a> summarizes what we're trying to
accomplish, but here's a quick start:</p>

<p>Toybox uses the standard three stage configure/make/install
<a href=code.html#building>build</a>, in this case "<b>make defconfig;
make; make install</b>". Type "<b>make help</b>" to
see available make targets.</p>

<p><u>The configure stage</u> is copied from the Linux kernel (in the "kconfig"
directory), and saves your selections in the file ".config" at the top
level. The "<b>make defconfig</b>" target selects the
maximum sane configuration (enabling all the commands and features that
aren't unfinished, or only intended as examples, or debug code...) and is
probably what you want. You can use "<b>make menuconfig</b>" to manually select
specific commands to include, through an interactive menu (cursor up and
down, enter to descend into a sub-menu, space to select an entry, ? to see
an entry's help text, esc to exit). The menuconfig help text is the
same as the command's "<b>--help</b>" output.</p>

<p><u>The "make" stage</u> creates a toybox binary (which is stripped, look in
generated/unstripped for the debug versions), and "<b>make install</b>" adds a bunch of
symlinks to toybox under the various command names. Toybox determines which
command to run based on the filename, or you can use the "toybox" name in which case the first
argument is the command to run (ala "toybox ls -l").</p>

<p>You can also build individual commands as <a href="#standalone">standalone
executables</a>, ala "make sed cat ls".
The "make change" target builds all of them, as in "change for a $20".</p>

<p><u>The main() function is in main.c</u> at the top level,
along with setup plumbing and selecting which command to run this time.
The function toybox_main() in the same file implements the "toybox"
multiplexer command that lists and selects the other commands.</p>

<p><u>The individual command implementations are under "toys"</u>, and are grouped
into categories (mostly based on which standard they come from, posix, lsb,
android...) The "pending" directory contains unfinished commands, and the
"examples" directory contains example code that aren't really useful commands.
Commands in those two directories
are _not_ selected by defconfig. (Most of the files in the pending directory
are third party submissions that have not yet undergone
<a href=cleanup.html>proper code review</a>.)</p>

<p><u>Common infrastructure shared between commands is under "lib"</u>. Most
commands call lib/args.c to parse their command line arguments before calling
the command's own main() function, which uses the option string in
the command's NEWTOY() macro. This is similar to the libc function getopt(),
but more powerful, and is documented at the top of lib/args.c. A NULL option
string prevents this code from being called for that command.</p>

<p><u>The build/install infrastructure is shell scripts under
"scripts"</u> (starting with scripts/make.sh and scripts/install.sh).
<u>These populate the "generated" directory</u> with headers
created from other files, which are <a href=code.html#generated>described</a>
in the code walkthrough. All the
build's temporary files live under generated, including the .o files built
from the .c files (in generated/obj). The "make clean" target deletes that
directory. ("make distclean" also deletes your .config and deletes the
kconfig binaries that process .config.)</p>

<p><u>Each command's .c file contains all the information for that command</u>, so
adding a command to toybox means adding a single file under "toys".
Usually you <a href=code.html#adding>start a new command</a> by copying an
existing command file to a new filename
(toys/examples/hello.c, toys/examples/skeleton.c, toys/posix/cat.c,
and toys/posix/true.c have all been used for this purpose) and then replacing
all instances of its old name with the new name (which should match the
new filename), and modifying the help text, argument string, and what the
code does. You might have to "make distclean" before your new command
shows up in defconfig or menuconfig.</p>

<p><u>The toybox test suite lives in the "tests" directory</u>, and is
driven by scripts/test.sh and scripts/runtest.sh. From the top
level you can "make tests" to test everything, or "make test_sed" to test a
single command's standalone version (which should behave identically,
but that's why we test). You can set TEST_HOST=1 to test the host version
instead of the toybox version (in theory they should work the same),
and VERBOSE=all to see diffs of the expected and actual output for all
failing tests. The default VERBOSE=fail stops at the first such failure.</p>

<hr /><h2><a name="when" />Q: When were historical toybox versions released?</h2>

<p>A: For vanilla releases, check the
<a href=https://github.com/landley/toybox/tags>date on the commit tag</a>
or <a href=https://landley.net/toybox/downloads/binaries/>the
example binaries</a> against the output of "toybox --version".
Between releases the --version
information is in "git describe --tags" format with "tag-count-hash" showing the
most recent commit tag, the number of commits since that tag, and
the hash of the current commit.</p>

<p>Android makes its own releases on its own
<a href=https://en.wikipedia.org/wiki/Android_version_history>schedule</a>
using its own version tags, but lists corresponding upstream toybox release
versions <a href=https://android.googlesource.com/platform/system/core/+/master/shell_and_utilities/README.md>here</a>. For more detail you can look up
<a href=https://android.googlesource.com/platform/external/toybox/+refs>AOSP's
git tags</a>. (The <a href=https://source.android.com/setup/start>Android Open Source Project</a> is the "upstream" android vendors
start form when making their own releases. Google's phones run AOSP versions
verbatim, other vendors tend to take those releases as starting points to
modify.)</p>

<p>If you want to find the vanilla toybox commit corresponding to an AOSP
toybox version, find the most recent commit in the android log that isn't from a
@google or @android address and search for it in the vanilla commit log.
(The timestamp should match but the hash will differ,
because each git hash includes the previous
git hash in the data used to generate it so all later commits have a different
hash if any of the tree's history differs; yes Linus Torvalds published 3 years
before Satoshi Nakamoto.) Once you've identified the vanilla commit's hash,
"git describe --tags $HASH" in the vanilla tree should give you the --version
info for that one.</p>

<hr /><h2><a name="bugs" />Q: Where do I report bugs?</h2>

<p>A: Ideally on the <a href=http://lists.landley.net/listinfo.cgi/toybox-landley.net>mailing list</a>, although <a href=mailto:[email protected]>emailing the
maintainer</a> is a popular if slightly less reliable alternative.
Issues submitted to <a href=https://github.com/landley/toybox>github</a>
are generally dealt with less promptly, but mostly get done eventually.
AOSP has its <a href=https://source.android.com/setup/contribute/report-bugs>own bug reporting mechanism</a> (although for toybox they usually forward them
to the mailing list) and Android vendors usually forward them to AOSP which
forwards them to the list.</p>

<p>Note that if we can't reproduce a bug, we probably can't fix it.
Not only does this mean providing enough information for us to see the
behavior ourselves, but ideally doing so in a reasonably current version.
The older it is the greater the chance somebody else found and fixed it
already, so the more out of date the version you're reporting a bug against
the less effort we're going to put into reproducing the problem.</p>

<hr /><h2><a name="b_links" />Q: What are those /b/number bug report
links in the git log?</h2>

<p>A: It's a Google thing. Replace /b/$NUMBER with
https://issuetracker.google.com/$NUMBER to read it outside the googleplex.</p>

<hr /><a name="opensource" /><h2>Q: What is the relationship between toybox and android?</h2>

<p>A: The <a href=about.html>about page</a> tries to explain that,
and Linux Weekly News has covered toybox's history a
<a href=https://lwn.net/Articles/202106/>little</a>
<a href=https://lwn.net/Articles/478308/>over</a>
<a href=https://lwn.net/Articles/616272/>the</a>
<a href=https://lwn.net/Articles/629362/>years</a>.</p>

<p>Toybox is a traditional open source project created and maintained
by hobbyist (volunteer) developers, originally for Linux but these days
also running on Android, BSD, and MacOS. The project started in 2006
and its original author (Rob Landley)
continues to maintain the open source project.</p>

<p>Android's base OS maintainer (Elliott Hughes, I.E. enh)
<a href=https://github.com/landley/toybox/commit/69a9f257234a>ported</a>
<a href=https://github.com/landley/toybox/commit/6a29bb1ebe62>toybox</a>
to Android in 2014, merged it into Android M (Marshmallow), and remains
Android's toybox maintainer. (He explained it in his own words in
<a href=http://androidbackstage.blogspot.com/2016/07/episode-53-adb-on-adb.html>this podcast</a>, starting either 18 or 20 minutes in depending how
much backstory you want.)</p>

<p>Android's policy for toybox development is to push patches to the
open source project (submitting them via the mailing list) then
"git pull" the public tree into Android's tree. To avoid merge conflicts, Android's
tree doesn't change any of the existing toybox files but instead adds <a href=https://android.googlesource.com/platform/external/toybox/+/refs/heads/master/Android.bp>parallel
build infrastructure</a> off to one side. (Toybox uses a make wrapper around bash
scripts, AOSP builds with soong/ninja instead and checks in a snapshot of the
generated/ directory to avoid running kconfig each build).
Android's changes to toybox going into the open source tree first
and being pulled from there into Android keeps the two trees in
sync, and makes sure each change undergoes full open source design review
and discussion.</p>

<p>Rob acknowledges Android is by far the largest userbase for the project,
but develops on a standard 64-bit Linux+glibc distro while building embedded
32-bit big-endian nommu musl systems requiring proper data alignment for work,
and is not a Google employee so does not have access
to the Google build cluster of powerful machines capable of running the full
AOSP build in a reasonable amount of time. Rob is working to get android
building under android (the list of toybox tools Android's build uses is
<a href=https://android.googlesource.com/platform/prebuilts/build-tools/+/refs/heads/master/path/linux-x86/>here</a>,
and what else it needs from its build environment is
<a href=https://android.googlesource.com/platform/build/soong/+/refs/heads/master/ui/build/paths/config.go>here</a>), and he hopes someday to not only make a usable development
environment out of it but also nudge the base OS towards a more granular
package management system allowing you to upgrade things like toybox without
a complete reinstall and reboot, plus the introduction of a "posix container"
within which you can not only run builds, but selinux lets you run binaries
you've just built). In the meantime, Rob tests static bionic
builds via the Android NDK when he remembers, but has limited time to work
on toybox because it's not his day job. (The products his company makes ship
toybox and they do sponsor the project's development, but it's one of many
responsibilities at work.)</p>

<p>Elliott is the Android base OS maintainer, in which role he manages
a team of engineers. He also has limited time for toybox, both because it's one
of many packages he's responsible for (he maintains bionic, used to maintain
dalvik...) and because he allowed himself to be promoted into management
and thus spends less time coding than he does sitting in meetings where testers
talk to security people about vendor issues.</p>

<p>Android has many other coders and security people who submit the occasional
toybox patch, but of the last 1000 commits at the <a href=https://github.com/landley/toybox/commit/88b34c4bd3f8>time
of writing</a> this FAQ entry, Elliott submitted 276 and all other google.com
or android.com addresses combined totaled 17. (Rob submitted 591, leaving
116 from other sources, but for both Rob and Elliott there's a lot of "somebody
else pointed out an issue, and then we wrote a patch". A lot of patches
from both "Author:" lines thank someone else for the suggestion in the
commit comment.)</p>

<hr /><a name="backporting" /><h2>Q: Will you backport fixes to old versions?</h2>

<p>A: Probably not. The easiest thing to do is get your issue fixed upstream
in the current release, then get the newest version of the
project built and running in the old environment.</p>

<p>Backporting fixes generally isn't something open source projects run by
volunteer developers do because the goal of the project's development community
is to extend and improve the project. We're happy to respond to our users'
needs, but if you're coming to the us for free tech support we're going
to ask you to upgrade to a current version before we try to diagnose your
problem.</p>

<p>The volunteers are happy to fix any bugs you point out in the current
versions because doing so helps everybody and makes the project better. We
want to make the current version work for you. But diagnosing, debugging, and
backporting fixes to old versions doesn't help anybody but you, so isn't
something we do for free. The cost of volunteer tech support is using a
reasonably current version of the project.</p>

<p>If you're using an old version built with an old
compiler on an old OS (kernel and libc), there's a fairly large chance
whatever problem you're
seeing already got fixed, and to get that fix all you have to do is upgrade
to a newer version. Diagnosing a problem that wasn't our bug means we spent
time that only helps you, without improving the project.
If you don't at least _try_ a current version, you're asking us for free
personalized tech support.</p>

<p>Reproducing bugs in current versions also makes our job easier.
The further back in time
you are, the more work it is for us digging back in the history to figure
out what we hadn't done yet in your version. If spot a problem in a git
build pulled 3 days ago, it's obvious what changed and easy to fix or back out.
If you ask about the current release version 3 months after it came out,
we may have to think a while to remember what we did and there are a number of
possible culprits, but it's still tractable. If you ask about 3 year old
code, we have to reconstruct the history and the problem could be anything,
there's a lot more ground to cover and we haven't seen it in a while.</p>

<p>As a rule of thumb, volunteers will generally answer polite questions about
a given version for about three years after its release before it's so old
we don't remember the answer off the top of our head. And if you want us to
put any _effort_ into tracking it down, we want you to put in a little effort
of your own by confirming it's still a problem with the current version
(I.E. we didn't fix it already). It's
also hard for us to fix a problem of yours if we can't reproduce it because
we don't have any systems running an environment that old.</p>

<p>If you don't want to upgrade, you have the complete source code and thus
the ability to fix it yourself, or can hire a consultant to do it for you. If
you got your version from a vendor who still supports the older version, they
can help you. But there are limits as to what volunteers will feel obliged to
do for you.</p>

<p>Commercial companies have different incentives. Your OS vendor, or
hardware vendor for preinstalled systems, may have their own bug reporting
mechanism and update channel providing backported fixes. And a paid consultant
will happily set up a special environment just to reproduce your problem.</p>

<hr /><h2><a name="install" />Q: How do I install toybox?</h2>

<p>A:
Multicall binaries like toybox behave differently based on the filename
used to call them, so if you "mv toybox ls; ./ls -l" it acts like ls. Creating
symlinks or hardlinks and adding them to the $PATH lets you run the
commands normally by name, so that's probably what you want to do.</p>

<p>If you already have a <a href=https://landley.net/toybox/downloads/binaries/>toybox binary</a>
you can install a tree of command symlinks to
<a href=http://git.musl-libc.org/cgit/musl/tree/include/paths.h>the
standard path</a>
locations (<b>export PATH=/bin:/usr/bin:/sbin:/usr/sbin</b>) by doing:</p>

<blockquote><p><b>for i in $(/bin/toybox --long); do ln -s /bin/toybox $i; done</b></p></blockquote>

<p>Or you can install all the symlinks in the same directory as the toybox binary
(<b>export PATH="$PWD:$PATH"</b>) via:</p>

<blockquote><p><b>for i in $(./toybox); do ln -s toybox $i; done</b></p></blockquote></p>

<p>When building from source, use the "<b>make install</b>" and
"<b>make install_flat</b>"
targets with an appropriate <b>PREFIX=/target/path</b> either
exported or on the make command line. When cross compiling,
"<b>make list</b>" outputs the command names enabled by defconfig.
For more information, see "<b>make help</b>".</p>

<p>The command name "toybox" takes the second argument as the name of the
command to run, so "./toybox ls -l" also behaves like ls. The "toybox"
name is special in that it can have a suffix (toybox-i686 or toybox-1.2.3)
and still be recognized, so you can have multiple versions of toybox in the
same directory.</p>

<p>When toybox doesn't recognize its
filename as a command, it dereferences one
level of symlink. So if your script needs "gsed" you can "ln -s sed gsed",
then when you run "gsed" toybox knows how to be "sed".</p>

<hr /><h2><a name="dotslash" />Q: What's this ./ on the front of commands in your examples?</h2>

<p>A: When you don't give a path to a command's executable file,
linux command shells search the directories listed in the $PATH envionment
variable (in order), which usually doesn't include the current directory
for security reasons. The
magic name "." indicates the current directory (the same way ".." means
the parent directory and starting with "/" means the root directory)
so "./file" gives a path to the executable file, and thus runs a command
out of the current directory where just typing "file" won't find it.
For historical reasons PATH is colon-separated, and treats an
empty entry (including leading/trailing colon) as "check the current
directory", so if you WANT to add the current directory to PATH you
can PATH="$PATH:" but doing so is a TERRIBLE idea.</p>

<p>Toybox's shell (toysh) checks for built-in commands before looking at the
$PATH (using the standard "bash builtin" logic just with lots more builtins),
so "ls" doesn't have to exist in your filesystem for toybox to find it. When
you give a path to a command the shell won't run the built-in version
but will run the file at that location. (But the multiplexer command
won't: "toybox /bin/ls" runs the built-in ls, you can't point it at an
arbitrary file out of the filesystem and have it run that. You could
"toybox nice /bin/ls" though.)</p>

<hr /><h2><a name="standalone" />Q: How do I make individual/standalone toybox command binaries?</h2>

<p>A: You can use almost<a href="#stand_foot"</a>*</a><a name="stand_back">
any command name as a make target (ala "make sed") or test the standalone versions individually
with the test_ prefix ("make test_sed"). You'll need to run the configure
step first (generally "make defconfig") so the .config file exists for
the build. For a list of currently available commands run
"make list".</p>

<p>The "make change" target (as in change for a $20) builds every command
standalone (in the "change" subdirectory). Note that this is collectively
about 10 times as large as the all-in-one multiplexer version (in disk space,
runtime memory, how long the build takes...)</p>

<p>As always, the Makefile is a thin wrapper around bash scripts actually
doing the work, you can just all "scripts/single.sh cat ls mv" directly
if you like.</p>

<p><a name="stand_foot"><a href="#stand_back">*</a> A few command names, like "help" and "test" have
other meanings to the Makefile, and you have to use scripts/single.sh or
"make change" to build them standalone.</p>

<hr /><h2><a name="hermetic">How do I build toybox on a system with a broken $PATH?</a></h2>

<p>Toybox can provide its own build prerequisites (I.E
perform a "hermetic" build) using the script <b>scripts/prereq/build.sh</b>
which is a canned minimal toybox build that basically does "cc *.c" against
saved headers to build the commands needed by the rest of the build.</p>

<p>At the moment, building toybox on mac requires homebrew to get a .config
file, ala:</p>

<blockquote><pre>
$ homebrew
$ make macos_defconfig
$ make clean
$ exit
</pre></blockquote>

<p>But the rest of the hermetic build works without it:</p>

<blockquote><pre>
$ scripts/prereq/build.sh #ignoring SO many warnings
$ mkdir prereq; mv toybox-prereq prereq/
$ for i in $(prereq/toybox-prereq); do ln -s toybox-prereq prereq/$i; done
$ PATH=$PWD/prereq:$PATH scripts/make.sh
$ ./toybox
</pre></blockquote>

<p>If you already have an appropriate .config file you can copy in you
don't need homebrew at all (and can skip the first section above).
Editing one up by hand for qnx and similar is currently left as an exercise
for the reader (but it's a fairly simple text file format).</p>

<p>The files in the scripts/prereq directory were created by
<b>scripts/recreate-prereq.sh</b> which records the commands used by
a toybox build, harvests stripped down headers, and writes a build.sh
to compile the appropriate source files. It's a couple dozen lines of
bash if you're interested.</p>

<p>At the moment toybox's full scripts/make.sh still requires bash
(until toysh is finished and promoted out of pending). Freebsd users
can invoke "/opt/usr/local/bin/bash scripts/make.sh" or similar
to work around their distro's policy insisting that /bin/env can be
trusted to live at a specific path but /bin/bash can't. (On Android both
env and sh live in /system/bin, which is at least internally consistent.)</p>

<p>Toybox does not yet provide "make" either. You can call scripts/make.sh
directly (and scripts/test.sh and scripts/single.sh) if you've got a .config,
but until kconfig/ is replaced defconfig/menuconfig still need gmake.</p>

<hr /><h2><a name="cross" />Q: How do I cross compile toybox?</h2>

<p>A: You need a compiler "toolchain" capable of producing binaries that
run on your target. A <a href=https://landley.net/toybox/downloads/binaries/toolchains>toolchain</a> is an
integrated suite of compiler, assembler, and linker, plus the standard
headers and
libraries necessary to build C programs. (And a few miscellaneous binaries like
nm and objdump that display info about <a href=https://en.wikipedia.org/wiki/Executable_and_Linkable_Format>ELF files</a>.)</p>

<p>Toybox supports the standard $CROSS_COMPILE prefix environnment variable,
same as the Linux kernel build uses. This is used to prefix all the tools
(target-cc, target-ld, target-strip) during the build, meaning the prefix
usually ends with a "-" that's easy to forget but kind of important
("target-cc" and "targetcc" are not the same name).</p>

<p>You can either provide a
full path in the CROSS_COMPILE string, or add the appropriate bin directory
to your $PATH. I.E:</p>

<blockquote>
<b><p>make LDFLAGS=--static CROSS_COMPILE=~/musl-cross-make/ccc/m68k-linux-musl-cross/bin/m68k-linux-musl- distclean defconfig toybox</p></b>
</blockquote>

<p>Is equivalent to:</p>

<blockquote><b><p>
export "PATH=~/musl-cross-make/ccc/m68k-linux-musl-cross/bin:$PATH"<br />
LDFLAGS=--static CROSS_COMPILE=m68k-linux-musl- make distclean defconfig toybox
</p></b></blockquote>

<p>Both of those examples use static linking so you can install just
the single file to target, or test them with "qemu-m68k toybox". Feel free
to dynamically link instead if you prefer, mkroot offers a "dynamic"
add-on to copy the compiler's shared libraries into the new root
filesystem.</p>

<p>Although you can individually override $CC and $STRIP and such,
providing the prefix twice applies it twice, ala
"CROSS_COMPILE=prefix- CC=prefix-cc" gives "prefix-prefix-cc".</p>

<p>Toybox's <a href=#mkroot>system builder</a> can use a simpler $CROSS
variable to specify the target name(s) to build for if you've installed
<a href=#cross2>compatible</a> cross compilers under the "ccc" directory.
Behind the scenes this uses wildcard expansion to set $CROSS_COMPILE to
an appropriate "path/prefix-".</p>

<hr /><h2><a name="targets">Q: What architectures does toybox support?</h2>

<p>Toybox runs on 64 bit and 32 bit processors, little endian and big endian,
tries to respect alignment, and will enable nommu support when fork() is
unavailable (or when TOYBOX_FORCE_NOMMU is enabled in the config to
work around broken nommu toolchains), but otherwise tries to be
processor agnostic (although some commands such as strace can't avoid
a processor-specific if/else staircase.).</p>

<P>Several commands (such as ps/top) are unavoidably full of Linux assumptions.
Some subset of the commands have been made to run on BSD and MacOS X, and
lib/portability.* and scripts/genconfig.sh exist to catch some known
variations.</p>
</p>

<p>Each release gets tested against two compilers (llvm, gcc), three C
libraries (bionic, musl, glibc), and a half-dozen different processor
types, in the following combinations:</p>

<a name="cross1" />
<p><a href="#cross1">1) gcc+glibc = host toolchain</a></p>

<p>Most Linux distros come with that as a host compiler, which is used by
default when you build normally
(<b>make distclean defconfig toybox</b>, or <b>make menuconfig</b> followed
by <b>make</b>).</p>

<p>You can use LDFLAGS=--static if you want static binaries, but static
glibc is hugely inefficient ("hello world" is 810k on x86-64) and throws a
zillion linker warnings because one of its previous maintainers
<a href=https://www.akkadia.org/drepper/no_static_linking.html>was insane</a>
(which meant at the time he refused to fix
<a href=https://elinux.org/images/2/2d/ELC2010-gc-sections_Denys_Vlasenko.pdf>obvious bugs</a>), plus it uses dlopen() at runtime to implement basic things like
<a href=https://stackoverflow.com/questions/15165306/compile-a-static-binary-which-code-there-a-function-gethostbyname>DNS lookup</a> (which is almost impossible
to support properly from a static binary because you wind up with two
instances of malloc() managing two heaps which corrupt as soon as a malloc()
from one is free()d into the other, although glibc added
<a href=https://stackoverflow.com/questions/14289488/use-dlsym-on-a-static-binary>improper support</a> which still requires the shared libraries to be
installed on the system alongside the static binary:
<a href=https://www.youtube.com/watch?v=Ih-3vK2qLls>in brief, avoid</a>).
These days glibc is <a href=https://blog.aurel32.net/175>maintained
by a committee</a> instead of a single
maintainer, if that's an improvement. (As with Windows and
Cobol, most people just try to get on with their lives.)</p>

<a name="cross2" />
<p><a href="#cross2">2) gcc+musl = musl-cross-make</a></p>

<p>These cross compilers are built from the
<a href=http://musl.libc.org/>musl-libc</a> maintainer's
<a href=https://github.com/richfelker/musl-cross-make>musl-cross-make</a>
project, built by running toybox's <a href=https://github.com/landley/toybox/blob/master/scripts/mcm-buildall.sh>scripts/mcm-buildall.sh</a> in that directory,
and then symlink the resulting "ccc" subdirectory into toybox where
"make root CROSS=" can find them, ala:</p>

<blockquote><b><pre>
cd ~
git clone https://github.com/landley/toybox
git clone https://github.com/richfelker/musl-cross-make
cd musl-cross-make
../toybox/scripts/mcm-buildall.sh # this takes a while
ln -s $(realpath ccc) ../toybox/ccc
</pre></b></blockquote>

<p>Since this takes a long time to run, and builds lots of targets
(cross and native), we've uploaded
<a href=downloads/binaries/toolchains/latest>the resulting binaries</a>
so you can wget and extract a tarball or two instead of
compiling them all yourself. (See the README in that directory for details.
Yes there's a big source tarball in there for license compliance reasons.)</p>

<p>Instead of CROSS= you can also specify a CROSS_COMPILE= prefix
in the same format the Linux kernel build uses. You can either provide a
full path in the CROSS_COMPILE string, or add the appropriate bin directory
to your $PATH. I.E:</p>

<blockquote>
<b><p>make LDFLAGS=--static CROSS_COMPILE=~/musl-cross-make/ccc/m68k-linux-musl-cross/bin/m68k-linux-musl- distclean defconfig toybox</p></b>
</blockquote>

<p>Is equivalent to:</p>

<blockquote><b><p>
export "PATH=~/musl-cross-make/ccc/m68k-linux-musl-cross/bin:$PATH"<br />
LDFLAGS=--static make distclean defconfig toybox CROSS=m68k-linux-musl-
</p></b></blockquote>

<p>Note: these examples use static linking because a dynamic musl binary
won't run on your host unless you install musl's libc.so into the system
libraries (which is an accident waiting to happen adding a second C library
to most glibc linux distribution) or play with $LD_LIBRARY_PATH.
(The <a href=https://github.com/landley/toybox/blob/master/scripts/root/dynamic>dynamic</a> package
in mkroot copies the shared libraries out of the toolchain to create a dynamic
linking environment in the root filesystem, but it's not nearly as well
tested.)</p>

<a name="cross3" />
<p><a href="#cross3">3) llvm+bionic = Android NDK</a></p>

<p>The <a href=https://developer.android.com/ndk/downloads>Android
Native Development Kit</a> provides an llvm toolchain with the bionic
libc used by Android. To turn it into something toybox can use, you
just have to add an appropriately prefixed "cc" symlink to the other
prefixed tools, ala:</p>

<blockquote><b><pre>
unzip android-ndk-r21b-linux-x86_64.zip
cd android-ndk-21b/toolchains/llvm/prebuilt/linux-x86_64/bin
ln -s x86_64-linux-android29-clang x86_64-linux-android-cc
PATH="$PWD:$PATH"
cd ~/toybox
make distclean
make LDFLAGS=--static CROSS_COMPILE=x86_64-linux-android- defconfig toybox
</pre></b></blockquote>

<p>Again, you need to static link unless you want to install bionic on your
host. Binaries statically linked against bionic are almost as big as with
glibc, but at least it doesn't have the dlopen() issues. (You still can't
sanely use dlopen() from a static binary, but bionic doesn't use dlopen()
internally to implement basic features.)</p>

<p>Note: although the resulting toybox will run in a standard
Linux system, even "hello world"
statically linked against bionic segfaults before calling main()
when /dev/null isn't present. This presents mkroot with a chicken and
egg problem for both chroot and qemu cases, because mkroot's init script
has to mount devtmpfs on /dev to provide /dev/null before the shell binary
can run mkroot's init script.
Since mkroot runs as a normal user, we can't "mknod dev/null" at build
time to create a "null" device in the filesystem we're packaging up so
initramfs doesn't start with an empty /dev, and the
<a href=https://lkml.org/lkml/2016/6/22/686>kernel</a>
<a href=https://lkml.org/lkml/2017/5/14/180>developers</a>
<a href=https://lkml.org/lkml/2017/9/13/651>repeatedly</a>
<a href=https://lkml.org/lkml/2020/5/14/1584>rejected</a> a patch to
make the Linux kernel honor DEVTMPFS_MOUNT in initramfs. Teaching toybox
cpio to accept synthetic filesystem metadata,
presumably in <a href=https://www.kernel.org/doc/Documentation/filesystems/ramfs-rootfs-initramfs.txt>get_init_cpio</a> format, remains a todo item.</p>

<hr /><h2><a name="system" />Q: What part of Linux/Android does toybox provide?</h2>

<p>A:
Toybox is one of three packages (linux, libc, command line) which together provide a bootable unix-style command line operating system.
Toybox provides the "command line" part, with a
<a href=https://en.wikipedia.org/wiki/Bash_(Unix_shell)>bash</a> compatible
<a href=https://en.wikipedia.org/wiki/Unix_shell>command line interpreter</a>
and over two hundred <a href=https://landley.net/toybox/help.html>commands</a>
to call from it, as documented in
<a href=https://pubs.opengroup.org/onlinepubs/9699919799.2008edition/>posix</a>,
the <a href=https://refspecs.linuxfoundation.org/LSB_4.1.0/LSB-Core-generic/LSB-Core-generic/cmdbehav.html>Linux Standard Base</a>, and the
<a href=https://man7.org/linux/man-pages/dir_section_1.html>Linux Manual
Pages</a>.</p>

<p>Toybox is not by itself a complete operating system, it's a set of standard command line utilities that run in an operating system.
Booting a simple system to a shell prompt requires a kernel to drive the hardware (such as Linux, or BSD with a Linux emulation layer), programs for the system to run (such as toybox's commands), and a C library ("libc") to connect them together.</p>

<p>Toybox has a policy of requiring no external dependencies other than the
kernel and C library (at least for defconfig builds). Our "software bill
of materials" (SBOM) defaults to just "the C library", both at build time
and and runtime. You can optionally enable support for
additional libraries in menuconfig (such as openssl, zlib, or selinux),
but toybox either provides its own built-in versions of such functionality
(which the libraries provide larger, more complex, often assembly optimized
alternatives to), or allows things like selinux support to cleanly drop
out.</p>

<p>Static linking (with the --static option) copies library contents
into the resulting binary, creating larger but more portable programs which
can run even if they're the only file in the filesystem. Otherwise,
the "dynamically" linked programs require each shared library file to be
present on the target system, either copied out of the toolchain or built
again from source (with potential version skew if they don't match the toolchain
versions exactly), plus a dynamic linker executable installed at a specific
absolute path. See the
<a href=https://www.man7.org/linux/man-pages/man1/ldd.1.html>ldd</a>,
<a href=https://www.man7.org/linux/man-pages/man8/ld.so.8.html>ld.so</a>,
and <a href=https://www.man7.org/linux/man-pages/man7/libc.7.html>libc</a>
man pages for details.</p>

<p>Most embedded systems will add another package to the kernel/libc/cmdline
above containing the dedicated "application" that the embedded system exists to
run, plus any other packages that application depends on.
Build systems add a native version of the toolchain packages so
they can compile additional software on the resulting system. Desktop systems
add a GUI and additional application packages like web browsers
and video players. A linux distro like Debian adds hundreds of packages.
Android adds around a thousand.</p>

<p>But all of these systems conceptually sit on a common three-package
"kernel/libc/cmdline" base (often inefficiently implemented and broken up
into more packages), and toybox aims to provide a simple, reproducible,
auditable version of the cmdline portion of that base.</p>

<hr /><h2><a name="mkroot" />Q: How do you build a working Linux system with toybox?</h2>

<p>A: Toybox has a built-in <a href=https://github.com/landley/toybox/blob/master/mkroot/mkroot.sh>system builder</a> called "<a href=https://github.com/landley/toybox/blob/master/mkroot/README>mkroot</a>", with the Makefile target "<b>make
root</b>". To enter the resulting root filesystem, "<b>sudo chroot
root/host/fs /init</b>". Type "exit" to get back out.</p>

<p>Prebuilt binary versions of these system images, suitable for running
under the emulator <a href=https://qemu.org>qemu</a>, are uploaded to
<a href=https://landley.net/bin/mkroot/latest>the website</a>
each release if you'd like to try before building from source.</p>

<p>You can cross compile simple three package (toybox+libc+linux) systems
configured to boot to a shell prompt under qemu by setting CROSS_COMPILE= to a
<a href=#cross>cross compiler</a> prefix (or by installing cross compilers
in the "ccc" subdirectory and specifying a target type with CROSS=)
and also pointing the build at a Linux kernel source directory, ala:</p>

<blockquote><p><b>make root CROSS=sh4 LINUX=~/linux</b></p></blockquote>

<p>Then you can <b>root/sh4/run-qemu.sh</b> to launch the emulator,
which boots the new Linux system (kernel and root filesystem) on a simulated
CPU with its own memory and I/O devices, connecting the
virtual serial console to the emulator's stdin and stdout.
You'll need the appropriate qemu-system-* emulator binary for the selected
architecture in your $PATH. Type "exit" when done to shut down the emulator,
similar to exiting the chroot version.</p>

<p>The build finds the <a href=#system>three packages</a> needed to produce
this system because 1) you're in a toybox source directory, 2) your cross
compiler has a libc built into it, 3) you tell it where to find a Linux kernel
source directory with LINUX= on the command line. If you don't say LINUX=,
it skips that part of the build and just produces a root filesystem directory
(root/$CROSS/fs or root/host/fs if no $CROSS target specified), which you
can chroot into if your architecture can run those binaries. (For PID other
than 1, the /init script at the top of the directory sets up and cleans up
the /proc mount points, so <b>chroot root/i686/fs /init</b> is a reasonable
"poke around and look at things" smoketest.)</p>

<p>The CROSS= shortcut expects a "ccc" symlink in the toybox source directory
pointing at a directory full of cross compilers. The ones I test this with are
built from the musl-libc maintainer's
<a href=https://github.com/richfelker/musl-cross-make>musl-cross-make</a>
project, built by running toybox's
<a href=https://github.com/landley/toybox/blob/master/scripts/mcm-buildall.sh>scripts/mcm-buildall.sh</a> in a musl-cross-make checkout directory,
and then symlinking the resulting "ccc" subdirectory into toybox where CROSS=
can find them:</p>

<blockquote><b><pre>
cd ~
git clone https://github.com/landley/toybox
git clone https://github.com/richfelker/musl-cross-make
cd musl-cross-make
../toybox/scripts/mcm-buildall.sh # this takes a while
ln -s $(realpath ccc) ../toybox/ccc
</pre></b></blockquote>

<p>If you don't want to do that, you can download <a href=http://landley.net/bin/toolchains/latest>prebuilt binary versions</a>
and extract them into a "ccc" subdirectory under the toybox source.</p>

<p>Once you've installed the cross compilers, "<b>make root CROSS=help</b>"
should list all the available cross compilers it recognizes under ccc,
something like:</p>

<blockquote><b><p>
aarch64 armv4l armv5l armv7l armv7m armv7r i486 i686 m68k microblaze mips mips64 mipsel or1k powerpc powerpc64 powerpc64le riscv32 riscv64 s390x sh2eb sh4 sh4eb x32 x86_64
</p></b></blockquote>

<p>(A long time ago I
<a href=http://landley.net/aboriginal/architectures.html>tried to explain</a>
what some of these architectures were.)</p>

<p>You can build all the targets at once, and can add additonal packages
to the build, by calling the script directly and listing packages on
the command line:</p>

<blockquote>
<p><b>mkroot/mkroot.sh CROSS=all LINUX=~/linux dropbear</b></p>
</blockquote>

<p>An example package build script (building the dropbear ssh server, adding a
port forward from 127.0.0.1:2222 to the qemu command line, and providing a
ssh2dropbear.sh convenience script to the output directory) is provided
in the mkroot/packages directory. If you add your own scripts elsewhere, just
give a path to them on the command line. (No, I'm not merging more package build
scripts, I <a href=https://speakerdeck.com/landley/developing-for-non-x86-targets-using-qemu?slide=78>learned that lesson</a> long ago. But if you
want to write your own, feel free.)</p>

<p>(Note: currently mkroot.sh cheats. If you don't have a .config it'll
make defconfig and add CONFIG_SH and CONFIG_ROUTE to it, because the new
root filesystem kinda needs those commands to function properly. If you already
have a .config that
_doesn't_ have CONFIG_SH in it, you won't get a shell prompt or be able to run
the init script without a shell. This is currently a problem because sh
and route are still in pending and thus not in defconfig, so "make root"
cheats and adds them. I'm working on it. tl;dr if make root doesn't work
"rm .config" and run it again, and all this should be fixed up in future when
those two commands are promoted out of pending so "make defconfig" would have
what you need anyway. It's designed to let yout tweak your config, which is
why it uses the .config that's there when there is one, but the default is
currently wrong because it's not quite finished yet. All this should be
cleaned up in a future release, before 1.0.)</p>

<hr /><h2><a name="cttyhack" />Q: Why doesn't toybox have cttyhack?</h2></li>

<p>A: Because it's unnecessary (it has "hack" in the name). Here's what
mkroot does in its PID 1 init script instead (after mounting /sys and /dev):</p>

<blockquote><p><b>
trap '' CHLD<br />
CONSOLE=$(sed '$s@.*/@@' /sys/class/tty/console/active)<br />
: ${HANDOFF:=/bin/sh}<br />
setsid -c &lt;&gt;/dev/$CONSOLE >&0 2>&1 $HANDOFF<br />
reboot -f &amp;<br />
sleep 5<br />
</b></p></blockquote>

<p>The "<b>trap</b>" tells the shell to accept and discard exiting child
processes (so zombies don't accumulate).
Child processes whose parents have already exited get reparented to init
(I.E. pid 1) and the shell script is sticking around as PID 1.
Setting SIGCHLD to SIG_IGN (which trap with an empty string does)
prevents them from waiting around in Z state to deliver their exit status
in case the parent ever gets around to calling wait().</p>

<p><b>$CONSOLE</b> fishes the underlying console device behind /dev/console out
of sysfs, because the linux kernel's /dev/console device can't act as a
controlling tty (for some reason). Since there may be more than one, and it
might or might not have a /dev/ prefix, we use <b>sed</b> to take the last
entry and remove any path.</p>

<p><b>$HANDOFF</b> is the child program to run, and the third line above
gives it the default value of /bin/sh if it wasn't already set on the
kernel command line. The bash ${NAME:=default value} syntax assigns a default
value to blank environment variables (see the bash man page) and : is a synonym
for the "<b>true</b>" command which ignores its arguments, so this combination is a
quick way to assign default values to blank variables. You can set $HANDOFF on
the kernel command line via "<b>KARGS='HANDOFF=cal' ./run-qemu.sh</b>"
since the <b>run-qemu.sh</b> script appends $KARGS to the end of the kernel
command line when launching QEMU, and unrecognized linux kernel command line
arguments with an = in them are treated as variable assignments exported into
PID 1's environment.</p>

<p>The "<b>setsid</b>" command runs a command in a new session (see "man 7
credentials") and the -c option makes stdin the controling TTY for the new
session. The first redirect points stdin at the new console device (the
<b>&lt;&gt;</b> redirect opens the file for both reading and writing at
the same time) and the second and third redirects duplicate the stdin
file descriptor to stdout and stderr. Redirects are guaranteed to be evaluated
from left to right, and all redirects happen before launching the command,
so -c grabs the new TTY device as the child's controlling tty.</p>

<p>When the child process setsid launched exits (usually by using the shell's
builtin "exit" command) the PID 1 shell script resumes and calls
"<b>reboot</b>" to exit qemu. Ordinarily the reboot command sends SIGTERM
to PID 1, but that won't do anything useful here, so we give it the -f option to
force it to call the reboot() syscall directly (see man 2 reboot). For
some reason the Linux reboot() syscall exits the process instead of blocking,
and if PID 1 exits the kernel panics, which aborts the reboot process, so
we background the reboot request into a child process and <b>sleep 5</b>
to give the reboot time to finish.</p>

<p>Toybox also has a <b>oneit</b> command that can do all this, and has a -3
option which hands off daemon management to a child process by writing each
exiting orphaned task's PID to the child's file descriptor 3 (the next
available on after stdin, stdout, and stderr). It can also respawn its
child (instead of halting or rebooting) when it exits, but you could add
a loop to the shell script easily enough.</p>
</li>
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