Perl Notes

Chapter 8.8 Checking a File's Status

It's often useful to know whether a file already exists, if it's readable, writeable, or executeable, and so on. You can combine the use of a conditional with a special set of codes that test for certain file states.

To check a files status:

Type -e to check if the files exists.

Or type -r to check if it's permissions are such that it can be read.

Or type -x to check if it's permissions are set such that it can be executed.

Or type -w to check if its permissions are set such tat it can be written to.

Or type -d to check if it's a directory.
Type the filename that you're interested in checking. You can use either a scalar variable or a constant. If you use, enclose it in quotation marks.

chdir ("../../logs"); if ( -e $desired_file) { open(LOG, $desired_file) || &ErrorMesage; @log_lines = <LOG>; close(LOG); $n=1; foreach $line (@log_lines) { print "Line # $n is $line"; $n++; } } else { print "Log does not exist."; }

File Test Operators Table

Operator	Meaning
-r	File is readable by effective uid/gid.
-w	File is writable by effective uid/gid.
-x	File is executable by effective uid/gid.
-o	File is owned by effective uid.
-R	File is readable by real uid/gid.
-W	File is writable by real uid/gid.
-X	File is executable by real uid/gid.
-O	File is owned by real uid.

-e	File exists.
-z	File has zero size.
-s	File has non-zero size (returns size).

-u	File has setuid bit set.
-g	File has setgid bit set.
-k	File has sticky bit set.

Operator	Meaning
-f	File is a plain file.
-d	File is a directory.
-l	File is a symbolic link.
-p	File is a named pipe (FIFO).
-S	File is a socket.
-b	File is a block special file.
-c	File is a character special file.
-t	Filehandle is opened to a tty.

-T	File is a text file.
-B	File is a binary file (opposite of -T).

-M	Age of file (at startup) in days since modification.
-A	Age of file (at startup) in days since last access.
-C	Age of file (at startup) in days since inode change

stat - get a file's status information

stat FILEHANDLE stat EXPR stat

Returns a 13-element list giving the status info for a file, either the file opened via FILEHANDLE, or named by EXPR. If EXPR is omitted, it stats $_. Returns a null list if the stat fails. Typically used as follows:

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

Not all fields are supported on all filesystem types. Here are the meaning of the fields:

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 since the epoch
9	mtime	last modify time since the epoch
10	ctime	inode change time (NOT creation time!) since the epoch
11	blksize	preferred block size for file system I/O
12	blocks	actual number of blocks allocated

(The epoch was at 00:00 January 1, 1970 GMT.)

If stat is passed the special filehandle consisting of an underline, no stat is done, but the current contents of the stat structure from the last stat or filetest are returned. Example:

if (-x $file && (($d) = stat(_)) && $d < 0) { print "$file is executable NFS file\n"; }

(This works on machines only for which the device number is negative under NFS.)

In scalar context, stat() returns a boolean value indicating success or failure, and, if successful, sets the information associated with the special filehandle _.

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