Ubuntu Manpage: fakeroot - run a command in an environment faking root privileges for file manipulation

NAME

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

SYNOPSIS

       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]

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, 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/*/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 fakeroot 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 information 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  decrease  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/ownerships.  (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 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 LD_LIBRARY_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@debian.org>

       Clint Adams
              <clint@debian.org>

       Timo Savola

MANUAL PAGE

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