file - determine file type
file [-vbczL] [-f namefile] [-m magicfiles] file [...]
This manual page documents version 3.22 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 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 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 /etc/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 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) and determine whether an unknown file should be
labelled as
``ASCII text'' or ``data''.
The options are as follows:
-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.
-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.
-L Cause symlinks to be followed, as the like-named option in ls(1).
(on systems that support symbolic links).
MAGIC Default magic number files.
/etc/magic default list of magic numbers
hexdump(1), od(1), strings(1), magic(5)
STANDARDS CONFORMANCE [Toc] [Back] 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)
g
In addition, in this veriion, if a pattern string contains a
backslash, n
it must be escaped. Fordexample
a
0 string tw Toolkit document
a
in an existing magic file would have to be changed to
n
0 string \begindata Andrew Toolkit document
r
SunOS releases 3.2 and later from Sun Microsystems include a
file 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
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 comparison purposes (rename it to /etc/magic.orig).
There has been a file command in every UNIX since at least
Research Version
4 (man page dated November, 1973). 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 F. Darwin
<[email protected]> 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.
Altered by Rob McMahon <[email protected]>, 1989, to extend the `&' operator
from simple ``x&y != 0'' to ``x&y op z''.
Altered by Guy Harris <[email protected]>, 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'' `&' operator, which
this version
of file supports, also handles ``x&y op
z'',
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.
Currently maintained by Christos Zoulas <chris[email protected]>.
Copyright (c) Ian F. Darwin, Toronto, Canada, 1986-1999.
Covered by the
standard Berkeley Software Distribution copyright; see the
file LEGAL.NOTICE
in the distribution.
The files tar.h and is_tar.c were written by John Gilmore
from his public-domain
tar program.
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 System 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 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 optimization 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.
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.
OpenBSD 3.6 July 30, 1997
[ Back ] |
