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file - determine file type

 

NAME

      file - determine file type
 

SYNOPSIS

      file [-bciknNsvzL] [-f namefile] [-F separator] [-m magicfiles] file ...
      file -C [-m magicfile]
 

DESCRIPTION

      This manual page documents version 3.41 of the file utility which tests
      each argument in an attempt to classify it.  There are three sets of
      tests, performed in this order: file system 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 printing characters and a few common control 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 for‐
      mats (core files, tar archives) that are known to contain binary data.
      When modifying the file /usr/share/misc/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 and change “shell commands text” to “shell script”.  Note that the
      file /usr/share/misc/magic is built mechanically from a large number of
      small files in the subdirectory Magdir in the source distribution of this
      program.
 
      The file system 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 sys‐
      tem 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 particu‐
      lar 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 exe‐
      cutable, 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 identifying these files is read
      from the compiled magic file /usr/share/misc/magic.mgc, or
      /usr/share/misc/magic if the compile file does not exist.
 
      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 “text”
      because they will be mostly readable on nearly any terminal; UTF-16 and
      EBCDIC are only “character data” because, while they contain text, it is
      text that will require translation before it can be read.  In addition,
      file 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.
 
      Once file has determined the character set used in a text-type file, it
      will attempt to determine in what language the file is written.  The lan‐
      guage tests look for particular strings (cf names.h) that can appear any‐
      where 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).
 
      Any file that cannot be identified as having been written in any of the
      character sets listed above is simply said to be “data”.
 

OPTIONS

      -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.
 
      -C      Write a magic.mgc output file that contains a pre-parsed version
              of file.
 
      -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.
 
      -F separator
              Use the specified separator character instead of ‘:’.
 
      -i      Causes the file command to output mime type strings rather than
              the more traditional human readable ones.  Thus it may say
              “text/plain; charset=us-ascii” rather than “ASCII text”.  In
              order for this option to work, file changes the way it handles
              files recognised by the command itself (such as many of the text
              file types, directories etc), and makes use of an alternative
              magic file.  (See FILES section, below).
 
      -k      Don’t stop at the first match, keep going.
 
      -m list
              Specify an alternate list of files containing magic numbers.
              This can be a single file, or a colon-separated list of files.
 
      -n      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.
 
      -N      Don’t pad output to align filenames nicely.
 
      -v      Print the version of the program and exit.
 
      -z      Try to look inside compressed files.
 
      -L      option causes symlinks to be followed, as the like-named option
              in ls(1).  (on systems that support symbolic links).
 
      -s      Normally, file only attempts to read and determine the type of
              argument files which stat(2) reports are ordinary files.  This
              prevents problems, because reading special files may have pecu‐
              liar consequences.  Specifying the -s option causes file to also
              read argument files which are block or character special files.
              This is useful for determining the file system types of the data
              in raw disk partitions, which are block special files.  This
              option also causes file to disregard the file size as reported by
              stat(2) since on some systems it reports a zero size for raw disk
              partitions.
 

FILES

      /usr/share/misc/magic.mgc   default compiled list of magic numbers
      /usr/share/misc/magic       default list of magic numbers
      /usr/share/misc/magic.mime  default list of magic numbers, used to output
                                  mime types when the -i option is specified.
 

ENVIRONMENT

      The environment variable MAGIC can be used to set the default magic num‐
      ber files.
      hexdump(1), od(1), strings(1), magic(5)
      This program is believed to exceed the System V Interface Definition,
      Fourth Edition (“SVID4”) 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) out‐
      put 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) com‐
      mand 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 con‐
      solidation 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 incor‐
      rect.  If your old file command uses a magic file, keep the old magic
      file around for comparison purposes (rename it to
      /usr/share/misc/magic.orig).
 

EXAMPLES

      $ 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}       # Linux
      /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 -s /dev/rwd0e                            # BSD
      /dev/rwd0e:
      Unix Fast File system (little-endian),
      last mounted on /usr,
      last written at Mon Feb 10 13:22:40 2003,
      clean flag 2,
      number of blocks 28754208,
      number of data blocks 27812712,
      number of cylinder groups 3566,
      block size 8192,
      fragment size 1024,
      minimum percentage of free blocks 5,
      rotational delay 0ms,
      disk rotational speed 60rps,
      TIME optimization
 
      $ file -i file.c file /dev/{wd0a,hda}
      file.c:    text/x-c
      file:      application/x-executable, dynamically linked (uses shared libs),
                 not stripped
      /dev/hda:  application/x-not-regular-file
      /dev/wd0a: application/x-not-regular-file
 

HISTORY

      There has been a file command in every UNIX since at least Research Ver‐
      sion 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 Darwin
      〈ian@darwinsys.com〉 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.  Contributions by the ‘&’ operator by Rob
      McMahon 〈cudcv@warwick.ac.uk〉, 1989.
 
      Guy Harris 〈guy@netapp.com〉, made many changes from 1993 to the present.
 
      Primary development and maintenance from 1990 to the present by Christos
      Zoulas 〈christos@astron.com〉.
 
      Altered by Chris Lowth 〈chris@lowth.com〉, 2000: Handle the -i option to
      output mime type strings and using an alternative magic file and internal
      logic.
 
      Altered by Eric Fischer 〈enf@pobox.com〉, July, 2000, to identify charac‐
      ter codes and attempt to identify the languages of non-ASCII files.
 
      The list of contributors to the Magdir directory (source for the
      /usr/share/misc/magic file) is too long to include here.  You know who
      you are; thank you.
      Copyright (c) Ian F. Darwin, Toronto, Canada, 1986-1999.  Covered by the
      standard Berkeley Software Distribution copyright; see the file
      LEGAL.NOTICE in the source distribution.
 
      The files tar.h and is_tar.c were written by John Gilmore from his pub‐
      lic-domain tar program, and are not covered by the above license.
 

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 Version 7 program of
      the same name, with the flexibility of the System V version.
 
      The file utility uses several algorithms that favor speed over accuracy,
      thus it can be misled about the contents of text files.
 
      The support for text files (primarily for programming languages) is sim‐
      plistic, 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 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?
 
      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.  The new support
      for multiple character codes makes it even slower.
 
      This manual page, and particularly this section, is too long.
 

AVAILABILITY

      You can obtain the original author’s latest version by anonymous FTP on
      ftp.astron.com in the directory /pub/file/file-X.YZ.tar.gz
 

Sections

Based on BSD UNIX
FreeBSD is an advanced operating system for x86 compatible (including Pentium and Athlon), amd64 compatible (including Opteron, Athlon64, and EM64T), UltraSPARC, IA-64, PC-98 and ARM architectures. It is derived from BSD, the version of UNIX developed at the University of California, Berkeley. It is developed and maintained by a large team of individuals. Additional platforms are in various stages of development.