magic(4) — Linux manual page

NAME | DESCRIPTION | SEE ALSO | BUGS | COLOPHON

MAGIC(4)              BSD Kernel Interfaces Manual              MAGIC(4)

NAME         top

     magic — file command's magic pattern file

DESCRIPTION         top

     This manual page documents the format of magic files as used by the
     file(1) command, version 5.40.  The file(1) command identifies the
     type of a file using, among other tests, a test for whether the
     file contains certain “magic patterns”.  The database of these
     “magic patterns” is usually located in a binary file in
     /usr/local/share/misc/magic.mgc or a directory of source text magic
     pattern fragment files in /usr/local/share/misc/magic.  The
     database specifies what patterns are to be tested for, what message
     or MIME type to print if a particular pattern is found, and
     additional information to extract from the file.

     The format of the source fragment files that are used to build this
     database is as follows: Each line of a fragment file specifies a
     test to be performed.  A test compares the data starting at a
     particular offset in the file with a byte value, a string or a
     numeric value.  If the test succeeds, a message is printed.  The
     line consists of the following fields:

     offset   A number specifying the offset (in bytes) into the file of
              the data which is to be tested.  This offset can be a
              negative number if it is:
              The first direct offset of the magic entry (at
                  continuation level 0), in which case it is interpreted
                  an offset from end end of the file going backwards.
                  This works only when a file descriptor to the file is
                  available and it is a regular file.
              A continuation offset relative to the end of the last
                  up-level field (&).

     type     The type of the data to be tested.  The possible values
              are:

              byte        A one-byte value.

              short       A two-byte value in this machine's native byte
                          order.

              long        A four-byte value in this machine's native
                          byte order.

              quad        An eight-byte value in this machine's native
                          byte order.

              float       A 32-bit single precision IEEE floating point
                          number in this machine's native byte order.

              double      A 64-bit double precision IEEE floating point
                          number in this machine's native byte order.

              string      A string of bytes.  The string type
                          specification can be optionally followed by
                          /[WwcCtbT]*.  The “W” flag compacts whitespace
                          in the target, which must contain at least one
                          whitespace character.  If the magic has n
                          consecutive blanks, the target needs at least
                          n consecutive blanks to match.  The “w” flag
                          treats every blank in the magic as an optional
                          blank.  The “c” flag specifies case
                          insensitive matching: lower case characters in
                          the magic match both lower and upper case
                          characters in the target, whereas upper case
                          characters in the magic only match upper case
                          characters in the target.  The “C” flag
                          specifies case insensitive matching: upper
                          case characters in the magic match both lower
                          and upper case characters in the target,
                          whereas lower case characters in the magic
                          only match upper case characters in the
                          target.  To do a complete case insensitive
                          match, specify both “c” and “C”.  The “t” flag
                          forces the test to be done for text files,
                          while the “b” flag forces the test to be done
                          for binary files.  The “T” flag causes the
                          string to be trimmed, i.e. leading and
                          trailing whitespace is deleted before the
                          string is printed.

              pstring     A Pascal-style string where the first
                          byte/short/int is interpreted as the unsigned
                          length.  The length defaults to byte and can
                          be specified as a modifier.  The following
                          modifiers are supported:
                          B  A byte length (default).
                          H  A 2 byte big endian length.
                          h  A 2 byte little endian length.
                          L  A 4 byte big endian length.
                          l  A 4 byte little endian length.
                          J  The length includes itself in its count.
                          The string is not NUL terminated.  “J” is used
                          rather than the more valuable “I” because this
                          type of length is a feature of the JPEG
                          format.

              date        A four-byte value interpreted as a UNIX date.

              qdate       An eight-byte value interpreted as a UNIX
                          date.

              ldate       A four-byte value interpreted as a UNIX-style
                          date, but interpreted as local time rather
                          than UTC.

              qldate      An eight-byte value interpreted as a UNIX-
                          style date, but interpreted as local time
                          rather than UTC.

              qwdate      An eight-byte value interpreted as a Windows-
                          style date.

              beid3       A 32-bit ID3 length in big-endian byte order.

              beshort     A two-byte value in big-endian byte order.

              belong      A four-byte value in big-endian byte order.

              bequad      An eight-byte value in big-endian byte order.

              befloat     A 32-bit single precision IEEE floating point
                          number in big-endian byte order.

              bedouble    A 64-bit double precision IEEE floating point
                          number in big-endian byte order.

              bedate      A four-byte value in big-endian byte order,
                          interpreted as a Unix date.

              beqdate     An eight-byte value in big-endian byte order,
                          interpreted as a Unix date.

              beldate     A four-byte value in big-endian byte order,
                          interpreted as a UNIX-style date, but
                          interpreted as local time rather than UTC.

              beqldate    An eight-byte value in big-endian byte order,
                          interpreted as a UNIX-style date, but
                          interpreted as local time rather than UTC.

              beqwdate    An eight-byte value in big-endian byte order,
                          interpreted as a Windows-style date.

              bestring16  A two-byte unicode (UCS16) string in big-
                          endian byte order.

              leid3       A 32-bit ID3 length in little-endian byte
                          order.

              leshort     A two-byte value in little-endian byte order.

              lelong      A four-byte value in little-endian byte order.

              lequad      An eight-byte value in little-endian byte
                          order.

              lefloat     A 32-bit single precision IEEE floating point
                          number in little-endian byte order.

              ledouble    A 64-bit double precision IEEE floating point
                          number in little-endian byte order.

              ledate      A four-byte value in little-endian byte order,
                          interpreted as a UNIX date.

              leqdate     An eight-byte value in little-endian byte
                          order, interpreted as a UNIX date.

              leldate     A four-byte value in little-endian byte order,
                          interpreted as a UNIX-style date, but
                          interpreted as local time rather than UTC.

              leqldate    An eight-byte value in little-endian byte
                          order, interpreted as a UNIX-style date, but
                          interpreted as local time rather than UTC.

              leqwdate    An eight-byte value in little-endian byte
                          order, interpreted as a Windows-style date.

              lestring16  A two-byte unicode (UCS16) string in little-
                          endian byte order.

              melong      A four-byte value in middle-endian (PDP-11)
                          byte order.

              medate      A four-byte value in middle-endian (PDP-11)
                          byte order, interpreted as a UNIX date.

              meldate     A four-byte value in middle-endian (PDP-11)
                          byte order, interpreted as a UNIX-style date,
                          but interpreted as local time rather than UTC.

              indirect    Starting at the given offset, consult the
                          magic database again.  The offset of the
                          indirect magic is by default absolute in the
                          file, but one can specify /r to indicate that
                          the offset is relative from the beginning of
                          the entry.

              name        Define a “named” magic instance that can be
                          called from another use magic entry, like a
                          subroutine call.  Named instance direct magic
                          offsets are relative to the offset of the
                          previous matched entry, but indirect offsets
                          are relative to the beginning of the file as
                          usual.  Named magic entries always match.

              use         Recursively call the named magic starting from
                          the current offset.  If the name of the
                          referenced begins with a ^ then the endianness
                          of the magic is switched; if the magic
                          mentioned leshort for example, it is treated
                          as beshort and vice versa.  This is useful to
                          avoid duplicating the rules for different
                          endianness.

              regex       A regular expression match in extended POSIX
                          regular expression syntax (like egrep).
                          Regular expressions can take exponential time
                          to process, and their performance is hard to
                          predict, so their use is discouraged.  When
                          used in production environments, their
                          performance should be carefully checked.  The
                          size of the string to search should also be
                          limited by specifying /<length>, to avoid
                          performance issues scanning long files.  The
                          type specification can also be optionally
                          followed by /[c][s][l].  The “c” flag makes
                          the match case insensitive, while the “s” flag
                          update the offset to the start offset of the
                          match, rather than the end.  The “l” modifier,
                          changes the limit of length to mean number of
                          lines instead of a byte count.  Lines are
                          delimited by the platforms native line
                          delimiter.  When a line count is specified, an
                          implicit byte count also computed assuming
                          each line is 80 characters long.  If neither a
                          byte or line count is specified, the search is
                          limited automatically to 8KiB.  ^ and $ match
                          the beginning and end of individual lines,
                          respectively, not beginning and end of file.

              search      A literal string search starting at the given
                          offset.  The same modifier flags can be used
                          as for string patterns.  The search expression
                          must contain the range in the form /number,
                          that is the number of positions at which the
                          match will be attempted, starting from the
                          start offset.  This is suitable for searching
                          larger binary expressions with variable
                          offsets, using \ escapes for special
                          characters.  The order of modifier and number
                          is not relevant.

              default     This is intended to be used with the test x
                          (which is always true) and it has no type.  It
                          matches when no other test at that
                          continuation level has matched before.
                          Clearing that matched tests for a continuation
                          level, can be done using the clear test.

              clear       This test is always true and clears the match
                          flag for that continuation level.  It is
                          intended to be used with the default test.

              der         Parse the file as a DER Certificate file.  The
                          test field is used as a der type that needs to
                          be matched.  The DER types are: eoc, bool,
                          int, bit_str, octet_str, null, obj_id,
                          obj_desc, ext, real, enum, embed, utf8_str,
                          rel_oid, time, res2, seq, set, num_str,
                          prt_str, t61_str, vid_str, ia5_str, utc_time,
                          gen_time, gr_str, vis_str, gen_str, univ_str,
                          char_str, bmp_str, date, tod, datetime,
                          duration, oid-iri, rel-oid-iri.  These types
                          can be followed by an optional numeric size,
                          which indicates the field width in bytes.

              guid        A Globally Unique Identifier, parsed and
                          printed as XXXXXXXX-XXXX-XXXX-XXXX-
                          XXXXXXXXXXXX.  It's format is a string.

              offset      This is a quad value indicating the current
                          offset of the file.  It can be used to
                          determine the size of the file or the magic
                          buffer.  For example the magic entries:

                                -0      offset  x       this file is %lld bytes
                                -0      offset  <=100   must be more than 100 \
                                    bytes and is only %lld

              For compatibility with the Single UNIX Standard, the type
              specifiers dC and d1 are equivalent to byte, the type
              specifiers uC and u1 are equivalent to ubyte, the type
              specifiers dS and d2 are equivalent to short, the type
              specifiers uS and u2 are equivalent to ushort, the type
              specifiers dI, dL, and d4 are equivalent to long, the type
              specifiers uI, uL, and u4 are equivalent to ulong, the
              type specifier d8 is equivalent to quad, the type
              specifier u8 is equivalent to uquad, and the type
              specifier s is equivalent to string.  In addition, the
              type specifier dQ is equivalent to quad and the type
              specifier uQ is equivalent to uquad.

              Each top-level magic pattern (see below for an explanation
              of levels) is classified as text or binary according to
              the types used.  Types “regex” and “search” are classified
              as text tests, unless non-printable characters are used in
              the pattern.  All other tests are classified as binary.  A
              top-level pattern is considered to be a test text when all
              its patterns are text patterns; otherwise, it is
              considered to be a binary pattern.  When matching a file,
              binary patterns are tried first; if no match is found, and
              the file looks like text, then its encoding is determined
              and the text patterns are tried.

              The numeric types may optionally be followed by & and a
              numeric value, to specify that the value is to be AND'ed
              with the numeric value before any comparisons are done.
              Prepending a u to the type indicates that ordered
              comparisons should be unsigned.

     test     The value to be compared with the value from the file.  If
              the type is numeric, this value is specified in C form; if
              it is a string, it is specified as a C string with the
              usual escapes permitted (e.g. \n for new-line).

              Numeric values may be preceded by a character indicating
              the operation to be performed.  It may be =, to specify
              that the value from the file must equal the specified
              value, <, to specify that the value from the file must be
              less than the specified value, >, to specify that the
              value from the file must be greater than the specified
              value, &, to specify that the value from the file must
              have set all of the bits that are set in the specified
              value, ^, to specify that the value from the file must
              have clear any of the bits that are set in the specified
              value, or ~, the value specified after is negated before
              tested.  x, to specify that any value will match.  If the
              character is omitted, it is assumed to be =.  Operators &,
              ^, and ~ don't work with floats and doubles.  The operator
              ! specifies that the line matches if the test does not
              succeed.

              Numeric values are specified in C form; e.g.  13 is
              decimal, 013 is octal, and 0x13 is hexadecimal.

              Numeric operations are not performed on date types,
              instead the numeric value is interpreted as an offset.

              For string values, the string from the file must match the
              specified string.  The operators =, < and > (but not &)
              can be applied to strings.  The length used for matching
              is that of the string argument in the magic file.  This
              means that a line can match any non-empty string (usually
              used to then print the string), with >\0 (because all non-
              empty strings are greater than the empty string).

              Dates are treated as numerical values in the respective
              internal representation.

              The special test x always evaluates to true.

     message  The message to be printed if the comparison succeeds.  If
              the string contains a printf(3) format specification, the
              value from the file (with any specified masking performed)
              is printed using the message as the format string.  If the
              string begins with “\b”, the message printed is the
              remainder of the string with no whitespace added before
              it: multiple matches are normally separated by a single
              space.

     An APPLE 4+4 character APPLE creator and type can be specified as:

           !:apple CREATYPE

     A MIME type is given on a separate line, which must be the next
     non-blank or comment line after the magic line that identifies the
     file type, and has the following format:

           !:mime  MIMETYPE

     i.e. the literal string “!:mime” followed by the MIME type.

     An optional strength can be supplied on a separate line which
     refers to the current magic description using the following format:

           !:strength OP VALUE

     The operand OP can be: +, -, *, or / and VALUE is a constant
     between 0 and 255.  This constant is applied using the specified
     operand to the currently computed default magic strength.

     Some file formats contain additional information which is to be
     printed along with the file type or need additional tests to
     determine the true file type.  These additional tests are
     introduced by one or more > characters preceding the offset.  The
     number of > on the line indicates the level of the test; a line
     with no > at the beginning is considered to be at level 0.  Tests
     are arranged in a tree-like hierarchy: if the test on a line at
     level n succeeds, all following tests at level n+1 are performed,
     and the messages printed if the tests succeed, until a line with
     level n (or less) appears.  For more complex files, one can use
     empty messages to get just the "if/then" effect, in the following
     way:

           0      string   MZ
           >0x18  leshort  <0x40   MS-DOS executable
           >0x18  leshort  >0x3f   extended PC executable (e.g., MS Windows)

     Offsets do not need to be constant, but can also be read from the
     file being examined.  If the first character following the last >
     is a ( then the string after the parenthesis is interpreted as an
     indirect offset.  That means that the number after the parenthesis
     is used as an offset in the file.  The value at that offset is
     read, and is used again as an offset in the file.  Indirect offsets
     are of the form: (( x [[.,][bBcCeEfFgGhHiIlmsSqQ]][+-][ y ]).  The
     value of x is used as an offset in the file.  A byte, id3 length,
     short or long is read at that offset depending on the
     [bBcCeEfFgGhHiIlmsSqQ] type specifier.  The value is treated as
     signed if “”, is specified or unsigned if “”.  is specified.  The
     capitalized types interpret the number as a big endian value,
     whereas the small letter versions interpret the number as a little
     endian value; the m type interprets the number as a middle endian
     (PDP-11) value.  To that number the value of y is added and the
     result is used as an offset in the file.  The default type if one
     is not specified is long.  The following types are recognized:

           Type    Sy Mnemonic   Sy Endian Sy Size
           bcBc    Byte/Char     N/A       1
           efg     Double        Little    8
           EFG     Double        Big       8
           hs      Half/Short    Little    2
           HS      Half/Short    Big       2
           i       ID3           Little    4
           I       ID3           Big       4
           m       Middle        Middle    4
           q       Quad          Little    8
           Q       Quad          Big       8

     That way variable length structures can be examined:

           # MS Windows executables are also valid MS-DOS executables
           0           string  MZ
           >0x18       leshort <0x40   MZ executable (MS-DOS)
           # skip the whole block below if it is not an extended executable
           >0x18       leshort >0x3f
           >>(0x3c.l)  string  PE\0\0  PE executable (MS-Windows)
           >>(0x3c.l)  string  LX\0\0  LX executable (OS/2)

     This strategy of examining has a drawback: you must make sure that
     you eventually print something, or users may get empty output (such
     as when there is neither PE\0\0 nor LE\0\0 in the above example).

     If this indirect offset cannot be used directly, simple
     calculations are possible: appending [+-*/%&|^]number inside
     parentheses allows one to modify the value read from the file
     before it is used as an offset:

           # MS Windows executables are also valid MS-DOS executables
           0           string  MZ
           # sometimes, the value at 0x18 is less that 0x40 but there's still an
           # extended executable, simply appended to the file
           >0x18       leshort <0x40
           >>(4.s*512) leshort 0x014c  COFF executable (MS-DOS, DJGPP)
           >>(4.s*512) leshort !0x014c MZ executable (MS-DOS)

     Sometimes you do not know the exact offset as this depends on the
     length or position (when indirection was used before) of preceding
     fields.  You can specify an offset relative to the end of the last
     up-level field using ‘&’ as a prefix to the offset:

           0           string  MZ
           >0x18       leshort >0x3f
           >>(0x3c.l)  string  PE\0\0    PE executable (MS-Windows)
           # immediately following the PE signature is the CPU type
           >>>&0       leshort 0x14c     for Intel 80386
           >>>&0       leshort 0x184     for DEC Alpha

     Indirect and relative offsets can be combined:

           0             string  MZ
           >0x18         leshort <0x40
           >>(4.s*512)   leshort !0x014c MZ executable (MS-DOS)
           # if it's not COFF, go back 512 bytes and add the offset taken
           # from byte 2/3, which is yet another way of finding the start
           # of the extended executable
           >>>&(2.s-514) string  LE      LE executable (MS Windows VxD driver)

     Or the other way around:

           0                 string  MZ
           >0x18             leshort >0x3f
           >>(0x3c.l)        string  LE\0\0  LE executable (MS-Windows)
           # at offset 0x80 (-4, since relative offsets start at the end
           # of the up-level match) inside the LE header, we find the absolute
           # offset to the code area, where we look for a specific signature
           >>>(&0x7c.l+0x26) string  UPX     \b, UPX compressed

     Or even both!

           0                string  MZ
           >0x18            leshort >0x3f
           >>(0x3c.l)       string  LE\0\0 LE executable (MS-Windows)
           # at offset 0x58 inside the LE header, we find the relative offset
           # to a data area where we look for a specific signature
           >>>&(&0x54.l-3)  string  UNACE  \b, ACE self-extracting archive

     If you have to deal with offset/length pairs in your file, even the
     second value in a parenthesized expression can be taken from the
     file itself, using another set of parentheses.  Note that this
     additional indirect offset is always relative to the start of the
     main indirect offset.

           0                 string       MZ
           >0x18             leshort      >0x3f
           >>(0x3c.l)        string       PE\0\0 PE executable (MS-Windows)
           # search for the PE section called ".idata"...
           >>>&0xf4          search/0x140 .idata
           # ...and go to the end of it, calculated from start+length;
           # these are located 14 and 10 bytes after the section name
           >>>>(&0xe.l+(-4)) string       PK\3\4 \b, ZIP self-extracting archive

     If you have a list of known values at a particular continuation
     level, and you want to provide a switch-like default case:

           # clear that continuation level match
           >18     clear
           >18     lelong  1       one
           >18     lelong  2       two
           >18     default x
           # print default match
           >>18    lelong  x       unmatched 0x%x

SEE ALSO         top

     file(1) - the command that reads this file.

BUGS         top

     The formats long, belong, lelong, melong, short, beshort, and
     leshort do not depend on the length of the C data types short and
     long on the platform, even though the Single UNIX Specification
     implies that they do.  However, as OS X Mountain Lion has passed
     the Single UNIX Specification validation suite, and supplies a
     version of file(1) in which they do not depend on the sizes of the
     C data types and that is built for a 64-bit environment in which
     long is 8 bytes rather than 4 bytes, presumably the validation
     suite does not test whether, for example long refers to an item
     with the same size as the C data type long.  There should probably
     be type names int8, uint8, int16, uint16, int32, uint32, int64, and
     uint64, and specified-byte-order variants of them, to make it
     clearer that those types have specified widths.

COLOPHON         top

     This page is part of the file (a file type guesser) project.
     Information about the project can be found at
     http://www.darwinsys.com/file/.  If you have a bug report for this
     manual page, see ⟨http://bugs.gw.com/my_view_page.php⟩.  This page
     was obtained from the project's upstream Git read-only mirror of
     the CVS repository ⟨https://github.com/glensc/file⟩ on 2021-04-01.
     (At that time, the date of the most recent commit that was found in
     the repository was 2021-04-01.)  If you discover any rendering
     problems in this HTML version of the page, or you believe there is
     a better or more up-to-date source for the page, or you have
     corrections or improvements to the information in this COLOPHON
     (which is not part of the original manual page), send a mail to
     man-pages@man7.org

BSD                            May 9, 2020                           BSD