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NAME
file - determine file type
SYNOPSIS
file [ -bciknsvzL ] [ -f namefile ] [ -m magicfiles ] file
file -C [ -m magicfile ]
DESCRIPTION
This manual page documents version 3.39 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 printing characters and a few common control characters
and is probably safe to read on an ASCII terminal), executable (the
file contains the result of compiling a program in a form understand-
able 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 /usr/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 and change ``shell commands text'' to ``shell
script''. Note that the file /usr/share/magic is built mechanically
from a large number of small files in the subdirectory Magdir in the
source distribution of this program.
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 partic-
ular fixed formats. The canonical example of this is a binary exe-
cutable (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 identi-
fying these files is read from the compiled magic file
/usr/share/magic.mgc , or /usr/share/magic if the compile file does not
exist.
If a file does not match any of the entries in the magic file, it is
examined to see if it seems to be a text file. ASCII, ISO-8859-x, non-
ISO 8-bit extended-ASCII character sets (such as those used on Macin-
tosh and IBM PC systems), UTF-8-encoded Unicode, UTF-16-encoded Uni-
code, and EBCDIC character sets can be distinguished by the different
ranges and sequences of bytes that constitute printable text in each
set. If a file passes any of these tests, its character set is
reported. ASCII, ISO-8859-x, UTF-8, and extended-ASCII files are iden-
tified as ``text'' because they will be mostly readable on nearly any
terminal; UTF-16 and EBCDIC are only ``character data'' because, while
they contain text, it is text that will require translation before it
can be read. In addition, file will attempt to determine other charac-
teristics of text-type files. If the lines of a file are terminated by
CR, CRLF, or NEL, instead of the Unix-standard LF, this will be
reported. Files that contain embedded escape sequences or overstriking
will also be identified.
Once file has determined the character set used in a text-type file, it
will attempt to determine in what language the file is written. 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 indicates 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).
Any file that cannot be identified as having been written in any of the
character sets listed above is simply said to be ``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.
-C Write a magic.mgc output file that contains a pre-parsed ver-
sion of file.
-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 stan-
dard 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; charset=us-ascii'' rather than ``ASCII text''. In
order for this option to work, file changes the way it handles
files recognised by the command itself (such as many of the
text file types, directories etc), and makes use of an alterna-
tive ``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 checking each file. This is
only useful if checking a list of files. It is intended to be
used by programs that 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 pecu-
liar consequences. Specifying the -s option causes file to
also read argument files which are block or character special
files. This is useful for determining the filesystem types of
the data in raw disk partitions, 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
/usr/share/magic.mgc - defaults compiled list of magic numbers
/usr/share/magic - default list of magic numbers
/usr/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(5) - 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 con-
tained 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 exten-
sion of the `' operator, used as, for example,
16 long0x7fffffff 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 con-
solidation 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 comparison purposes (rename it to
/usr/share/magic.orig).
EXAMPLES
$ file file.c file /dev/{wd0a,hda}
file.c: C program text
file: ELF 32-bit LSB executable, Intel 80386, version 1 (SYSV),
dynamically linked (uses shared libs), stripped
/dev/wd0a: block special (0/0)
/dev/hda: block special (3/0)
$ file -s /dev/wd0{b,d}
/dev/wd0b: data
/dev/wd0d: x86 boot sector
$ 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/{wd0a,hda}
file.c: text/x-c
file: application/x-executable, dynamically linked (uses shared libs),
not stripped
/dev/hda: application/x-not-regular-file
/dev/wd0a: application/x-not-regular-file
HISTORY
There has been a file command in every UNIX since at least Research
Version 4 (man page dated November, 1973). The System V version intro-
duced 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
ian@darwinsys.com 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 inadequacies and provided
some magic file entries. Contributions by the `' operator by Rob McMa-
hon, cudcv@warwick.ac.uk, 1989.
Guy Harris, guy@netapp.com, made many changes from 1993 to the present.
Primary development and maintenance from 1990 to the present by Chris-
tos Zoulas (christos@astron.com).
Altered by Chris Lowth, chris@lowth.com, 2000: Handle the ``-i'' option
to output mime type strings and using an alternative magic file and
internal logic.
Altered by Eric Fischer (enf@pobox.com), July, 2000, to identify char-
acter codes and attempt to identify the languages of non-ASCII files.
The list of contributors to the "Magdir" directory (source for the
/etc/magic file) is too long to include here. You know who you are;
thank you.
LEGAL NOTICE
Copyright (c) Ian F. Darwin, Toronto, Canada, 1986-1999. Covered by
the standard Berkeley Software Distribution copyright; see the file
LEGAL.NOTICE in the source distribution.
The files tar.h and is_tar.c were written by John Gilmore from his pub-
lic-domain tar program, and are not covered by the above license.
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 environ-
ments) for faster startup. Then the program would run as fast as the
Version 7 program of the same name, with the flexibility of the System
V version.
File uses several algorithms that favor speed over accuracy, thus it
can be misled about the contents of text files.
The support for text 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 keywords 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
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. ``News-
groups:'' 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. The new sup-
port for multiple character codes makes it even slower.
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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