searching files for hex or oct values

Applications Programming
1

I have a set of files without extensions. How can I programatically tell if a file is in gzip format? The gzip file format spec

RFC 1952 GZIP File Format Specification version 4.3

states that gzip files have certain hex/oct values at the beginning of the file.

1st byte = 0x1f in hex, \037 in octal
2nd byte = 0x8b in hex, \213 in octal

How can I search for these values to determine if a file is truly gzip format?

Thanks.

2 
od -c infile.gz| head
0000000 037 213  \b  \b   � 201   Q   L  \0 003   i   n   f   i   l   e
0000020  \0  \v   N   -   �   �   �   q   �   -   �   t 001 221   ~ 211
3

Here is one way of doing it using C


#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <string.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>

#define GZIP_MAGIC "\037\213"     /* Magic header for gzip files, 1F 8B */

int
main(int argc, char *argv[])
{
    int fd;
    char magic[2];
    ssize_t ret;

    if (argc < 2) {
        fprintf (stderr, "ERROR: A filename must be specified.\n");
        exit(1);
    }

    fd = open(argv[1], O_RDONLY);
    if (fd < 0) {
        fprintf (stderr, "ERROR: cannot open %s\n", argv[1]);
        exit(1);
    }

    if ((ret = read(fd, &magic, 2)) == -1) {
        fprintf (stderr, "ERROR: cannot read %s\n", argv[1]);
        exit(1);
    }

    if (memcmp(magic, GZIP_MAGIC, 2) == 0) {
        fprintf (stdout, "%s is a gzipped file\n", argv[1]);
    } else {
        fprintf (stdout, "%s is not a gzipped file\n", argv[1]);
    }

    close(fd);
    exit(0);
}