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NAME
file - determine file type
SYNOPSIS
file [ -bciknsvzL ] [ -f namefile ] [ -m magicfiles ] file
...
DESCRIPTION
This manual page documents version 3.31 of the file command.
File tests each argument in an attempt to classify it.
There are three sets of tests, performed in this order:
filesystem tests, magic number tests, and language tests.
The first test that succeeds causes the file type to be
printed.
The type printed will usually contain one of the words text
(the file contains only ASCII characters and is probably
safe to read on an ASCII terminal), executable (the file
contains the result of compiling a program in a form under-
standable to some UNIX kernel or another), or data meaning
anything else (data is usually `binary' or non-printable).
Exceptions are well-known file formats (core files, tar
archives) that are known to contain binary data. When modi-
fying the file /opt/sfw/share/magic or the program itself,
preserve these keywords . People depend on knowing that all
the readable files in a directory have the word ``text''
printed. Don't do as Berkeley did - change ``shell commands
text'' to ``shell script''.
The filesystem tests are based on examining the return from
a stat(2) system call. The program checks to see if the
file is empty, or if it's some sort of special file. Any
known file types appropriate to the system you are running
on (sockets, symbolic links, or named pipes (FIFOs) on those
systems that implement them) are intuited if they are
defined in the system header file sys/stat.h.
The magic number tests are used to check for files with data
in particular fixed formats. The canonical example of this
is a binary executable (compiled program) a.out file, whose
format is defined in a.out.h and possibly exec.h in the
standard include directory. These files have a `magic
number' stored in a particular place near the beginning of
the file that tells the UNIX operating system that the file
is a binary executable, and which of several types thereof.
The concept of `magic number' has been applied by extension
to data files. Any file with some invariant identifier at a
small fixed offset into the file can usually be described in
this way. The information in these files is read from the
magic file /opt/sfw/share/magic.
If an argument appears to be an ASCII file, file attempts to
guess its language. The language tests look for particular
strings (cf names.h) that can appear anywhere in the first
few blocks of a file. For example, the keyword .br indi-
cates that the file is most likely a troff(1) input file,
just as the keyword struct indicates a C program. These
tests are less reliable than the previous two groups, so
they are performed last. The language test routines also
test for some miscellany (such as tar(1) archives) and
determine whether an unknown file should be labelled as
`ascii text' or `data'.
OPTIONS
-b Do not prepend filenames to output lines (brief
mode).
-c Cause a checking printout of the parsed form of the
magic file. This is usually used in conjunction
with -m to debug a new magic file before installing
it.
-f namefile
Read the names of the files to be examined from
namefile (one per line) before the argument list.
Either namefile or at least one filename argument
must be present; to test the standard input, use
``-'' as a filename argument.
-i Causes the file command to output mime type strings
rather than the more traditional human readable
ones. Thus it may say "text/plain, ASCII" rather
than "ASCII text". In order for this option to work,
file changes the way it handles files recognised by
the command it's self (such as many of the text file
types, directories etc), and makes use of an alter-
native "magic" file. (See "FILES" section, below).
-k Don't stop at the first match, keep going.
-m list Specify an alternate list of files containing magic
numbers. This can be a single file, or a colon-
separated list of files.
-n Force stdout to be flushed after check a file. This
is only useful if checking a list of files. It is
intended to be used by programs want filetype output
from a pipe.
-v Print the version of the program and exit.
-z Try to look inside compressed files.
-L option causes symlinks to be followed, as the like-
named option in ls(1). (on systems that support
symbolic links).
-s Normally, file only attempts to read and determine
the type of argument files which stat(2) reports are
ordinary files. This prevents problems, because
reading special files may have peculiar conse-
quences. Specifying the -s option causes file to
also read argument files which are block or charac-
ter special files. This is useful for determining
the filesystem types of the data in raw disk parti-
tions, which are block special files. This option
also causes file to disregard the file size as
reported by stat(2) since on some systems it reports
a zero size for raw disk partitions.
FILES
/opt/sfw/share/magic - default list of magic numbers
/opt/sfw/share/magic.mime - default list of magic numbers,
used to output mime types when the -i option is specified.
ENVIRONMENT
The environment variable MAGIC can be used to set the
default magic number files.
SEE ALSO
magic(4) - description of magic file format.
strings(1), od(1), hexdump(1) - tools for examining non-
textfiles.
STANDARDS CONFORMANCE
This program is believed to exceed the System V Interface
Definition of FILE(CMD), as near as one can determine from
the vague language contained therein. Its behaviour is
mostly compatible with the System V program of the same
name. This version knows more magic, however, so it will
produce different (albeit more accurate) output in many
cases.
The one significant difference between this version and Sys-
tem V is that this version treats any white space as a del-
imiter, so that spaces in pattern strings must be escaped.
For example,
>10 string language impress (imPRESS data)
in an existing magic file would have to be changed to
>10 string language\ impress (imPRESS data)
In addition, in this version, if a pattern string contains a
backslash, it must be escaped. For example
0 string \begindata Andrew Toolkit document
in an existing magic file would have to be changed to
0 string \\begindata Andrew Toolkit document
SunOS releases 3.2 and later from Sun Microsystems include a
file(1) command derived from the System V one, but with some
extensions. My version differs from Sun's only in minor
ways. It includes the extension of the `&' operator, used
as, for example,
>16 long&0x7fffffff >0 not stripped
MAGIC DIRECTORY
The magic file entries have been collected from various
sources, mainly USENET, and contributed by various authors.
Christos Zoulas (address below) will collect additional or
corrected magic file entries. A consolidation of magic file
entries will be distributed periodically.
The order of entries in the magic file is significant.
Depending on what system you are using, the order that they
are put together may be incorrect. If your old file command
uses a magic file, keep the old magic file around for com-
parison purposes (rename it to /opt/sfw/share/magic.orig).
EXAMPLES
$ file file.c file /dev/hda
file.c: C program text
file: ELF 32-bit LSB executable, Intel 80386, version 1,
dynamically linked, not stripped
/dev/hda: block special
$ file -s /dev/hda{,1,2,3,4,5,6,7,8,9,10}
/dev/hda: x86 boot sector
/dev/hda1: Linux/i386 ext2 filesystem
/dev/hda2: x86 boot sector
/dev/hda3: x86 boot sector, extended partition table
/dev/hda4: Linux/i386 ext2 filesystem
/dev/hda5: Linux/i386 swap file
/dev/hda6: Linux/i386 swap file
/dev/hda7: Linux/i386 swap file
/dev/hda8: Linux/i386 swap file
/dev/hda9: empty
/dev/hda10: empty
$ file -i file.c file /dev/hda
file.c: text/x-c
file: application/x-executable, dynamically linked (uses shared libs), not stripped
/dev/hda: application/x-not-regular-file
HISTORY
There has been a file command in every UNIX since at least
Research Version 6 (man page dated January, 1975). The
System V version introduced one significant major change:
the external list of magic number types. This slowed the
program down slightly but made it a lot more flexible.
This program, based on the System V version, was written by
Ian Darwin without looking at anybody else's source code.
John Gilmore revised the code extensively, making it better
than the first version. Geoff Collyer found several inade-
quacies and provided some magic file entries. The program
has undergone continued evolution since.
AUTHOR
Written by Ian F. Darwin, UUCP address {utzoo |
ihnp4}!darwin!ian, Internet address ian@sq.com, postal
address: P.O. Box 603, Station F, Toronto, Ontario, CANADA
M4Y 2L8.
Altered by Rob McMahon, cudcv@warwick.ac.uk, 1989, to extend
the `&' operator from simple `x&y != 0' to `x&y op z'.
Altered by Guy Harris, guy@netapp.com, 1993, to:
put the ``old-style'' `&' operator back the way it was,
because 1) Rob McMahon's change broke the previous
style of usage, 2) the SunOS ``new-style'' `&' opera-
tor, which this version of file supports, also handles
`x&y op z', and 3) Rob's change wasn't documented in
any case;
put in multiple levels of `>';
put in ``beshort'', ``leshort'', etc. keywords to look
at numbers in the file in a specific byte order, rather
than in the native byte order of the process running
file.
Changes by Ian Darwin and various authors including Christos
Zoulas (christos@astron.com), 1990-1999.
Altered by Chris Lowth, chris@lowth.com, 2000: Handle the
"-i" option to output mime type strings and using an alter-
native magic file and internal logic.
LEGAL NOTICE
Copyright (c) Ian F. Darwin, Toronto, Canada, 1986, 1987,
1988, 1989, 1990, 1991, 1992, 1993.
This software is not subject to and may not be made subject
to any license of the American Telephone and Telegraph Com-
pany, Sun Microsystems Inc., Digital Equipment Inc., Lotus
Development Inc., the Regents of the University of
California, The X Consortium or MIT, or The Free Software
Foundation.
This software is not subject to any export provision of the
United States Department of Commerce, and may be exported to
any country or planet.
Permission is granted to anyone to use this software for any
purpose on any computer system, and to alter it and redis-
tribute it freely, subject to the following restrictions:
1. The author is not responsible for the consequences of use
of this software, no matter how awful, even if they arise
from flaws in it.
2. The origin of this software must not be misrepresented,
either by explicit claim or by omission. Since few users
ever read sources, credits must appear in the documentation.
3. Altered versions must be plainly marked as such, and must
not be misrepresented as being the original software. Since
few users ever read sources, credits must appear in the
documentation.
4. This notice may not be removed or altered.
A few support files (getopt, strtok) distributed with this
package are by Henry Spencer and are subject to the same
terms as above.
A few simple support files (strtol, strchr) distributed with
this package are in the public domain; they are so marked.
The files tar.h and is_tar.c were written by John Gilmore
from his public-domain tar program, and are not covered by
the above restrictions.
BUGS
There must be a better way to automate the construction of
the Magic file from all the glop in Magdir. What is it?
Better yet, the magic file should be compiled into binary
(say, ndbm(3) or, better yet, fixed-length ASCII strings for
use in heterogenous network environments) for faster
startup. Then the program would run as fast as the Version
7 program of the same name, with the flexibility of the Sys-
tem V version.
File uses several algorithms that favor speed over accuracy,
thus it can be misled about the contents of ASCII files.
The support for ASCII files (primarily for programming
languages) is simplistic, inefficient and requires
recompilation to update.
There should be an ``else'' clause to follow a series of
continuation lines.
The magic file and keywords should have regular expression
support. Their use of ASCII TAB as a field delimiter is
ugly and makes it hard to edit the files, but is entrenched.
It might be advisable to allow upper-case letters in key-
words for e.g., troff(1) commands vs man page macros. Regu-
lar expression support would make this easy.
The program doesn't grok FORTRAN. It should be able to fig-
ure FORTRAN by seeing some keywords which appear indented at
the start of line. Regular expression support would make
this easy.
The list of keywords in ascmagic probably belongs in the
Magic file. This could be done by using some keyword like
`*' for the offset value.
Another optimisation would be to sort the magic file so that
we can just run down all the tests for the first byte, first
word, first long, etc, once we have fetched it. Complain
about conflicts in the magic file entries. Make a rule that
the magic entries sort based on file offset rather than
position within the magic file?
The program should provide a way to give an estimate of
``how good'' a guess is. We end up removing guesses (e.g.
``From '' as first 5 chars of file) because they are not as
good as other guesses (e.g. ``Newsgroups:'' versus "Return-
Path:"). Still, if the others don't pan out, it should be
possible to use the first guess.
This program is slower than some vendors' file commands.
This manual page, and particularly this section, is too
long.
AVAILABILITY
You can obtain the original author's latest version by
anonymous FTP on ftp.astron.com in the directory
/pub/file/file-X.YY.tar.gz
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