FILE(1) FILE(1)
NAME
file - determine file type
SYNOPSIS
file [ -vczL ] [ -f namefile ] [ -m magicfiles ] file ...
DESCRIPTION
This manual page documents version 3.22 of the file com-
mand. 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 prob-
ably safe to read on an ASCII terminal), executable (the
file contains the result of compiling a program in a form
understandable 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 modifying the file /etc/magic or the program
itself, preserve these keywords . People depend on know-
ing 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 informa-
tion in these files is read from the magic file
/etc/magic.
If an argument appears to be an ASCII file, file attempts
to guess its language. The language tests look for par-
ticular strings (cf names.h) that can appear anywhere in
the first few blocks of a file. For example, the keyword
.br indicates that the file is most likely a troff(1)
input file, just as the keyword struct indicates a C pro-
gram. 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
-v Print the version of the program and exit.
-m list Specify an alternate list of files containing
magic numbers. This can be a single file, or a
colon-separated list of files.
-z Try to look inside compressed files.
-c Cause a checking printout of the parsed form of
the magic file. This is usually used in conjunc-
tion 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.
-L option causes symlinks to be followed, as the
like-named option in ls(1). (on systems that sup-
port symbolic links).
FILES
/etc/magic - default list of magic numbers
ENVIRONMENT
The environment variable MAGIC can be used to set the
default magic number files.
SEE ALSO
magic(5) - description of magic file format.
strings(1), od(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
System V is that this version treats any white space as a
delimiter, 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 `&' opera-
tor, 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 consolida-
tion of magic file entries will be distributed periodi-
cally.
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 comparison purposes (rename it to /etc/magic.orig).
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 bet-
ter than the first version. Geoff Collyer found several
inadequacies 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@auspex.com, 1993, to:
put the ``old-style'' `&' operator back the way it
was, because 1) Rob McMahon's change broke the pre-
vious style of usage, 2) the SunOS ``new-style''
`&' operator, 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 Chris-
tos Zoulas (christos@deshaw.com), 1990-1997.
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 sub-
ject to any license of the American Telephone and Tele-
graph Company, Sun Microsystems Inc., Digital Equipment
Inc., Lotus Development Inc., the Regents of the Univer-
sity 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
redistribute it freely, subject to the following restric-
tions:
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 documenta-
tion.
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 Ver-
sion 7 program of the same name, with the flexibility of
the System V version.
File uses several algorithms that favor speed over accu-
racy, 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 recom-
pilation 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.
Regular expression support would make this easy.
The program doesn't grok FORTRAN. It should be able to
figure 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 off-
set 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.deshaw.com in the directory
/pub/file/file-X.YY.tar.gz
Copyrighted but distributable FILE(1)