fakeroot(1) Debian manual fakeroot(1)
fakeroot - run a command in an environment faking root privileges for file manipulation
fakeroot [-l|--lib library] [--faked faked-binary] [-i load-file] [-s save-file]
[-u|--unknown-is-real ] [-b|--fd-base ] [-h|--help ] [-v|--version ] [--] [command]
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 ar-
chives directly, without 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 really been root. These wrapper functions are in a shared library /usr/lib/libfake-
root.so* which is loaded through the LD_PRELOAD mechanism of the dynamic loader. (See
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
fakeroot versions). If those tests fail (for example because you have certain libc5 pro-
grams 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 fake-
root. Especially configure and friends don't like it when the system suddenly behaves dif-
ferently from what they expect. (or, they randomly unset some environment variables, some
of which fakeroot needs).
-l library, --lib library
Specify an alternative wrapper library.
Specify an alternative binary to use as faked.
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.
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 information without needing to be root. See
/usr/share/doc/fakeroot/README.saving for more details.
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.
Use the real ownership of files previously unknown to fakeroot instead of pretend-
ing 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.
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.
$ fakeroot /bin/bash
# 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 /
$ 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).
fakeroot is a regular, non-setuid program. It does not enhance a user's privileges, or
decrease the system's security.
/usr/lib/libfakeroot/libfakeroot.so* The shared library containing the wrapper functions.
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/own-
erships. (assuming the daemon and connecting program were started by the same
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
intercepted 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
Every command executed within fakeroot needs to be linked to the same version of
the C library as fakeroot itself.
fakeroot doesn't wrap open(), create(), etc. So, if user joost does either
ls -al foo
or the other way around,
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 prob-
lems, 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 num-
ber of functions wrapped by fakeroot as small as possible, to limit the likelihood
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.
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).
fakeroot is distributed under the GNU General Public License. (GPL 2.0 or greater).
mostly by J.H.M. Dassen <firstname.lastname@example.org> Rather a lot mods/additions by joost and
faked(1) dpkg-buildpackage(1), debuild(1) /usr/share/doc/fakeroot/DEBUG
Debian Project 6 August 2004 fakeroot(1)