Explain please what is a magic file in unix [closed]
We can create magic files with file -C -m filename . Also there is the /usr/share/file/magic folder with a list of magic files on my OS X machine. But what is this? Would somebody explain, why should we have magic files? How should we create a magic file? I read the man pages ( man file and man magic ), but it is still not clear.
1 Answer 1
In Unix a file is just a sequence of bytes, however some files have more structure. The file command can look at the sequence of bytes and tell you things.for example if the first 4bytes are \177 E L F then it will tell you it is an elf file. It will then look at more bytes and tell you if it is a core file, an executable or an object file. If the bytes 6 thru 9 are JFIF then it is a picture file and again the program can read more and tell you about it.
A lot of this is driven by data tables rather than being compiled into the program. These data tables are what is in the magic files.
So the reason why you have magic files is to allow you to say file * and get the program to tell you what you have in the current directory.
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Linux file magic file
This manual page documents the format of the magic file as used by the file (1) command, version 4.17. The file command 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/file/magic specifies 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 offset in the file with a 1-byte, 2-byte, or 4-byte numeric value or a 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 data which 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. The string type specification can be optionally followed by /[Bbc]*. The «B» 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 «b» flag treats every blank in the target as an optional blank. Finally the «c» flag, specifies case insensitive matching: lowercase characters in the magic match both lower and upper case characters in the targer, whereas upper case characters in the magic, only much uppercase characters in the target. pstring A pascal style string where the first byte is interpreted as the an unsigned length. The string is not NUL terminated. date A four-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. 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. beldate A four-byte value (on most systems) in big-endian byte order, interpreted as a UNIX-style date, but interpreted as local time rather than UTC. bestring16 A two-byte unicode (UCS16) string in big-endian byte order. 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. leldate A four-byte value (on most systems) in little-endian byte order, interpreted as a UNIX-style date, but interpreted as local time rather than UTC. lestring16 A two-byte unicode (UCS16) string in little-endian byte order. melong A four-byte value (on most systems) in middle-endian (PDP-11) byte order. medate A four-byte value (on most systems) in middle-endian (PDP-11) byte order, interpreted as a UNIX date. meldate A four-byte value (on most systems) in middle-endian (PDP-11) byte order, interpreted as a UNIX-style date, but interpreted as local time rather than UTC. regex A regular expression match in extended POSIX regular expression syntax (much like egrep). The type specification can be optionally followed by /c for case-insensitive matches. The regular expression is always tested against the first N lines, where N is the given offset, thus it is only useful for (single-byte encoded) text. ^ and $ will 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. It must be followed by / which specifies how many matches shall be attempted (the range). This is suitable for searching larger binary expressions with variable offsets, using \ escapes for special characters.
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 = . For all tests except string and regex, operation ! 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. For string values, the byte string from the file 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 argument in the magic file. This means that a line can match any string, and then 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 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.
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 a 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, untile 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 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 [.[bslBSL]][+-][ y ]). The value of x is used as an offset in the file. A byte, short or long is read at that offset depending on the [bslBSLm] type specifier. 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.
That way variable length structures can be examined:
# MS Windows executables are also valid MS-DOS executables 0 string MZ >0x18 leshort 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 one drawback: You must make sure that you eventually print something, or users may get empty output (like, when there is neither PE\0\0 nor LE\0\0 in the above example)
If this indirect offset cannot be used as-is, there are simple calculations possible: appending [+-*/%&|^] 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 >(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 uplevel 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 >(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)
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 uplevel 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
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
Finally, if you have to deal with offset/length pairs in your file, even the second value in a parenthesed 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
BUGS
The formats long , belong , lelong , melong , short , beshort , leshort , date , bedate , medate , ledate , beldate , leldate , and meldate are system-dependent; perhaps they should be specified as a number of bytes (2B, 4B, etc), since the files being recognized typically come from a system on which the lengths are invariant.