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 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).
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).
fakeroot is a regular, non-setuid program. It does not enhance a user's privileges, or decrease the system's security.
/usr/lib/*/libfakeroot-*.so The shared library containing the wrapper functions.
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'.
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 <email@example.com> Rather a lot mods/additions by joost and Clint.
faked(1) dpkg-buildpackage(1), debuild(1) /usr/share/doc/fakeroot/DEBUG