#lang magic
#lang magic is a Racket implementation of the mini language used by the Unix file command. This is a language designed to write tests to determine a file's type. Thousands of tests written in the magic language can be seamlessly incorporated into Racket applications using #lang magic. In addition, compared to the file command, Racket provides a secure environment for executing these tests.
Requirements
- Racket version 7.4.0.902 or higher.
- The brag Racket package. Install with
raco pkg install brag. - #lang magic is currently being developed and tested with Racket on Linux. It has also been tested briefly on Windows and didn't exhibit any obvious problems. I will occassionaly spot check the Windows version, but I'm not currently testing every commit against Windows.
Description
For the curious, The man pages for 'magic' describes the magic language in considerable, but not exhaustive, detail. A code sample to check for Microsoft executables provides the flavor of the language:
# MS Windows executables are also valid MS-DOS executables
0 string MZ
>0x18 leshort <0x40
>>(4.s*512) leshort 0x014c COFF executable (MS-DOS, DJGPP)
>>(4.s*512) leshort !0x014c 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)
>>>&0 leshort 0x14c for Intel 80386
>>>&0 leshort 0x184 for DEC Alpha
>>>&0 leshort 0x8664 for AMD64
>>(0x3c.l) string LX\0\0 LX executable (OS/2)
In #lang magic the code sample above compiles to one magic query. A #lang magic Racket module consists of 1 or more such queries. New queries start on a line without a preceding '>'. Every #lang magic module provides two functions: magic-query and magic-query-run-all. These functions are thunks that can be passed to with-input-from-file to test the file against the queries in the module. magic-query replicates the default behavior of the file command. It stops and returns true after the first query match or returns false if no queries pass. A query is matched if the test on the first line of the query succeeds. A matched query will run until completion, but even if later tests in the query fail, the query is still considered a match if the first test passes. magic-query-run-all, on the other hand, will always test the file against every query in the module. Both functions print the messages for each successful test to the current output port.
This is a brief summary, so consult 'man magic' for a complete explanation of magic syntax. Windows users can google the Linux man page for 'magic'. The tests/ directory also contains sample magic files and calls to them from test.rkt.
Status
There is still a lot of work to do. Some of the macros need revising and should probably be rewritten with syntax/parse. The lexer could be improved as well. And of course there are missing features, some of which are documented in the TODO file.
Goals
The goal is to get as close to 100% compatibility as possible with Ian Darwin's version(available at https://www.darwinsys.com/file/) found in most Linux and BSD distributions.