#!/usr/local/bin/perl

#

# watch_a_file.perl -- use lsof -F output to watch a specific file

#		       (or file system)

#

# usage:	watch_a_file.perl file_name

## Interrupt handler

sub interrupt { wait; print "\n"; exit 0; }

## Start main program

$Pn = "watch_a_file";

# Check file argument.

if ($#ARGV != 0) { print "$#ARGV\n"; die "$Pn usage: file_name\n"; }

$fnm = $ARGV[0];

if (! -r $fnm) { die "$Pn: can't read $fnm\n"; }

# Do setup.

$RPT = 15;				# lsof repeat time

$| = 1;					# unbuffer output

$SIG{'INT'} = 'interrupt';		# catch interrupt

# Set path to lsof.

if (($LSOF = &isexec("../lsof")) eq "") {	# Try .. first

    if (($LSOF = &isexec("lsof")) eq "") {	# Then try . and $PATH

	print "can't execute $LSOF\n"; exit 1

    }

}

# Read lsof -nPF output from a pipe and gather the PIDs of the processes

# and file descriptors to watch.

open(P, "$LSOF -nPFpf $fnm|") || die "$Pn: can't pipe to $LSOF\n";

$curpid = -1;

$pids = "";

while (
) {

    chop;

    if (/^p(.*)/) { $curpid = $1; next; }	# Identify process.

    if (/^f/) {

	if ($curpid > 0) {

	    if ($pids eq "") { $pids = $curpid; }

	    else { $pids = $pids . "," . $curpid; }

	    $curpid = -1;

	}

    }

}

close(P);

wait;

if ($pids eq "") { die "$Pn: no processes using $fnm located.\n"; }

print "watch_file: $fnm being used by processes:\n\t$pids\n\n";

# Read repeated lsof output from a pipe and display.

$pipe = "$LSOF -ap $pids -r $RPT $fnm";

open(P, "$pipe|") || die "$Pn: can't pipe: $pipe\n";

while (
) { print $_; }

close(P);

print "$Pn: unexpected EOF from \"$pipe\"\n";

exit 1;

## isexec($path) -- is $path executable

#

# $path   = absolute or relative path to file to test for executabiity.

#	    Paths that begin with neither '/' nor '.' that arent't found as

#	    simple references are also tested with the path prefixes of the

#	    PATH environment variable.  

sub

isexec {

    my ($path) = @_;

    my ($i, @P, $PATH);

    $path =~ s/^\s+|\s+$//g;

    if ($path eq "") { return(""); }

    if (($path =~ m#^[\/\.]#)) {

	if (-x $path) { return($path); }

	return("");

    }

    $PATH = $ENV{PATH};

    @P = split(":", $PATH);

    for ($i = 0; $i <= $#P; $i++) {

	if (-x "$P[$i]/$path") { return("$P[$i]/$path"); }

    }

    return("");

}