File Operations

chmod(LIST)

chmod LIST

Changes the permissions of a list of files. The first element of the list must be the numerical mode. Returns the number of files successfully changed.

	$cnt = chmod 0755, 'foo', 'bar';
	chmod 0755, @executables;

chown(LIST)

chown LIST

Changes the owner (and group) of a list of files. The first two elements of the list must be the NUMERICAL uid and gid, in that order. Returns the number of files successfully changed.

	$cnt = chown $uid, $gid, 'foo', 'bar';
	chown $uid, $gid, @filenames;

Here's an example that looks up non-numeric uids in the passwd file:

	print "User: ";
	$user = <STDIN>;
	chop($user);
	print "Files: "
	$pattern = <STDIN>;
	chop($pattern);
	open(pass, '/etc/passwd')
		|| die "Can't open passwd: $!\n";
	while (<pass>) {
		($login,$pass,$uid,$gid) = split(/:/);
		$uid{$login} = $uid;
		$gid{$login} = $gid;
	}
	@ary = <${pattern}>;	# get filenames
	if ($uid{$user} eq '') {
		die "$user not in passwd file";
	}
	else {
		chown $uid{$user}, $gid{$user}, @ary;
	}

link(OLDFILE,NEWFILE)

Creates a new filename linked to the old filename. Returns 1 for success, 0 otherwise.

lstat(FILEHANDLE)

lstat FILEHANDLE

lstat(EXPR)

lstat SCALARVARIABLE

Does the same thing as the stat() function, but stats a symbolic link instead of the file the symbolic link points to. If symbolic links are unimplemented on your system, a normal stat is done.

mkdir(FILENAME,MODE)

Creates the directory specified by FILENAME, with permissions specified by MODE (as modified by umask). If it succeeds it returns 1, otherwise it returns 0 and sets $! (errno).

readlink(EXPR)

readlink EXPR

Returns the value of a symbolic link, if symbolic links are implemented. If not, gives a fatal error. If there is some system error, returns the undefined value and sets $! (errno). If EXPR is omitted, uses $_.

recv(SOCKET,SCALAR,LEN,FLAGS)

Receives a message on a socket. Attempts to receive LENGTH bytes of data into variable SCALAR from the specified SOCKET filehandle. Returns the address of the sender, or the undefined value if there's an error. SCALAR will be grown or shrunk to the length actually read. Takes the same flags as the system call of the same name.

rename(OLDNAME,NEWNAME)

Changes the name of a file. Returns 1 for success, 0 otherwise. Will not work across filesystem boundaries.

rmdir(FILENAME)

rmdir FILENAME

Deletes the directory specified by FILENAME if it is empty. If it succeeds it returns 1, otherwise it returns 0 and sets $! (errno). If FILENAME is omitted, uses $_.

select(RBITS,WBITS,EBITS,TIMEOUT)

This calls the select system call with the bitmasks specified, which can be constructed using fileno() and vec(), along these lines:

	$rin = $win = $ein = '';
	vec($rin,fileno(STDIN),1) = 1;
	vec($win,fileno(STDOUT),1) = 1;
	$ein = $rin | $win;

If you want to select on many filehandles you might wish to write a subroutine:

	sub fhbits {
	    local(@fhlist) = split(' ',$_[0]);
	    local($bits);
	    for (@fhlist) {
		vec($bits,fileno($_),1) = 1;
	    }
	    $bits;
	}
	$rin = &fhbits('STDIN TTY SOCK');

The usual idiom is:

	($nfound,$timeleft) =
	  select($rout=$rin, $wout=$win, $eout=$ein, $timeout);

or to block until something becomes ready:

	$nfound = select($rout=$rin, $wout=$win,
				$eout=$ein, undef);

Any of the bitmasks can also be undef. The timeout, if specified, is in seconds, which may be fractional. NOTE: not all implementations are capable of returning the $timeleft. If not, they always return $timeleft equal to the supplied $timeout.

stat(FILEHANDLE)

stat FILEHANDLE

stat(EXPR)

stat SCALARVARIABLE

Returns a 13-element array giving the statistics for a file, either the file opened via FILEHANDLE, or named by EXPR. 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);

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

symlink(OLDFILE,NEWFILE)

Creates a new filename symbolically linked to the old filename. Returns 1 for success, 0 otherwise. On systems that don't support symbolic links, produces a fatal error at run time. To check for that, use eval:

	$symlink_exists = (eval 'symlink("","");', $@ eq '');

truncate(FILEHANDLE,LENGTH)

truncate(EXPR,LENGTH)

Truncates the file opened on FILEHANDLE, or named by EXPR, to the specified length. Produces a fatal error if truncate isn't implemented on your system.

unlink(LIST)

unlink LIST

Deletes a list of files. Returns the number of files successfully deleted.

	$cnt = unlink 'a', 'b', 'c';
	unlink @goners;
	unlink <*.bak>;

Note: unlink will not delete directories unless you are superuser and the -U flag is supplied to perl. Even if these conditions are met, be warned that unlinking a directory can inflict damage on your filesystem. Use rmdir instead.

utime(LIST)

utime LIST

Changes the access and modification times on each file of a list of files. The first two elements of the list must be the NUMERICAL access and modification times, in that order. Returns the number of files successfully changed. The inode modification time of each file is set to the current time. Example of a "touch" command:

	#!/usr/bin/perl
	$now = time;
	utime $now, $now, @ARGV;