MAN page from RedHat Other file-3.26-2.i686.rpm
MAGIC
Section: Devices and Network Interfaces (4)
Updated: Public Domain
Index NAME
magic - file command's magic number file
DESCRIPTION
This manual page documents the format of the magic file asused by the
file(1)command, version 3.26. The
filecommand identifies the type of a file using,among other tests,a test for whether the file begins with a certain
magic number.The file
/usr/share/magicspecifies what magic numbers are to be tested for,what message to print if a particular magic number is found,and additional information to extract from the file.
Each line of the file specifies a test to be performed.A test compares the data starting at a particular offsetin the file with a 1-byte, 2-byte, or 4-byte numeric value ora string. 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 datawhich is to be tested.
- type
- The type of the data to be tested. The possible values are:
- byte
- A one-byte value.
- short
- A two-byte value (on most systems) in this machine's native byte order.
- long
- A four-byte value (on most systems) in this machine's native byte order.
- string
- A string of bytes.
- date
- A four-byte value interpreted as a unix date.
- beshort
- A two-byte value (on most systems) in big-endian byte order.
- belong
- A four-byte value (on most systems) in big-endian byte order.
- bedate
- A four-byte value (on most systems) in big-endian byte order,interpreted as a unix date.
- leshort
- A two-byte value (on most systems) in little-endian byte order.
- lelong
- A four-byte value (on most systems) in little-endian byte order.
- ledate
- A four-byte value (on most systems) in little-endian byte order,interpreted as a unix date.
The numeric types may optionally be followed by&and a numeric value,to specify that the value is to be AND'ed with thenumeric value before any comparisons are done. Prepending auto the type indicates that ordered comparisons should be unsigned.
- test
- The value to be compared with the value from the file. If the type isnumeric, this valueis specified in C form; if it is a string, it is specified as a C stringwith the usual escapes permitted (e.g. \n for new-line).
- Numeric valuesmay 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 specifiedvalue,>,to specify that the value from the file must be greater than the specifiedvalue,&,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, orx,to specify that any value will match. If the character is omitted,it is assumed to be=.
- Numeric values are specified in C form; e.g.13is decimal,013is octal, and0x13is hexadecimal.
- For string values, the byte string from thefile must match the specified byte string. The operators=,<and>(but not&)can be applied to strings.The length used for matching is that of the string argumentin the magic file. This means that a line can match any string, andthen presumably print that string, by doing>\0(because all strings are greater than the null string).
- message
- The message to be printed if the comparison succeeds. If the stringcontains aprintf(3S)format specification, the value from the file (with any specified maskingperformed) is printed using the message as the format string.
Some file formats contain additional information which is to be printedalong with the file type. A line which begins with the character>indicates additional tests and messages to be printed. 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.Each line at leveln+1is under the control of the line at levelnmost closely preceding it in the magic file.If the test on a line at levelnsucceeds, the tests specified in all the subsequent lines at leveln+1are performed, and the messages printed if the tests succeed. The nextline at levelnterminates this.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 inthe file. The value at that offset is read, and is used again as an offsetin the file. Indirect offsets are of the form:((x[.[bslBSL]][+-][y]).The value of xis used as an offset in the file. A byte, short or long is read at that offsetdepending on the [bslBSL] type specifier. The capitalized types interpret the number as a big endianvalue, whereas the small letter versions interpet the number as a littleendian value. To that number the value ofyis added and the result is used as an offset in the file. The default typeif one is not specified is long.
Sometimes you do not know the exact offset as this depends on the length ofpreceding fields. You can specify an offset relative to the end of thelast uplevel field (of course this may only be done for sublevel tests, i.e.test beginning with >). Such a relative offset is specified using&as a prefix to the offset.
BUGS
The formats
long,
belong,
lelong,
short,
beshort,
leshort,
date,
bedate,and
ledateare system-dependent; perhaps they should be specified as a numberof bytes (2B, 4B, etc), since the files being recognized typically come froma system on which the lengths are invariant.
There is (currently) no support for specified-endian data to be used inindirect offsets.
SEE ALSO
file(1)- the command that reads this file.
Index
- NAME
- DESCRIPTION
- BUGS
- SEE ALSO
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