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doc/file.man
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Christos Zoulas
PR/474: Add a limit to the size of regex searches to prevent performance
11 сен 2015, 20:24
11 сен 2015, 20:24b9bbc26
.\" $File: file.man,v 1.117 2015/06/03 19:51:27 christos Exp $
.Dd September 11, 2015
.Dt FILE __CSECTION__
.Os
.Sh NAME
.Nm file
.Nd determine file type
.Sh SYNOPSIS
.Nm
.Bk -words
.Op Fl bcEhiklLNnprsvzZ0
.Op Fl Fl apple
.Op Fl Fl extension
.Op Fl Fl mime-encoding
.Op Fl Fl mime-type
.Op Fl e Ar testname
.Op Fl F Ar separator
.Op Fl f Ar namefile
.Op Fl m Ar magicfiles
.Op Fl P Ar name=value
.Ar
.Ek
.Nm
.Fl C
.Op Fl m Ar magicfiles
.Nm
.Op Fl Fl help
.Sh DESCRIPTION
This manual page documents version __VERSION__ of the
.Nm
command.
.Pp
.Nm
tests each argument in an attempt to classify it.
There are three sets of tests, performed in this order:
filesystem tests, magic tests, and language tests.
The
.Em first
test that succeeds causes the file type to be printed.
.Pp
The type printed will usually contain one of the words
.Em text
(the file contains only
printing characters and a few common control
characters and is probably safe to read on an
.Dv ASCII
terminal),
.Em executable
(the file contains the result of compiling a program
in a form understandable to some
.Tn UNIX
kernel or another),
or
.Em data
meaning anything else (data is usually
.Dq binary
or non-printable).
Exceptions are well-known file formats (core files, tar archives)
that are known to contain binary data.
When modifying magic files or the program itself, make sure to
.Em "preserve these keywords" .
Users depend on knowing that all the readable files in a directory
have the word
.Dq text
printed.
Don't do as Berkeley did and 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
.In sys/stat.h .
.Pp
The magic tests are used to check for files with data in
particular fixed formats.
The canonical example of this is a binary executable (compiled program)
.Dv a.out
file, whose format is defined in
.In elf.h ,
.In a.out.h
and possibly
.In exec.h
in the standard include directory.
These files have a
.Dq "magic number"
stored in a particular place
near the beginning of the file that tells the
.Tn UNIX
operating system
that the file is a binary executable, and which of several types thereof.
The concept of a
.Dq "magic"
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 identifying these files is read from the compiled
magic file
.Pa __MAGIC__.mgc ,
or the files in the directory
.Pa __MAGIC__
if the compiled file does not exist.
In addition, if
.Pa $HOME/.magic.mgc
or
.Pa $HOME/.magic
exists, it will be used in preference to the system magic files.
.Pp
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 Macintosh and IBM PC systems),
UTF-8-encoded Unicode, UTF-16-encoded Unicode, 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 identified
as
.Dq text
because they will be mostly readable on nearly any terminal;
UTF-16 and EBCDIC are only
.Dq character data
because, while
they contain text, it is text that will require translation
before it can be read.
In addition,
.Nm
will attempt to determine other characteristics 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.
.Pp
Once
.Nm
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.
.In 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
.Em 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).
.Pp
Any file that cannot be identified as having been written
in any of the character sets listed above is simply said to be
.Dq data .
.Sh OPTIONS
.Bl -tag -width indent
.It Fl Fl apple
Causes the file command to output the file type and creator code as
used by older MacOS versions. The code consists of eight letters,
the first describing the file type, the latter the creator.
.It Fl b , Fl Fl brief
Do not prepend filenames to output lines (brief mode).
.It Fl C , Fl Fl compile
Write a
.Pa magic.mgc
output file that contains a pre-parsed version of the magic file or directory.
.It Fl c , Fl Fl checking-printout
Cause a checking printout of the parsed form of the magic file.
This is usually used in conjunction with the
.Fl m
flag to debug a new magic file before installing it.
.It Fl E
On filesystem errors (file not found etc), instead of handling the error
as regular output as POSIX mandates and keep going, issue an error message
and exit.
.It Fl e , Fl Fl exclude Ar testname
Exclude the test named in
.Ar testname
from the list of tests made to determine the file type.
Valid test names are:
.Bl -tag -width compress
.It apptype
.Dv EMX
application type (only on EMX).
.It ascii
Various types of text files (this test will try to guess the text
encoding, irrespective of the setting of the
.Sq encoding
option).
.It encoding
Different text encodings for soft magic tests.
.It tokens
Ignored for backwards compatibility.
.It cdf
Prints details of Compound Document Files.
.It compress
Checks for, and looks inside, compressed files.
.It elf
Prints ELF file details.
.It soft
Consults magic files.
.It tar
Examines tar files.
.El
.It Fl Fl extension 
Print a slash-separated list of valid extensions for the file type found.
.It Fl F , Fl Fl separator Ar separator
Use the specified string as the separator between the filename and the
file result returned.
Defaults to
.Sq \&: .
.It Fl f , Fl Fl files-from 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.
Please note that 
.Ar namefile 
is unwrapped and the enclosed filenames are processed when this option is
encountered and before any further options processing is done.
This allows one to process multiple lists of files with different command line
arguments on the same
.Nm
invocation.
Thus if you want to set the delimiter, you need to do it before you specify
the list of files, like:
.Dq Fl F Ar @ Fl f Ar namefile ,
instead of:
.Dq Fl f Ar namefile Fl F Ar @ .
.It Fl h , Fl Fl no-dereference
option causes symlinks not to be followed
(on systems that support symbolic links).
This is the default if the environment variable
.Dv POSIXLY_CORRECT
is not defined.
.It Fl i , Fl Fl mime
Causes the file command to output mime type strings rather than the more
traditional human readable ones.
Thus it may say
.Sq text/plain; charset=us-ascii
rather than
.Dq ASCII text .
.It Fl Fl mime-type , Fl Fl mime-encoding
Like
.Fl i ,
but print only the specified element(s).
.It Fl k , Fl Fl keep-going
Don't stop at the first match, keep going.
Subsequent matches will be
have the string
.Sq "\[rs]012\- "
prepended.
(If you want a newline, see the
.Fl r
option.)
The magic pattern with the highest strength (see the
.Fl l
option) comes first.
.It Fl l , Fl Fl list
Shows a list of patterns and their strength sorted descending by
.Xr magic 4
strength
which is used for the matching (see also the
.Fl k
option).
.It Fl L , Fl Fl dereference
option causes symlinks to be followed, as the like-named option in
.Xr ls 1
(on systems that support symbolic links).
This is the default if the environment variable
.Ev POSIXLY_CORRECT
is defined.
.It Fl m , Fl Fl magic-file Ar magicfiles
Specify an alternate list of files and directories containing magic.
This can be a single item, or a colon-separated list.
If a compiled magic file is found alongside a file or directory,
it will be used instead.
.It Fl N , Fl Fl no-pad
Don't pad filenames so that they align in the output.
.It Fl n , Fl Fl no-buffer
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.
.It Fl p , Fl Fl preserve-date
On systems that support
.Xr utime 3
or
.Xr utimes 2 ,
attempt to preserve the access time of files analyzed, to pretend that
.Nm
never read them.
.It Fl P , Fl Fl parameter Ar name=value
Set various parameter limits.
.Bl -column "elf_phnum" "Default" "XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX" -offset indent
.It Sy "Name" Ta Sy "Default" Ta Sy "Explanation"
.It Li indir Ta 15 Ta recursion limit for indirect magic
.It Li name Ta 30 Ta use count limit for name/use magic
.It Li elf_notes Ta 256 Ta max ELF notes processed
.It Li elf_phnum Ta 128 Ta max ELF program sections processed
.It Li elf_shnum Ta 32768 Ta max ELF sections processed
.It Li regex Ta 8192 Ta length limit for regex searches
.El
.It Fl r , Fl Fl raw
Don't translate unprintable characters to \eooo.
Normally
.Nm
translates unprintable characters to their octal representation.
.It Fl s , Fl Fl special-files
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 , Fl Fl version
Print the version of the program and exit.
.It Fl z , Fl Fl uncompress
Try to look inside compressed files.
.It Fl Z , Fl Fl uncompress-noreport
Try to look inside compressed files, but report information about the contents
only not the compression.
.It Fl 0 , Fl Fl print0
Output a null character
.Sq \e0
after the end of the filename.
Nice to
.Xr cut 1
the output.
This does not affect the separator, which is still printed.
.It Fl -help
Print a help message and exit.
.El
.Sh FILES
.Bl -tag -width __MAGIC__.mgc -compact
.It Pa __MAGIC__.mgc
Default compiled list of magic.
.It Pa __MAGIC__
Directory containing default magic files.
.El
.Sh ENVIRONMENT
The environment variable
.Ev MAGIC
can be used to set the default magic file name.
If that variable is set, then
.Nm
will not attempt to open
.Pa $HOME/.magic .
.Nm
adds
.Dq Pa .mgc
to the value of this variable as appropriate.
However,
.Pa file
has to exist in order for
.Pa file.mime
to be considered.
The environment variable
.Ev POSIXLY_CORRECT
controls (on systems that support symbolic links), whether
.Nm
will attempt to follow symlinks or not.
If set, then
.Nm
follows symlink, otherwise it does not.
This is also controlled by the
.Fl L
and
.Fl h
options.
.Sh SEE ALSO
.Xr magic __FSECTION__ ,
.Xr hexdump 1 ,
.Xr od 1 ,
.Xr strings 1 ,
.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 behavior 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.
.\" URL: http://www.opengroup.org/onlinepubs/009695399/utilities/file.html
.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,
.Bd -literal -offset indent
\*[Gt]10	string	language impress\ 	(imPRESS data)
.Ed
.Pp
in an existing magic file would have to be changed to
.Bd -literal -offset indent
\*[Gt]10	string	language\e impress	(imPRESS data)
.Ed
.Pp
In addition, in this version, if a pattern string contains a backslash,
it must be escaped.
For example
.Bd -literal -offset indent
0	string		\ebegindata	Andrew Toolkit document
.Ed
.Pp
in an existing magic file would have to be changed to
.Bd -literal -offset indent
0	string		\e\ebegindata	Andrew Toolkit document
.Ed
.Pp
SunOS releases 3.2 and later from Sun Microsystems include a
.Nm
command derived from the System V one, but with some extensions.
This version differs from Sun's only in minor ways.
It includes the extension of the
.Sq \*[Am]
operator, used as,
for example,
.Bd -literal -offset indent
\*[Gt]16	long\*[Am]0x7fffffff	\*[Gt]0		not stripped
.Ed
.Sh 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 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
.Nm
command uses a magic file,
keep the old magic file around for comparison purposes
(rename it to
.Pa __MAGIC__.orig ) .
.Sh EXAMPLES
.Bd -literal -offset indent
$ 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
/dev/hda:    application/x-not-regular-file
/dev/wd0a:   application/x-not-regular-file

.Ed
.Sh HISTORY
There has been a
.Nm
command in every
.Dv 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 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
.Aq ian@darwinsys.com
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.
Contributions by the
.Sq \*[Am]
operator by Rob McMahon, 
.Aq cudcv@warwick.ac.uk ,
1989.
.Pp
Guy Harris, 
.Aq guy@netapp.com ,
made many changes from 1993 to the present.
1989.
.Pp
Primary development and maintenance from 1990 to the present by
Christos Zoulas
.Aq christos@astron.com .
.Pp
Altered by Chris Lowth
.Aq chris@lowth.com ,
2000: handle the
.Fl i
option to output mime type strings, using an alternative
magic file and internal logic.
.Pp
Altered by Eric Fischer
.Aq enf@pobox.com ,
July, 2000,
to identify character codes and attempt to identify the languages
of non-ASCII files.
.Pp
Altered by Reuben Thomas
.Aq rrt@sc3d.org ,
2007-2011, to improve MIME support, merge MIME and non-MIME magic,
support directories as well as files of magic, apply many bug fixes,
update and fix a lot of magic, improve the build system, improve the
documentation, and rewrite the Python bindings in pure Python.
.Pp
The list of contributors to the
.Sq magic
directory (magic files)
is too long to include here.
You know who you are; thank you.
Many contributors are listed in the source files.
.Sh LEGAL NOTICE
Copyright (c) Ian F. Darwin, Toronto, Canada, 1986-1999.
Covered by the standard Berkeley Software Distribution copyright; see the file
COPYING in the source distribution.
.Pp
The files
.Pa tar.h
and
.Pa is_tar.c
were written by John Gilmore from his public-domain
.Xr tar 1
program, and are not covered by the above license.
.Sh RETURN CODE
.Nm
returns 0 on success, and non-zero on error.
.Sh BUGS
.Pp
Please report bugs and send patches to the bug tracker at
.Pa http://bugs.gw.com/
or the mailing list at
.Aq file@mx.gw.com
(visit
.Pa http://mx.gw.com/mailman/listinfo/file
first to subscribe).
.Sh TODO
.Pp
Fix output so that tests for MIME and APPLE flags are not needed all
over the place, and actual output is only done in one place.
This needs a design.
Suggestion: push possible outputs on to a list, then pick the
last-pushed (most specific, one hopes) value at the end, or
use a default if the list is empty.
This should not slow down evaluation.
.Pp
The handling of
.Dv MAGIC_CONTINUE
and printing \e012- between entries is clumsy and complicated; refactor
and centralize.
.Pp
Some of the encoding logic is hard-coded in encoding.c and can be moved
to the magic files if we had a !:charset annotation
.Pp
Continue to squash all magic bugs.
See Debian BTS for a good source.
.Pp
Store arbitrarily long strings, for example for %s patterns, so that
they can be printed out.
Fixes Debian bug #271672.
This can be done by allocating strings in a string pool, storing the
string pool at the end of the magic file and converting all the string
pointers to relative offsets from the string pool.
.Pp
Add syntax for relative offsets after current level (Debian bug #466037).
.Pp
Make file -ki work, i.e. give multiple MIME types.
.Pp
Add a zip library so we can peek inside Office2007 documents to
print more details about their contents.
.Pp
Add an option to print URLs for the sources of the file descriptions.
.Pp
Combine script searches and add a way to map executable names to MIME
types (e.g. have a magic value for !:mime which causes the resulting
string to be looked up in a table).
This would avoid adding the same magic repeatedly for each new
hash-bang interpreter.
.Pp
When a file descriptor is available, we can skip and adjust the buffer
instead of the hacky buffer management we do now.
.Pp
Fix
.Dq name
and
.Dq use
to check for consistency at compile time (duplicate 
.Dq name ,
.Dq use
pointing to undefined
.Dq name
).
Make 
.Dq name
/
.Dq use 
more efficient by keeping a sorted list of names.
Special-case ^ to flip endianness in the parser so that it does not
have to be escaped, and document it.
.Pp
If the offsets specified internally in the file exceed the buffer size
(
.Dv HOWMANY
variable in file.h), then we don't seek to that offset, but we give up.
It would be better if buffer managements was done when the file descriptor
is available so move around the file.
One must be careful though because this has performance (and thus security
considerations).
.Sh AVAILABILITY
You can obtain the original author's latest version by anonymous FTP
on
.Pa ftp.astron.com
in the directory
.Pa /pub/file/file-X.YZ.tar.gz .