NetBSD/usr.bin/file/file.1

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.TH FILE 1 "Copyright but distributable"
.SH NAME
.I file
\- determine file type
.SH SYNOPSIS
.B file
[
.B -c
]
[
.B -f
namefile ]
[
.B -m 
magicfile ]
file ...
.SH DESCRIPTION
.I 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
.I first
test that succeeds causes the file type to be printed.
.PP
The type printed will usually contain one of the words
.B text
(the file contains only ASCII characters and is 
probably safe to read on an ASCII terminal),
.B executable
(the file contains the result of compiling a program
in a form understandable to some \s-1UNIX\s0 kernel or another),
or
.B 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
.I /etc/magic
or the program itself, 
.B "preserve these keywords" .
People depend on knowing that all the readable files in a directory
have the word ``text'' printed.
Don't do as one computer vendor did \- change ``shell commands text''
to ``shell script''.
.PP
The filesystem tests are based on examining the return from a
.I 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 and symbolic links on 4.2BSD, named pipes (FIFOs) on System V)
are intuited if they are defined in
the system header file
.I 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)
.I a.out
file, whose format is defined in 
.I a.out.h
and possibly
.I 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 \s-1UNIX\s0 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
.I /etc/magic .
.PP
If an argument appears to be an
.SM ASCII 
file,
.I file
attempts to guess its language.
The language tests look for particular strings (cf \fInames.h\fP)
that can appear anywhere in the first few blocks of a file.
For example, the keyword
.I .br
indicates that the file is most likely a troff input file,
just as the keyword 
.I 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 
.I tar
archives) and determine whether an unknown file should be
labelled as `ascii text' or `data'. 
.PP
Use
.B -m
.I file
to specify an alternate file of magic numbers.
.PP
The
.B -c
option causes a checking printout of the parsed form of the magic file.
This is usually used in conjunction with 
.B -m
to debug a new magic file before installing it.
.PP
The 
.B -f
.I namefile
option specifies that the names of the files to be examined
are to be read (one per line) from 
.I namefile
before the argument list.
Either 
.I namefile
or at least one filename argument must be present;
to test the standard input, use ``-'' as a filename argument.
.SH FILES
.I /etc/magic
\- default list of magic numbers
.SH SEE ALSO
.IR Magic (FILES)
\- description of magic file format.
.br
.IR Strings (1), " od" (1)
\- tools for examining non-textfiles.
.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,
.br
>10	string	language impress\ 	(imPRESS data)
.br
in an existing magic file would have to be changed to
.br
>10	string	language\e impress	(imPRESS data)
.PP
The Sun Microsystems implementation of System V compatibility
includes a file(1) command that has some extentions.
My version differs from Sun's only in minor ways.
The significant one is the `&' operator, which Sun's program expects as,
for example,
.br
>16	long&0x7fffffff	>0		not stripped
.br
would be entered in my version as
.br
>16	long	&0x7fffffff	not stripped
.br
which is a little less general; it simply tests (location 16)&0x7ffffff
and returns its truth value as a C expression.
.SH MAGIC DIRECTORY
The magic file entries have been collected from various sources,
mainly USENET, and contributed by various authors.
Ian Darwin (address below) will collect additional
or corrected magic file entries.
A consolidation of magic file entries 
will be distributed periodically.
.PP
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
.I file
command uses a magic file,
keep the old magic file around for comparison purposes
(rename it to 
.IR /etc/magic.orig ).
.SH HISTORY
There has been a 
.I 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.
.PP
This program, based on the System V version,
was written by Ian Darwin without looking at anybody else's source code.
.PP
John Gilmore revised the code extensively, making it better than
the first version.
Geoff Collyer found several inadequacies
and provided some magic file entries.
The program has undergone continued evolution since.
.SH NOTICE
Copyright (c) Ian F. Darwin,  1986 and 1987.
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.
.PP
.I Strtok.c
and
.I getopt.c
written by and copyright by Henry Spencer, utzoo!henry.
.PP
This software is not subject to any license of the American Telephone
and Telegraph Company or of the Regents of the University of California.
.PP
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 restrictions:
.PP 
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.
.PP
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.
.PP
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.
.PP
4. This notice may not be removed or altered.
.PP
A few support files (\fIgetopt\fP, \fIstrtok\fP)
distributed with this package
are by Henry Spencer and are subject to the same terms as above.
.PP
A few simple support files (\fIstrtol\fP, \fIstrchr\fP)
distributed with this package
are in the public domain; they are so marked.
.PP
The files
.I tar.h
and
.I is_tar.c
were written by John Gilmore from his public-domain
.I tar
program, and are not covered by the above restrictions.
.SH BUGS
There must be a way to automate the construction of the Magic
file from all the glop in magdir. What is it?
.PP
.I File
uses several algorithms that favor speed over accuracy,
thus it can be misled about the contents of ASCII files.
.PP
The support for ASCII files (primarily for programming languages)
is simplistic, inefficient and requires recompilation to update.
.PP
Should there be an ``else'' clause to follow a series of continuation lines?
.PP
Is it worthwhile to implement recursive file inspection,
so that compressed files, uuencoded, etc., can say ``compressed
ascii text'' or ``compressed executable'' or ``compressed tar archive"
or whatever? 
.PP
The magic file and keywords should have regular expression support.
.PP
It might be advisable to allow upper-case letters in keywords
for e.g., troff 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 
.I ascmagic
probably belongs in the Magic file.
This could be done by using some keyword like `*' for the offset value.
.PP
The program should malloc the magic file structures,
rather than using a fixed-size array as at present.
.PP
The magic file should be compiled into binary 
(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.
But then there would have to be yet another magic number for the 
.I magic.out
file.
.PP
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?
.PP
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.  
.PP
Perhaps the program should automatically try all tests with
byte-swapping done, to avoid having to figure out the byte-swapped values
when constructing the magic file.
Of course this will run more slowly, so it should probably be
an option (-a?).
.PP
This manual page, and particularly this section, is too long.