path: root/usr.bin/file/file.1

.\" $OpenBSD: file.1,v 1.29 2009/08/16 09:41:08 sobrado Exp $
.\" $FreeBSD: src/usr.bin/file/file.1,v 1.16 2000/03/01 12:19:39 sheldonh Exp $
.\"
.\" Copyright (c) Ian F. Darwin 1986-1995.
.\" Software written by Ian F. Darwin and others;
.\" maintained 1995-present by Christos Zoulas and others.
.\"
.\" Redistribution and use in source and binary forms, with or without
.\" modification, are permitted provided that the following conditions
.\" are met:
.\" 1. Redistributions of source code must retain the above copyright
.\"    notice immediately at the beginning of the file, without modification,
.\"    this list of conditions, and the following disclaimer.
.\" 2. Redistributions in binary form must reproduce the above copyright
.\"    notice, this list of conditions and the following disclaimer in the
.\"    documentation and/or other materials provided with the distribution.
.\"
.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
.\" ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE FOR
.\" ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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.Dd $Mdocdate: August 16 2009 $
.Dt FILE 1
.Os
.Sh NAME
.Nm file
.Nd determine file type
.Sh SYNOPSIS
.Nm file
.Op Fl bckLNnrsvz
.Op Fl F Ar separator
.Op Fl f Ar namefile
.Op Fl m Ar magicfiles
.Bk -words
.Ar
.Ek
.Nm file
.Op Fl m Ar magicfiles
.Fl C
.Sh DESCRIPTION
The
.Nm
utility
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.
.Pp
The type printed will usually contain one of the words
.Dq text
(the file contains only
.Tn ASCII
characters and is probably safe to read on an
.Tn ASCII
terminal),
.Dq executable
(the file contains the result of compiling a program
in a form understandable to some
.Ux
kernel or another),
or
.Dq data
meaning anything else (data is usually binary or non-printable).
.Pp
Exceptions are well-known file formats (core files, tar archives)
that are known to contain binary data.
When modifying the file
.Pa /etc/magic
or the program itself,
.Em "preserve these keywords" .
.Pp
People depend on knowing that all the readable files in a directory
have the word
.Dq text
printed.
Don't do as Berkeley did; change
.Dq shell commands text
to
.Dq shell script .
.Pp
The filesystem tests are based on examining the return from a
.Xr 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
.Aq Pa sys/stat.h .
.Pp
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)
.Pa a.out
file, whose format is defined in
.Aq Pa a.out.h
and possibly
.Aq Pa exec.h
in the standard include directory and is explained in
.Xr a.out 5 .
These files have a
.Dq magic number
stored in a particular place
near the beginning of the file that tells the
.Ux
operating system
that the file is a binary executable, and which of several types thereof.
.Pp
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
.Pa /etc/magic .
.Pp
If an argument appears to be an
.Tn ASCII
file,
.Nm
attempts to guess its language.
The language tests look for particular strings (cf
.Pa names.h )
that can appear anywhere in the first few blocks of a file.
For example, the keyword
.Em .br
indicates that the file is most likely a
.Xr troff 1
input file, just as the keyword
.Li 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
.Xr tar 1
archives) and determine whether an unknown file should be
labelled as
.Dq ASCII text
or
.Dq data .
.Pp
The options are as follows:
.Bl -tag -width Ds
.It Fl b
Do not prepend filenames to output lines (brief mode).
.It Fl C
For each magic number file, write a
.Pa magic.mgc
output file that contains a preparsed (compiled) version of it.
.It Fl c
Cause a checking printout of the parsed form of the magic file.
This is usually used in conjunction with
.Fl m
to debug a new magic file before installing it.
.It Fl F Ar separator
Use the specified string as the separator between the filename and
the file result returned.
Defaults to
.Sq \&: .
.It Fl f Ar namefile
Read the names of the files to be examined from
.Ar namefile
(one per line)
before the argument list.
Either
.Ar namefile
or at least one filename argument must be present;
to test the standard input, use
.Sq -
as a filename argument.
.It Fl k
Don't stop at the first match, keep going.
.It Fl L
Cause symlinks to be followed, as the like-named option in
.Xr ls 1
(on systems that support symbolic links).
.It Fl m Ar magicfiles
Specify an alternate list,
.Ar magicfiles ,
of files containing magic numbers.
This can be a single file or a colon-separated list of files.
If a compiled magic file is found alongside, it will be used instead.
.It Fl N
Don't pad filenames so that they align in the output.
.It Fl n
Force
.Em 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.
.It Fl r
Don't translate unprintable characters to
.Sq \e Ns Em ooo .
Normally
.Nm
translates unprintable characters to their octal representation
(raw mode).
.It Fl s
Normally,
.Nm
only attempts to read and determine the type of argument files which
.Xr stat 2
reports are ordinary files.
This prevents problems, because reading special files may have peculiar
consequences.
Specifying the
.Fl s
option causes
.Nm
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
.Nm
to disregard the file size as reported by
.Xr stat 2 ,
since on some systems it reports a zero size for raw disk partitions.
.It Fl v
Print the version of the program and exit.
.It Fl z
Try to look inside files that have been run through
.Xr compress 1 .
.El
.Sh ENVIRONMENT
.Bl -tag -width indent
.It Ev MAGIC
Default magic number files, separated by colon characters.
.Nm
adds
.Dq .mgc
to the value of this variable as appropriate.
.El
.Sh FILES
.Bl -tag -width /etc/magic -compact
.It Pa /etc/magic
default list of magic numbers
.El
.Sh SEE ALSO
.Xr compress 1 ,
.Xr hexdump 1 ,
.Xr ls 1 ,
.Xr od 1 ,
.Xr strings 1 ,
.Xr a.out 5 ,
.Xr magic 5
.Sh 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.
.Pp
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,
.Pp
>10     string  language impress\       (imPRESS data)
.Pp
in an existing magic file would have to be changed to
.Pp
>10     string  language\e impress      (imPRESS data)
.Pp
In addition, in this version, if a pattern string contains a backslash,
it must be escaped.
For example
.Pp
0       string          \ebegindata     Andrew Toolkit document
.Pp
in an existing magic file would have to be changed to
.Pp
0       string          \e\ebegindata   Andrew Toolkit document
.Pp
SunOS releases 3.2 and later from Sun Microsystems include a
.Nm 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
.Ql &
operator, used as,
for example,
.Pp
>16     long&0x7fffffff >0              not stripped
.Sh MAGIC DIRECTORY
The magic file entries have been collected from various sources,
mainly USENET, and contributed by various authors.
.An 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
.Nm
command uses a magic file,
keep the old magic file around for comparison purposes
(rename it to
.Pa /etc/magic.orig ) .
.Sh HISTORY
There has been a
.Nm
command in every
.Ux
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.
.Pp
This program, based on the System V version, was written by
.An Ian F. Darwin
without looking at anybody else's source code.
.Pp
.An John Gilmore
revised the code extensively, making it better than
the first version.
.An Geoff Collyer
found several inadequacies
and provided some magic file entries.
Contributions to the
.Ql &
operator by
.An Rob McMahon ,
1989.
.Pp
.An Guy Harris
made many changes from 1993 to the present.
.Pp
Primary development and maintenance from 1990 to the present by
.An Christos Zoulas Aq christos@zoulas.com .
.Pp
Altered by
.An Chris Lowth ,
2000, to optionally report MIME types.
This required an alternative magic file, and is not available in
.Ox .
.Pp
Altered by
.An Eric Fischer ,
July, 2000, to identify character codes and attempt to identify the
languages of non-ASCII files.
.Pp
The list of contributors to the
.Dq magdir
directory (source for the
.Pa /etc/magic
file) is too long to include here.
You know who you are; thank you.
.Sh 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 distribution.
.Pp
The files
.Pa tar.h
and
.Pa is_tar.c
were written by
.An John Gilmore
from his public-domain
.Nm tar
program, and are not covered by the above license.
.Sh 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,
.Xr ndbm 3
or, better yet, fixed-length
.Tn 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.
.Pp
.Nm
uses several algorithms that favor speed over accuracy;
thus it can be misled about the contents of
.Tn ASCII
files.
.Pp
The support for
.Tn ASCII
files (primarily for programming languages)
is simplistic, inefficient and requires recompilation to update.
.Pp
There should be an
.Dq else
clause to follow a series of continuation lines.
.Pp
The magic file and keywords should have regular expression support.
Their use of
.Tn ASCII TAB
as a field delimiter is ugly and makes
it hard to edit the files, but is entrenched.
.Pp
It might be advisable to allow upper-case letters in keywords
for e.g.,
.Xr troff 1
commands vs man page macros.
Regular expression support would make this easy.
.Pp
The program doesn't grok \s-2FORTRAN\s0.
It should be able to figure \s-2FORTRAN\s0 by seeing some keywords which
appear indented at the start of line.
Regular expression support would make this easy.
.Pp
The list of keywords in
.Em ascmagic
probably belongs in the Magic file.
This could be done by using some keyword like
.Ql *
for the offset value.
.Pp
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?
.Pp
The program should provide a way to give an estimate
of
.Dq how good
a guess is.
We end up removing guesses (e.g.,
.Dq From\ \&
as first 5 chars of file) because
they are not as good as other guesses (e.g.,
.Dq Newsgroups:
versus
.Qq Return-Path: ) .
Still, if the others don't pan out, it should be
possible to use the first guess.
.Pp
This program is slower than some vendors'
.Nm
commands.
.Pp
This manual page, and particularly this section, is too long.
.Sh AVAILABILITY
You can obtain the original author's latest version by anonymous FTP
on
.Em ftp.astron.com
in the directory
.Pa /pub/file/file-X.YY.tar.gz .