file

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file - determine file type


SYNOPSIS

       file [ -c ] [ -z ] [ -L ] [ -f namefile ] [ -m magicfile ]
       file ...


DESCRIPTION

       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 prob-
       ably 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 know-
       ing 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  informa-
       tion   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 par-
       ticular 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  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
       archives)  and determine whether an unknown file should be
       labelled as `ascii text' or `data'.

       Use -m file to specify an alternate file of magic numbers.

       The -z tries to look inside compressed files.

       The  -c  option  causes  a checking printout of the parsed
       form of the magic file.  This is usually used in  conjunc-
       tion  with  -m to debug a new magic file before installing
       it.

       The -f namefile option specifies that  the  names  of  the
       files  to  be  examined are to be read (one per line) from
       namefile 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.

       The -L option causes symlinks to be followed, as the like-
       named option in ls(1).


FILES

       /etc/magic - default list of magic numbers


SEE ALSO

       magic(1) - description of magic file format.
       Strings(1), od(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  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)
       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 extension of the  `&'  opera-
       tor, used as, for example,
       >16  long&0x7fffffff     >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  consolida-
       tion  of  magic  file entries will be distributed periodi-
       cally.

       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).


HISTORY

       There has been a 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.

       This program, based on the System V version,  was  written
       by  Ian  Darwin  without  looking at anybody else's source
       code.

       John Gilmore revised the code extensively, making it  bet-
       ter  than  the first version.  Geoff Collyer found several
       inadequacies and provided some magic  file  entries.   The
       program has undergone continued evolution since.


AUTHOR

       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.

       Altered by  Rob  McMahon,  cudcv@warwick.ac.uk,  1989,  to
       extend  the `&' operator from simple `x&y != 0' to `x&y op
       z'.

       Altered by Guy Harris, guy@auspex.com, 1993, to:

              put the ``old-style'' `&' operator back the way  it
              was, because 1) Rob McMahon's change broke the pre-
              vious style of usage, 2)  the  SunOS  ``new-style''
              `&'  operator, which this version of file supports,
              also handles `x&y op z', and 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.

       Changes by Ian Darwin and various authors including Chris-
       tos Zoulas (christos@ee.cornell.edu), 1990-1992.


LEGAL NOTICE

       Copyright  (c) Ian F. Darwin, Toronto, Canada, 1986, 1987,
       1988, 1989, 1990, 1991, 1992, 1993.

       This software is not subject to and may not be  made  sub-
       ject  to  any  license of the American Telephone and Tele-
       graph Company, Sun Microsystems  Inc.,  Digital  Equipment
       Inc.,  Lotus  Development Inc., the Regents of the Univer-
       sity of California, The X Consortium or MIT, or  The  Free
       Software Foundation.

       This  software  is  not subject to any export provision of
       the United States  Department  of  Commerce,  and  may  be
       exported to any country or planet.

       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 restric-
       tions:

       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.

       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  documenta-
       tion.

       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.

       4. This notice may not be removed or altered.

       A few support files (getopt, strtok) distributed with this
       package  are  by Henry Spencer and are subject to the same
       terms as above.

       A few simple support files  (strtol,  strchr)  distributed
       with  this  package  are in the public domain; they are so
       marked.

       The files tar.h and is_tar.c were written by John  Gilmore
       from his public-domain tar program, and are not covered by
       the above restrictions.


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 environments)  for  faster
       startup.   Then  the program would run as fast as the Ver-
       sion 7 program of the same name, with the  flexibility  of
       the System V version.

       File  uses  several algorithms that favor speed over accu-
       racy, 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  recom-
       pilation 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 key-
       words for e.g., troff commands vs man page macros.   Regu-
       lar 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 off-
       set 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.


AVAILABILITY

       You  can  obtain  the  original author's latest version by
       anonymous FTP on  tesla.ee.cornell.edu  in  the  directory
       /pub/file-X.YY.tar.gz
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