in reply to Unix File Type with stat

Yes, 0170000 and 0007777 are octal masks, the first for file types, the second for permissions. Here is the complete list of masks and constants from 'man 2 stat': 

  The following flags are defined for the st_mode field:

    S_IFMT     0170000   bitmask for the file type bitfields
    S_IFSOCK   0140000   socket
    S_IFLNK    0120000   symbolic link
    S_IFREG    0100000   regular file
    S_IFBLK    0060000   block device
    S_IFDIR    0040000   directory
    S_IFCHR    0020000   character device
    S_IFIFO    0010000   fifo
    S_ISUID    0004000   set UID bit
    S_ISGID    0002000   set GID bit (see below)
    S_ISVTX    0001000   sticky bit (see below)
    S_IRWXU    00700     mask for file owner permissions
    S_IRUSR    00400     owner has read permission
    S_IWUSR    00200     owner has write permission
    S_IXUSR    00100     owner has execute permission
    S_IRWXG    00070     mask for group permissions
    S_IRGRP    00040     group has read permission
    S_IWGRP    00020     group has write permission
    S_IXGRP    00010     group has execute permission
    S_IRWXO    00007     mask for permissions for others (not in group
+)
    S_IROTH    00004     others have read permission
    S_IWOTH    00002     others have write permisson
    S_IXOTH    00001     others have execute permission

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Octal is the popular notation for these, because sets of permissions occupy three bits.

After Compline,
Zaxo

Replies are listed 'Best First'.
Re: Unix File Type with stat
by Anonymous Monk on Sep 10, 2002 at 12:55 UTC
    I am wanting to see if my file is a link without having to rerun the stat operator. I saw an note in O'Reilly Perl book that stated that when returning info on a link, stat would return the filetype of the actual file (- , d, etc...). So maybe I should re-phrase my question to: Is there a way to have an algorithm that will tell me what type of file it is without having to check each specific bit. The following code returns a - for the file which is a link in both the $filelink and the $filetype variables. 
    
# hard code a test link file to check
$source_file = "/temp/test/source/dir1/hello1_link.txt";

# define filetype array with potential first bits of mode
@ftypes  = qw(. p c ? d ? b ? - ? l ? s ? ? ?);
$ftypes0 = '';

# Get File Statistics Using Scalar context

($dev,$inode,$mode,$nlink,$uid,$gid,$rdev,$size,
 $atime,$mtime,$ctime,$blksize,$blocks) = stat($source_file);

$perms    = $mode & 07777;
$octperms = sprintf("%lo", $perms);
$filetype = $ftypes[($mode & 0170000)>>12];
$filelink = $ftypes[($mode & 0120000)>>12];

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    And while I am admitting how much I don't know, what does the >>12 do in the array reference? Thanks.
[reply]
[d/l]
      I can answer the last part about the bit shift operator.
      >> 12 takes the number to the left and shifts it 12 bits to the right.
      << 12 would shift it 12 bits to the left.
      In octal there are 3 bits per character shown (is this still called a nibble? Or is that only 4 bits, I don't play in octal that often :)
      So this will bitwise and $mode with 0170000 and then shift it down so that the bottom 4 zero's go away.
      0170000 >> 12 = 017
      Hope this helps!
[reply]

      You should use the lstat builtin if you don't want to resolve soft links.

      If you use Fcntl qw(:mode); you can get the constants by name. That will be easier to read and remember.

      ++Helter for the other answers.

      After Compline,
      Zaxo

[reply]
Re: Unix File Type with stat
by Anonymous Monk on Sep 10, 2002 at 12:38 UTC
    What does the $mode & 07777 do? Could you explain it in layman's terms. I am not familiar with this type of expression as I am very new to Perl.
[reply]
      I'm not sure there is a "laymans" terms explanation for this. A single & is a bitwise and. This means it will go through the left and right, compare them bit by bit and the result is determined by the and function:
      src1   src2   dest
0      0      0
0      1      0
1      0      0
1      1      1

      [download]
      In your example src1 is $mode, src2 is 07777 and dest is $perms. This is mostly used to zero out bits, or mask off portions of bits.
      The bitwise or operator, |, is used to set bits in a bit-string.
      Hope this helped.
[reply]
[d/l]

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