Unix Tip: Displaying File Attributes with the Perl stat Command

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A lot of information is available about individual files on a Unix system. For example, the ls -l command will display the permissions matrix and ls -i will display a file's inode. But, if we want to list all three of the file's dates (atime, ctime and mtime) or retrieve one of the attributes in a script, one of the versatile tools is a command included in Perl -- the stat command.



The stat command retrieves thirteen different pieces of information about a file and is most often used like this command in which each of the file descriptors is assigned to a variable that reflects its value:

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

Numbered 0 through 12, each of these parameters is defined here: 
  0 dev      the device number of the filesystem
  1 ino      the inode number
  2 mode     the file mode  (type and permissions)
  3 nlink    the number of (hard) links to the file
  4 uid      the numeric user ID of file's owner
  5 gid      the numeric group ID of file's owner
  6 rdev     the device identifier (special files only)
  7 size     the total size of the file, in bytes
  8 atime    the last access time in seconds since the epoch
  9 mtime    the last modify time in seconds since the epoch
 10 ctime    the inode change time in seconds since the epoch (*)
 11 blksize  the preferred block size for file system I/O
 12 blocks   the actual number of blocks allocated

If you are only interested in using one of the file's attribute, on the other hand, you could use this syntax and avoid the "possible typo" messages that you would get from failing to make subsequent use of the other twelve values:

$mode = (stat($filename))[2];



What you're going to notice right away, however, when you use the stat command is that not all of the attributes are going to be extracted in a form that makes them easy to use. If we retrieve the mode value, using the command shown above, for example, and then print the value, we're going to see something like this:



33188



This is because the mode field contains the file type along with its permissions matrix, so you still need to extract the information you want. This can be done by logically anding the field with the number 07777 like this:

printf "Permissions are %04o\n", $mode & 07777; 

The number 33188 is 0100644 in octal and that number ANDed with 07777 yields 644. This printf statement will, therefore, print the phrase "Permissions are 0644".

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