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fakeroot(1) Debian manual fakeroot(1)

NAME
fakeroot - run a command in an environment faking root privileges for file manipulation

DESCRIPTION
fakeroot runs a command in an environment wherein it appears to have root privileges for file
manipulation. This is useful for allowing users to create archives (tar, ar, .deb etc.) with
files in them with root permissions/ownership. Without fakeroot one would need to have root
privileges to create the constituent files of the archives with the correct permissions and
ownership, and then pack them up, or one would have to construct the archives directly, with‐
out using the archiver.

fakeroot works by replacing the file manipulation library functions (chmod(2), stat(2) etc.)
by ones that simulate the effect the real library functions would have had, had the user re‐
ally been root. These wrapper functions are in a shared library /usr/lib/*/libfakeroot-*.so
or similar location on your platform. The shared object is loaded through the LD_PRELOAD
mechanism of the dynamic loader. (See ld.so(8))

If you intend to build packages with fakeroot, please try building the fakeroot package
first: the "debian/rules build" stage has a few tests (testing mostly for bugs in old fake‐
root versions). If those tests fail (for example because you have certain libc5 programs on
your system), other packages you build with fakeroot will quite likely fail too, but possibly
in much more subtle ways.

Also, note that it's best not to do the building of the binaries themselves under fakeroot.
Especially configure and friends don't like it when the system suddenly behaves differently
from what they expect. (or, they randomly unset some environment variables, some of which
fakeroot needs).

OPTIONS
-l library, --lib library
Specify an alternative wrapper library.

--faked binary
Specify an alternative binary to use as faked.

[--] command
Any command you want to be ran as fakeroot. Use ‘--’ if in the command you have other
options that may confuse fakeroot's option parsing.

-s save-file
Save the fakeroot environment to save-file on exit. This file can be used to restore
the environment later using -i. However, this file will leak and fakeroot will behave
in odd ways unless you leave the files touched inside the fakeroot alone when outside
the environment. Still, this can be useful. For example, it can be used with rsync(1)
to back up and restore whole directory trees complete with user, group and device in‐
formation without needing to be root. See /usr/share/doc/fakeroot/README.saving for
more details.

-i load-file
Load a fakeroot environment previously saved using -s from load-file. Note that this
does not implicitly save the file, use -s as well for that behaviour. Using the same
file for both -i and -s in a single fakeroot invocation is safe.

-u, --unknown-is-real
Use the real ownership of files previously unknown to fakeroot instead of pretending
they are owned by root:root.

-b fd Specify fd base (TCP mode only). fd is the minimum file descriptor number to use for
TCP connections; this may be important to avoid conflicts with the file descriptors
used by the programs being run under fakeroot.

-h Display help.

-v Display version.

EXAMPLES
Here is an example session with fakeroot. Notice that inside the fake root environment file
manipulation that requires root privileges succeeds, but is not really happening.

$ whoami
joost
$ fakeroot /bin/bash
# whoami
root
# mknod hda3 b 3 1
# ls -ld hda3
brw-r--r-- 1 root root 3, 1 Jul 2 22:58 hda3
# chown joost:root hda3
# ls -ld hda3
brw-r--r-- 1 joost root 3, 1 Jul 2 22:58 hda3
# ls -ld /
drwxr-xr-x 20 root root 1024 Jun 17 21:50 /
# chown joost:users /
# chmod a+w /
# ls -ld /
drwxrwxrwx 20 joost users 1024 Jun 17 21:50 /
# exit
$ ls -ld /
drwxr-xr-x 20 root root 1024 Jun 17 21:50 //
$ ls -ld hda3
-rw-r--r-- 1 joost users 0 Jul 2 22:58 hda3

Only the effects that user joost could do anyway happen for real.

fakeroot was specifically written to enable users to create Debian GNU/Linux packages (in the
deb(5) format) without giving them root privileges. This can be done by commands like dpkg-
buildpackage -rfakeroot or debuild -rfakeroot (actually, -rfakeroot is default in debuild
nowadays, so you don't need that argument).

SECURITY ASPECTS
fakeroot is a regular, non-setuid program. It does not enhance a user's privileges, or de‐
crease the system's security.

FILES
/usr/lib/*/libfakeroot-*.so The shared library containing the wrapper functions.

ENVIRONMENT
FAKEROOTKEY
The key used to communicate with the fakeroot daemon. Any program started with the
right LD_PRELOAD and a FAKEROOTKEY of a running daemon will automatically connect to
that daemon, and have the same "fake" view of the file system's permissions/owner‐
ships. (assuming the daemon and connecting program were started by the same user).

LD_LIBRARY_PATH

LD_PRELOAD
Fakeroot is implemented by wrapping system calls. This is accomplished by setting
LD_LIBRARY_PATH=/usr/lib/fakeroot and LD_PRELOAD=libfakeroot.so.0. That library is
loaded before the system's C library, and so most of the library functions are inter‐
cepted by it. If you need to set either LD_LIBRARY_PATH or LD_PRELOAD from within a
fakeroot environment, it should be set relative to the given paths, as in LD_LI����
BRARY_PATH=$LD_LIBRARY_PATH:/foo/bar/

LIMITATIONS
Library versions
Every command executed within fakeroot needs to be linked to the same version of the C
library as fakeroot itself.

open()/create()
fakeroot doesn't wrap open(), create(), etc. So, if user joost does either

touch foo
fakeroot
ls -al foo

or the other way around,

fakeroot
touch foo
ls -al foo

fakeroot has no way of knowing that in the first case, the owner of foo really should
be joost while the second case it should have been root. For the Debian packaging,
defaulting to giving all "unknown" files uid=gid=0, is always OK. The real way around
this is to wrap open() and create(), but that creates other problems, as demonstrated
by the libtricks package. This package wrapped many more functions, and tried to do a
lot more than fakeroot . It turned out that a minor upgrade of libc (from one where
the stat() function didn't use open() to one with a stat() function that did (in some
cases) use open()), would cause unexplainable segfaults (that is, the libc6 stat()
called the wrapped open(), which would then call the libc6 stat(), etc). Fixing them
wasn't all that easy, but once fixed, it was just a matter of time before another
function started to use open(), never mind trying to port it to a different operating
system. Thus I decided to keep the number of functions wrapped by fakeroot as small as
possible, to limit the likelihood of ‘collisions’.

GNU configure (and other such programs)
fakeroot, in effect, is changing the way the system behaves. Programs that probe the
system like GNU configure may get confused by this (or if they don't, they may stress
fakeroot so much that fakeroot itself becomes confused). So, it's advisable not to run
"configure" from within fakeroot. As configure should be called in the "debian/rules
build" target, running "dpkg-buildpackage -rfakeroot" correctly takes care of this.

BUGS
It doesn't wrap open(). This isn't bad by itself, but if a program does open("file",
O_WRONLY, 000), writes to file "file", closes it, and then again tries to open to read the
file, then that open fails, as the mode of the file will be 000. The bug is that if root does
the same, open() will succeed, as the file permissions aren't checked at all for root. I
choose not to wrap open(), as open() is used by many other functions in libc (also those that
are already wrapped), thus creating loops (or possible future loops, when the implementation
of various libc functions slightly change).

COPYING
fakeroot is distributed under the GNU General Public License. (GPL 2.0 or greater).

AUTHORS
joost witteveen
<joostje AT debian.org>

Clint Adams
<clint AT debian.org>

Timo Savola

MANUAL PAGE
mostly by J.H.M. Dassen <jdassen AT debian.org> Rather a lot mods/additions by joost and Clint.

SEE ALSO
faked(1) dpkg-buildpackage(1), debuild(1) /usr/share/doc/fakeroot/DEBUG

Debian Project 5 October 2014 fakeroot(1)