就像福尔摩斯说的，太阳底下就没啥新东西，不过就是来来回回地重复罢了。我怎敢例外！ 所以这里不过是stat函数的perldoc的内容。加上适当理解和注释等等，以其能更好被理解和取用。随着“作案”手法的改进，不定期更新，争取成为连大侦探也会虎躯一震的有用存在。
先来看看文件的测试：
Operator Meaning
-r $file True if $file is a readable file.（是否可读）
-w $file True if $file is a writeable file.（是否可写）
-x $file True if $file is an executable file.（是否可执行）
-o $file True if $file is owned by effective uid.（effective 用户）
以上都是针对 uid,而非ruid
uid effective user ID
ruid real user ID
存在情况判断：
-e $file True if file exists.（测试文件是否存在，存在为真）
-z $file True if file is zero in size.（文件是否存在且为空。对目录而言，永远为假）
-s $file True if $file has nonzero size. Returns the size of the file in bytes.
文件是否不空。返回值是文件大小，单位字节。如果返回值为0，说明文件为空。
类型判断
-f $file True if $file is a plain file.(是否为普通文件）
-d $file True if $file is a directory file.（是否为目录）
-l $file True if $file is a symbolic link.（是否为软链接，即字符链接）
-p $file True if $file is a named pipe or FIFO.（是否为命名管道，即FIFO)
管道读写数据的原理是先进先出(First In First Out) 所以叫做FIFO
-S $file True if $file is a socket.（是否为 socket)
-b $file True if $file is a block special file.(是否为块设备）
-c $file True if $file is a character special file.（是否为字符设备文件）
-T $file True if $file is a text file.（文本文件）
-B $file True if file is a binary file.（二进制文件）
所有权测试：
-u $file True if $file has a setuid bit set.（是否设置了setuid）
-g $file True if $file has a setgid bit set.（是否设置了setgid）
-k $file True if $file has a sticky bit set. （是否设置了sticky
-t $file True if filehandle is opened to a tty.(文件句柄是否为TTY设备(该测试只对文件句柄有效))
-M $file Age of the file in days since modified.(最后一次修改距离目前的天数 mtime）
-A $file Age of the file in days since last accessed.（最后一次访问距离目前的天数 atime）
-C $file Age of the file in days since the inode changed.（最后一次inode修改距离目前的天数 ctime）
接下来说说stat:
#### stat 的值
my ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,$atime,$mtime,$ctime,$blksize,$blocks)
= stat($filename);
返回的是文件的13个参数值：（Returns a 13-element list giving the status info for a file:）
0 dev device number of filesystem
1 ino inode number
2 mode file mode (type and permissions)
3 nlink number of (hard) links to the file
4 uid numeric user ID of file's owner
5 gid numeric group ID of file's owner
6 rdev the device identifier (special files only)
7 size total size of file, in bytes
8 atime last access time in seconds since the epoch
9 mtime last modify time in seconds since the epoch
10 ctime inode change time in seconds since the epoch (*)
11 blksize preferred I/O size in bytes for interacting with the
file (may vary from file to file)
12 blocks actual number of system-specific blocks allocated
on disk (often, but not always, 512 bytes each)
!!!!!(The epoch was at 00:00 January 1, 1970 GMT.)
0 dev 设备号 驱动器号
1 ino 索引节号
2 mode 文件的类型与权限
3 nlink 链接（硬链接）数
4 uid 文件所有者的用户ID（UID）
5 gid 文件所有者的组ID（GID）
6 rdev 特殊文件信息 驱动器号
7 size 文件大小（以字节计）文件大小（以字节计）
8 atime 上次访问的时间
9 mtime 上次修改的时间
10 ctime Inode修改时间 可以算做文件的创建时间
11 blksz 磁盘块的大小
12 blocks 文件中的块的数量
也可以这么用：
use File::stat;
my $mtime = stat($filename)->mtime;
一个栗子：
mode，包括文件类型和文件权限。如果只需要文件权限，需要把文件类型的部分，用位运算符的与操作(&）屏蔽掉（you should mask off the file type portion and (s)printf using a "%o" if you want to see the real permissions.）
my @array=stat("Perl_Note.txt");
print "atime is:",$array[8],"\n";
print "mtime is:",$array[9],"\n";
print "inode time is:",$array[10],"\n";
print "mode is:",$array[2],"\n";
printf "Permissions are %04o\n", $array[2] & 07777;
【output]:
atime is:1571997054
mtime is:1571996794
inode time is:1571996794
mode is:33279
Permissions are 0777
Mode： 33279(十进制decimal)->100777(八进制octal)-> 001-000-000-111-111-111(二进制 binary)
其中开始部分并不是权限，我们需要把这部分屏蔽掉（通过与 111-111-111-111 进行位与操作，即07777).
注意，777是十进制，0777才是八进制。
（777 is decimal, not octal. If you want only the last nine bits, then AND the number with octal 777: 0777）
时间模式并不友好吧？ 可以用localtime 改成易读的模式（参考：
perl:好用的localtime函数。）
print "Atime is (Readable):",scalar localtime($array[8]),"\n";
print "mtime is (Readable):",scalar localtime($array[9]),"\n";
print "Inode time is (Readable):",scalar localtime($array[10]),"\n";
【output】
Atime is (Readable):Sun Oct 27 09:56:07 2019
mtime is (Readable):Fri Oct 25 17:59:48 2019
Inode time is (Readable):Sun Oct 27 09:15:14 2019
man 2 stat about mode:
S_IFMT 0170000 bit mask for the file type bit fields
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-group-ID 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 permission
S_IXOTH 00001 others have execute permission
(Note that the leading 0 means those numbers are octal numbers.)
You can see seven fields in the mode word.
S_IFMT file type
S_ISUID set UID bit
S_ISGID set-group-ID bit
S_ISVTX sticky bit
S_IRWXU owner permissions
S_IRWXG group permissions
S_IRWXO other permissions
另一个栗子：
找出12小时内修改了的文件，
用stat 函数。利用现在时间和修改时间减，得出修改的间隔。都是是从1970年起的秒数。
和12小时的秒数相比，查找修改时间符合要求的：
my $lmtimedate = (stat $myfile)[9];
my $now = time();
my $howLongAgo = $now - $lmtimedate; #Since epoch time is just a number of seconds, it's easy to calculate short distances in time. 60*60 = 1 hour * 12 = 12 hours.
if ($howLongAgo <= (60*60*12))
{
print "$myfile was edited less than 12 hours ago.\n";
}
