update-binfmts - maintain registry of executable binary formats
update-binfmts [options] --install name path spec
update-binfmts [options] --remove name path
update-binfmts [options] --import [name]
update-binfmts [options] --display [name]
update-binfmts [options] --enable [name]
update-binfmts [options] --disable [name]
Versions 2.1.43 and later of the Linux kernel have contained the
binfmt_misc module. This enables a system administrator to register
interpreters for various binary formats based on a magic number or their
file extension, and cause the appropriate interpreter to be invoked
whenever a matching file is executed. Think of it as a more flexible
version of the #! executable interpreter mechanism, or as something which
can behave a little like "associations" in certain other operating
systems (though in GNU/Linux the tendency is to keep this sort of thing
somewhere else, like your file manager). update-binfmts manages a
persistent database of these interpreters.
When each package providing a registered interpreter is installed,
changed, or removed, update-binfmts is called to update information about
that interpreter. update-binfmts is usually called from the postinst or
prerm scripts in Debian packages.
Exactly one action must be specified; this may be accompanied by any one
of the common options.
Specifies the name of the current package, to be used by package
post-installation and pre-removal scripts. System administrators
installing binary formats for local use should probably ignore this
When installing new formats, the --import action should be used
Specifies the administrative directory, when this is to be
different from the default of /var/lib/binfmts.
Specifies the directory from which packaged binary formats are
imported, when this is to be different from the default of
Specifies the directory in which binary format registration
instructions are cached, when this is to be different from the
default of /var/cache/binfmts.
Don’t do anything, just demonstrate what would be done.
Display some usage information.
Display version information.
--install name path spec
Install a binary format identified by name with interpreter path
into the database. After registration, this format will be used
when the kernel tries to execute a file matching spec (see BINARY
FORMAT SPECIFICATIONS below).
--install will attempt to enable this binary format in the kernel
as well as adding it to its own database; see --enable below.
You cannot install a format with any of the names ".", "..",
"register", or "status", as these are used by the filesystem or the
--remove name path
Remove the binary format identified by name with interpreter path
from the database. This will also attempt to disable the binary
format in the kernel; see --disable below.
Import a packaged format file called name, or import all format
files currently on the system if no name is given. If name is not
a full path, it is assumed to be a file in the import directory
(/usr/share/binfmts by default). See FORMAT FILES below for the
required contents of these files.
For packages, this is preferable to using the --install option, as
a format file can be installed without update-binfmts needing to be
Display any information held in the database about the binary
format identifier name, or about all known binary formats if no
name is given. Also show whether displayed binary formats are
enabled or disabled.
Enable binary format name, or all known binary formats if no name
is given, in the kernel, thus enabling direct execution of matching
files. You must have binfmt_misc compiled into the kernel or
loaded as a module for this to work.
Disable binary format name, or all known binary formats if no name
is given, in the kernel, thus disabling direct execution of
matching files. You must have binfmt_misc compiled into the kernel
or loaded as a module for this to work.
BINARY FORMAT SPECIFICATIONS
This matches all files with the magic number byte-sequence.
Hexadecimal escapes may be included in the byte-sequence by
preceding them with \x, for example ‘\x0a’ for a linefeed.
Remember to protect such escapes with quotes or an additional
backslash to prevent their interpretation by the shell.
Also see --offset and --mask.
This is the offset of the magic/mask in the file, counted in bytes.
The default is 0. Only valid with --magic.
This mask will be logically-ANDed with the string to be checked
against the magic number given with --magic. The default is all
0xff, i.e. no effect. Only valid with --magic.
This matches all files whose names end in ".extension".
Hexadecimal escapes are not recognized here. Extension matching is
If this option is used, a userspace detector program will be used
to check whether the file is suitable for this interpreter. This
may be used when the binary format is more complex than can be
handled by the kernel’s format specifications alone. The program
should return an exit code of zero if the file is appropriate and
--credentials yes, --credentials no
Whether to keep the credentials of the original binary to run the
interpreter; this is typically useful to run setuid binaries, but
has security implications.
A format file is a sequence of options, one per line, corresponding
roughly to the options given to an --install command. Each option
consists of a key, followed by whitespace, followed by a value.
The package option should be set to the current package. The interpreter
option should be set to the path to the interpreter that will handle this
binary format. The magic, offset, mask, extension, detector, and
credentials options correspond to the command-line options of the same
0 The requested action was successfully performed.
2 Problems were encountered whilst parsing the command line or
performing the action.
This format file can be used with an interpreter capable of handling Java
This corresponds roughly to the following command:
update-binfmts --package javawrapper \
--install javawrapper /usr/bin/javawrapper \
If you’re not careful, you can break your system with update-binfmts. An
easy way to do this is to register an ELF binary as a handler for ELF,
which will almost certainly cause your system to hang immediately; even
if it doesn’t, you won’t be able to run update-binfmts to fix it. In the
future update-binfmts may have some checks to prevent this sort of thing
happening accidentally, though of course you can still manipulate the
binfmt_misc kernel module directly.
update-binfmts is copyright (c) 2000-2002 Colin Watson
〈firstname.lastname@example.org〉. See the GNU General Public License version 2 or
later for copying conditions.
You can find the GNU GPL in /usr/share/common-licenses/GPL on any modern
Richard Guenther wrote the binfmt_misc kernel module.
Ian Jackson wrote update-alternatives and dpkg-divert, from which this
program borrows heavily.