update-binfmts Man page

UPDATE-BINFMTS(8) Debian System Manager’s Manual UPDATE-BINFMTS(8)


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] update-binfmts [options] –find [path]


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 when‐
ever 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 data‐
base 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.



–package package-name
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

–admindir directory
Specifies the administrative directory, when this is to be differ‐
ent from the default of /var/lib/binfmts.

–importdir directory
Specifies the directory from which packaged binary formats are
imported, when this is to be different from the default of

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

–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 “.”, “..”, “reg‐
ister”, or “status”, as these are used by the filesystem or the
binfmt_misc module.

–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 [name] 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 [name] Display any information held in the database about the binary for‐
mat identifier name, or about all known binary formats if no name
is given. Also show whether displayed binary formats are enabled
or disabled.

–enable [name] 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 [name] Disable binary format name, or all known binary formats if no name
is given, in the kernel, thus disabling direct execution of match‐
ing files. You must have binfmt_misc compiled into the kernel or
loaded as a module for this to work.

–find [path] Print the list of interpreters that will be tried in sequence when
attempting to execute path, one per line. The first one for which
execvp(3) succeeds will be used.

Note that if multiple formats match an executable, then the order
is in general not defined, and may not be preserved between
update-binfmts operations, so you should generally try to ensure
that this option prints at most one line for any given path. The
exception to this is that any format with a userspace detector will
be run before any format without a userspace detector.

–magic byte-sequence
This matches all files with the magic number byte-sequence. Hexa‐
decimal escapes may be included in the byte-sequence by preceding
them with \x, for example ‘\x0a’ for a linefeed. Remember to pro‐
tect such escapes with quotes or an additional backslash to prevent
their interpretation by the shell.

Also see –offset and –mask.

–offset offset
This is the offset of the magic/mask in the file, counted in bytes.
The default is 0. Only valid with –magic.

–mask byte-sequence
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.

–extension extension
This matches all files whose names end in “.extension”. Hexadeci‐
mal escapes are not recognized here. Extension matching is case-

–detector path
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 han‐
dled by the kernel’s format specifications alone. The program
should return an exit code of zero if the file is appropriate and
non-zero otherwise.

–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.

–preserve yes, –preserve no
Whether to preserve the original argv[0] when running the inter‐
preter, rather than overwriting it with the full path to the

A format file is a sequence of options, one per line, corresponding
roughly to the options given to an –install command. Each option con‐
sists 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,
credentials, and preserve options correspond to the command-line options
of the same names.

0 The requested action was successfully performed.

2 Problems were encountered whilst parsing the command line or per‐
forming the action.

This format file can be used with an interpreter capable of handling Java
.class files:

package javawrapper
interpreter /usr/bin/javawrapper
magic \xca\xfe\xba\xbe

This corresponds roughly to the following command:

update-binfmts –package javawrapper \
–install javawrapper /usr/bin/javawrapper \
–magic ‘\xca\xfe\xba\xbe’

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, 2001, 2002, 2003, 2004, 2005, 2006,
2007, 2008, 2009, 2010, 2011 Colin Watson . See the
GNU General Public License version 3 or later for copying conditions.

You can find the GNU GPL v3 in /usr/share/common-licenses/GPL-3 on any
modern Debian system.

Richard Guenther wrote the binfmt_misc kernel module.

Ian Jackson wrote update-alternatives and dpkg-divert, from which this
program borrows heavily.

Debian January 24, 2011 Debian