I have a Perl CGI script that isn't working and I don't know how to start narrowing down the problem. What can I do?
Note: I'm adding the question because I really want to add my very lengthy answer to Stack Overflow. I keep externally linking to it in other answers and it deserves to be here. Don't be shy about editing my answer if you have something to add.
This answer is intended as a general framework for working through
problems with Perl CGI scripts and originally appeared on Perlmonks as Troubleshooting Perl CGI Scripts. It is not a complete guide to every
problem that you may encounter, nor a tutorial on bug squashing. It
is just the culmination of my experience debugging CGI scripts for twenty (plus!) years. This page seems to have had many different homes, and I seem
to forget it exists, so I'm adding it to the StackOverflow. You
can send any comments or suggestions to me at
bdfoy#cpan.org. It's also community wiki, but don't go too nuts. :)
Are you using Perl's built in features to help you find problems?
Turn on warnings to let Perl warn you about questionable parts of your code. You can do this from the command line with the -w switch so you don't have to change any code or add a pragma to every file:
% perl -w program.pl
However, you should force yourself to always clear up questionable code by adding the warnings pragma to all of your files:
use warnings;
If you need more information than the short warning message, use the diagnostics pragma to get more information, or look in the perldiag documentation:
use diagnostics;
Did you output a valid CGI header first?
The server is expecting the first output from a CGI script to be the CGI header. Typically that might be as simple as print "Content-type: text/plain\n\n"; or with CGI.pm and its derivatives, print header(). Some servers are sensitive to error output (on STDERR) showing up before standard output (on STDOUT).
Try sending errors to the browser
Add this line
use CGI::Carp 'fatalsToBrowser';
to your script. This also sends compilation errors to the browser window. Be sure to remove this before moving to a production environment, as the extra information can be a security risk.
What did the error log say?
Servers keep error logs (or they should, at least).
Error output from the server and from your script should
show up there. Find the error log and see what it says.
There isn't a standard place for log files. Look in the
server configuration for their location, or ask the server
admin. You can also use tools such as CGI::Carp
to keep your own log files.
What are the script's permissions?
If you see errors like "Permission denied" or "Method not
implemented", it probably means that your script is not
readable and executable by the web server user. On flavors
of Unix, changing the mode to 755 is recommended:
chmod 755 filename. Never set a mode to 777!
Are you using use strict?
Remember that Perl automatically creates variables when
you first use them. This is a feature, but sometimes can
cause bugs if you mistype a variable name. The pragma
use strict will help you find those sorts of
errors. It's annoying until you get used to it, but your
programming will improve significantly after awhile and
you will be free to make different mistakes.
Does the script compile?
You can check for compilation errors by using the -c
switch. Concentrate on the first errors reported. Rinse,
repeat. If you are getting really strange errors, check to
ensure that your script has the right line endings. If you
FTP in binary mode, checkout from CVS, or something else that
does not handle line end translation, the web server may see
your script as one big line. Transfer Perl scripts in ASCII
mode.
Is the script complaining about insecure dependencies?
If your script complains about insecure dependencies, you
are probably using the -T switch to turn on taint mode, which is
a good thing since it keeps you have passing unchecked data to the shell. If
it is complaining it is doing its job to help us write more secure scripts. Any
data originating from outside of the program (i.e. the environment)
is considered tainted. Environment variables such as PATH and
LD_LIBRARY_PATH
are particularly troublesome. You have to set these to a safe value
or unset them completely, as I recommend. You should be using absolute
paths anyway. If taint checking complains about something else,
make sure that you have untainted the data. See perlsec
man page for details.
What happens when you run it from the command line?
Does the script output what you expect when run from the
command line? Is the header output first, followed by a
blank line? Remember that STDERR may be merged with STDOUT
if you are on a terminal (e.g. an interactive session), and
due to buffering may show up in a jumbled order. Turn on
Perl's autoflush feature by setting $| to a
true value. Typically you might see $|++; in
CGI programs. Once set, every print and write will
immediately go to the output rather than being buffered.
You have to set this for each filehandle. Use select to
change the default filehandle, like so:
$|++; #sets $| for STDOUT
$old_handle = select( STDERR ); #change to STDERR
$|++; #sets $| for STDERR
select( $old_handle ); #change back to STDOUT
Either way, the first thing output should be the CGI header
followed by a blank line.
What happens when you run it from the command line with a CGI-like environment?
The web server environment is usually a lot more limited
than your command line environment, and has extra
information about the request. If your script runs fine
from the command line, you might try simulating a web server
environment. If the problem appears, you have an
environment problem.
Unset or remove these variables
PATH
LD_LIBRARY_PATH
all ORACLE_* variables
Set these variables
REQUEST_METHOD (set to GET, HEAD, or POST as appropriate)
SERVER_PORT (set to 80, usually)
REMOTE_USER (if you are doing protected access stuff)
Recent versions of CGI.pm ( > 2.75 ) require the -debug flag to
get the old (useful) behavior, so you might have to add it to
your CGI.pm imports.
use CGI qw(-debug)
Are you using die() or warn?
Those functions print to STDERR unless you have redefined
them. They don't output a CGI header, either. You can get
the same functionality with packages such as CGI::Carp
What happens after you clear the browser cache?
If you think your script is doing the right thing, and
when you perform the request manually you get the right
output, the browser might be the culprit. Clear the cache
and set the cache size to zero while testing. Remember that
some browsers are really stupid and won't actually reload
new content even though you tell it to do so. This is
especially prevalent in cases where the URL path is the
same, but the content changes (e.g. dynamic images).
Is the script where you think it is?
The file system path to a script is not necessarily
directly related to the URL path to the script. Make sure
you have the right directory, even if you have to write a
short test script to test this. Furthermore, are you sure
that you are modifying the correct file? If you don't see
any effect with your changes, you might be modifying a
different file, or uploading a file to the wrong place.
(This is, by the way, my most frequent cause of such trouble
;)
Are you using CGI.pm, or a derivative of it?
If your problem is related to parsing the CGI input and you
aren't using a widely tested module like CGI.pm, CGI::Request,
CGI::Simple or CGI::Lite, use the module and get on with life.
CGI.pm has a cgi-lib.pl compatibility mode which can help you solve input
problems due to older CGI parser implementations.
Did you use absolute paths?
If you are running external commands with
system, back ticks, or other IPC facilities,
you should use an absolute path to the external program.
Not only do you know exactly what you are running, but you
avoid some security problems as well. If you are opening
files for either reading or writing, use an absolute path.
The CGI script may have a different idea about the current
directory than you do. Alternatively, you can do an
explicit chdir() to put you in the right place.
Did you check your return values?
Most Perl functions will tell you if they worked or not
and will set $! on failure. Did you check the
return value and examine $! for error messages? Did you check
$# if you were using eval?
Which version of Perl are you using?
The latest stable version of Perl is 5.28 (or not, depending on when this was last edited). Are you using an older version? Different versions of Perl may have different ideas of warnings.
Which web server are you using?
Different servers may act differently in the same
situation. The same server product may act differently with
different configurations. Include as much of this
information as you can in any request for help.
Did you check the server documentation?
Serious CGI programmers should know as much about the
server as possible - including not only the server features
and behavior, but also the local configuration. The
documentation for your server might not be available to you
if you are using a commercial product. Otherwise, the
documentation should be on your server. If it isn't, look
for it on the web.
Did you search the archives of comp.infosystems.www.authoring.cgi?
This use to be useful but all the good posters have either died or wandered off.
It's likely that someone has had your problem before,
and that someone (possibly me) has answered it in this
newsgroup. Although this newsgroup has passed its heyday, the collected wisdom from the past can sometimes be useful.
Can you reproduce the problem with a short test script?
In large systems, it may be difficult to track down a bug
since so many things are happening. Try to reproduce the problem
behavior with the shortest possible script. Knowing the problem
is most of the fix. This may be certainly time-consuming, but you
haven't found the problem yet and you're running out of options. :)
Did you decide to go see a movie?
Seriously. Sometimes we can get so wrapped up in the problem that we
develop "perceptual narrowing" (tunnel vision). Taking a break,
getting a cup of coffee, or blasting some bad guys in [Duke Nukem,Quake,Doom,Halo,COD] might give you
the fresh perspective that you need to re-approach the problem.
Have you vocalized the problem?
Seriously again. Sometimes explaining the problem aloud
leads us to our own answers. Talk to the penguin (plush toy) because
your co-workers aren't listening. If you are interested in this
as a serious debugging tool (and I do recommend it if you haven't
found the problem by now), you might also like to read The Psychology
of Computer Programming.
I think CGI::Debug is worth mentioning as well.
Are you using an error handler while you are debugging?
die statements and other fatal run-time and compile-time errors get
printed to STDERR, which can be hard to find and may be conflated with
messages from other web pages at your site. While you're debugging your
script, it's a good idea to get the fatal error messages to display in your
browser somehow.
One way to do this is to call
use CGI::Carp qw(fatalsToBrowser);
at the top of your script. That call will install a $SIG{__DIE__} handler (see perlvar)display fatal errors in your browser, prepending it with a valid header if necessary. Another CGI debugging trick that I used before I ever heard of CGI::Carp was to
use eval with the DATA and __END__ facilities on the script to catch compile-time errors:
#!/usr/bin/perl
eval join'', <DATA>;
if ($#) { print "Content-type: text/plain:\n\nError in the script:\n$#\n; }
__DATA__
# ... actual CGI script starts here
This more verbose technique has a slight advantage over CGI::Carp in that it will catch more compile-time errors.
Update: I've never used it, but it looks like CGI::Debug, as Mikael S
suggested, is also a very useful and configurable tool for this purpose.
I wonder how come no-one mentioned the PERLDB_OPTS option called RemotePort; although admittedly, there aren't many working examples on the web (RemotePort isn't even mentioned in perldebug) - and it was kinda problematic for me to come up with this one, but here it goes (it being a Linux example).
To do a proper example, first I needed something that can do a very simple simulation of a CGI web server, preferably through a single command line. After finding Simple command line web server for running cgis. (perlmonks.org), I found the IO::All - A Tiny Web Server to be applicable for this test.
Here, I'll work in the /tmp directory; the CGI script will be /tmp/test.pl (included below). Note that the IO::All server will only serve executable files in the same directory as CGI, so chmod +x test.pl is required here. So, to do the usual CGI test run, I change directory to /tmp in the terminal, and run the one-liner web server there:
$ cd /tmp
$ perl -MIO::All -e 'io(":8080")->fork->accept->(sub { $_[0] < io(-x $1 ? "./$1 |" : $1) if /^GET \/(.*) / })'
The webserver command will block in the terminal, and will otherwise start the web server locally (on 127.0.0.1 or localhost) - afterwards, I can go to a web browser, and request this address:
http://127.0.0.1:8080/test.pl
... and I should observe the prints made by test.pl being loaded - and shown - in the web browser.
Now, to debug this script with RemotePort, first we need a listener on the network, through which we will interact with the Perl debugger; we can use the command line tool netcat (nc, saw that here: Perl如何remote debug?). So, first run the netcat listener in one terminal - where it will block and wait for connections on port 7234 (which will be our debug port):
$ nc -l 7234
Then, we'd want perl to start in debug mode with RemotePort, when the test.pl has been called (even in CGI mode, through the server). This, in Linux, can be done using the following "shebang wrapper" script - which here also needs to be in /tmp, and must be made executable:
cd /tmp
cat > perldbgcall.sh <<'EOF'
#!/bin/bash
PERLDB_OPTS="RemotePort=localhost:7234" perl -d -e "do '$#'"
EOF
chmod +x perldbgcall.sh
This is kind of a tricky thing - see shell script - How can I use environment variables in my shebang? - Unix & Linux Stack Exchange. But, the trick here seems to be not to fork the perl interpreter which handles test.pl - so once we hit it, we don't exec, but instead we call perl "plainly", and basically "source" our test.pl script using do (see How do I run a Perl script from within a Perl script?).
Now that we have perldbgcall.sh in /tmp - we can change the test.pl file, so that it refers to this executable file on its shebang line (instead of the usual Perl interpreter) - here is /tmp/test.pl modified thus:
#!./perldbgcall.sh
# this is test.pl
use 5.10.1;
use warnings;
use strict;
my $b = '1';
my $a = sub { "hello $b there" };
$b = '2';
print "YEAH " . $a->() . " CMON\n";
$b = '3';
print "CMON " . &$a . " YEAH\n";
$DB::single=1; # BREAKPOINT
$b = '4';
print "STEP " . &$a . " NOW\n";
$b = '5';
print "STEP " . &$a . " AGAIN\n";
Now, both test.pl and its new shebang handler, perldbgcall.sh, are in /tmp; and we have nc listening for debug connections on port 7234 - so we can finally open another terminal window, change directory to /tmp, and run the one-liner webserver (which will listen for web connections on port 8080) there:
cd /tmp
perl -MIO::All -e 'io(":8080")->fork->accept->(sub { $_[0] < io(-x $1 ? "./$1 |" : $1) if /^GET \/(.*) / })'
After this is done, we can go to our web browser, and request the same address, http://127.0.0.1:8080/test.pl. However, now when the webserver tries to execute the script, it will do so through perldbgcall.sh shebang - which will start perl in remote debugger mode. Thus, the script execution will pause - and so the web browser will lock, waiting for data. We can now switch to the netcat terminal, and we should see the familiar Perl debugger text - however, output through nc:
$ nc -l 7234
Loading DB routines from perl5db.pl version 1.32
Editor support available.
Enter h or `h h' for help, or `man perldebug' for more help.
main::(-e:1): do './test.pl'
DB<1> r
main::(./test.pl:29): $b = '4';
DB<1>
As the snippet shows, we now basically use nc as a "terminal" - so we can type r (and Enter) for "run" - and the script will run up do the breakpoint statement (see also In perl, what is the difference between $DB::single = 1 and 2?), before stopping again (note at that point, the browser will still lock).
So, now we can, say, step through the rest of test.pl, through the nc terminal:
....
main::(./test.pl:29): $b = '4';
DB<1> n
main::(./test.pl:30): print "STEP " . &$a . " NOW\n";
DB<1> n
main::(./test.pl:31): $b = '5';
DB<1> n
main::(./test.pl:32): print "STEP " . &$a . " AGAIN\n";
DB<1> n
Debugged program terminated. Use q to quit or R to restart,
use o inhibit_exit to avoid stopping after program termination,
h q, h R or h o to get additional info.
DB<1>
... however, also at this point, the browser locks and waits for data. Only after we exit the debugger with q:
DB<1> q
$
... does the browser stop locking - and finally displays the (complete) output of test.pl:
YEAH hello 2 there CMON
CMON hello 3 there YEAH
STEP hello 4 there NOW
STEP hello 5 there AGAIN
Of course, this kind of debug can be done even without running the web server - however, the neat thing here, is that we don't touch the web server at all; we trigger execution "natively" (for CGI) from a web browser - and the only change needed in the CGI script itself, is the change of shebang (and of course, the presence of the shebang wrapper script, as executable file in the same directory).
Well, hope this helps someone - I sure would have loved to have stumbled upon this, instead of writing it myself :)
Cheers!
For me, I use log4perl . It's quite useful and easy.
use Log::Log4perl qw(:easy);
Log::Log4perl->easy_init( { level => $DEBUG, file => ">>d:\\tokyo.log" } );
my $logger = Log::Log4perl::get_logger();
$logger->debug("your log message");
Honestly you can do all the fun stuff above this post.
ALTHOUGH, the simplest and most proactive solution I found was to just "print it".
In example:
(Normal code)
`$somecommand`;
To see if it's doing what I really want it to do:
(Trouble shooting)
print "$somecommand";
It will probably also be worth mentioning that Perl will always tell you on what line the error occurs when you execute the Perl script from the command line. (An SSH Session for example)
I will usually do this if all else fails. I will SSH into the server and manually execute the Perl script. For example:
% perl myscript.cgi
If there is a problem then Perl will tell you about it. This debugging method does away with any file permission related issues or web browser or web server issues.
You may run the perl cgi-script in terminal using the below command
$ perl filename.cgi
It interpret the code and provide result with HTML code.
It will report the error if any.
I'm trying to reconfigure some .config variables to generate a modified kernel with wifi support enabled. The native layer/recipe for the kernel is located in this directory:
meta-layer/recipes-kernel/linux/linux-yocto_3.19.bb
First I reconfigure the native kernel to add wifi support (for example, adding CONFIG_WLAN=y):
$ bitbake linux-yocto -c menuconfig
After that, I generate a "fragment.cfg" file:
$ bitbake linux-yocto -c diffconfig
I have created this directory into my custom-layer:
custom-layer/recipes-kernel/linux/linux-yocto/
I have copied the "fragment.cfg file into this directory:
$ cp fragment.cfg custom-layer/recipes-kernel/linux/linux-yocto/
I have created an append file to customize the native kernel recipe:
custom-layer/recipes-kernel/linux/linux-yocto_3.19.bbappend
This is the content of this append file:
FILESEXTRAPATHS_prepend:="${THISDIR}/${PN}:"
SRC_URI += "file://fragment.cfg"
After that I execute the kernel compilation:
$ bitbake linux-yocto -c compile -f
After this command, "fragment.cfg" file can be found into this working directory:
tmp/work/platform/linux-yocto/3.19-r0
However none of the expected variables is active on the .config file (for example, CONFIG_WLAN is not set).
How can I debug this issue? What is supposed I'm doing wrong?
When adding this configuration you want to use append in your statement such as:
SRC_URI_append = "file://fragment.cfg"
After analyzing different links and solutions proposed on different resources, I finally found the link https://community.freescale.com/thread/376369 pointing to a nasty but working patch, consisting in adding this function at the end of append file:
do_configure_append() {
cat ${WORKDIR}/*.cfg >> ${B}/.config
}
It works, but I expected Yocto managing all this stuff. It would be nice to know what is wrong with the proposed solution. Thank you in advance!
If your recipe is based on kernel.bbclass then fragments will not work. You need to inherit kernel-yocto.bbclass
You can also use merge_config.sh scripts which is present in kernel sources. I did something like this:
do_configure_append () {
${S}/scripts/kconfig/merge_config.sh -m -O ${WORKDIR}/build ${WORKDIR}/build/.config ${WORKDIR}/*.cfg
}
Well, unfortunately, not a real answer... As I haven't been digging deep enough.
This was working alright for me on a Daisy-based build, however, when updating the build system to Jethro or Krogoth, I get the same issue as you.
Issue:
When adding a fragment like
custom-layer/recipes-kernel/linux/linux-yocto/cdc-ether.cfg
The configure step of the linux-yocto build won't find it. However, if you move it to:
custom-layer/recipes-kernel/linux/linux-yocto/${MACHINE}/cdc-ether.cfg
it'll work as expected. And it's a sligthly less hackish way of getting it to work.
If anyone comes by, this is working on jethro and sumo:
FILESEXTRAPATHS_prepend := "${THISDIR}/${PN}:"
SRC_URI_append = " \
file://fragment.cfg \
"
FILESEXTRAPATHS documentation says:
Extends the search path the OpenEmbedded build system uses when looking for files and patches as it processes recipes and append files. The directories BitBake uses when it processes recipes are defined by the FILESPATH variable, and can be extended using FILESEXTRAPATHS.
I'm working on some code with a partner. Our make files differ slightly courtesy of different build setups. Because of this, so far we have not been tracking this file. However it would be nice to have at least one of ours tracked. The problem is, when that is done and the other person runs hg update, their copy gets update and the code won't compile.
Is there a way to track the file, but have it such that you can update the working directory selectively? Or is there some other way I should deal with this problem?
This is a slight variant of the standard "how do I deal with a config file" question. The standard answer in SVN, Mercurial, and Git is: don't track the file, instead track <file>.example. Then each user copies that over to <file> and tweaks it as needed.
But Makefiles are a bit smarter than config files: they execute code and can include other files. In which case, it starts making sense to track the Makefile normally and have it include another local file if it's present that overrides the default rules. For instance, the following will work with GNU Make:
# pull in any local user tweaks
-include Makefile.local
MQ extension is the best and The Right Way (tm) to do it (not easiest, but...)
Store common part of file in repo, individual personalisation - in own MQ-patches
Is it possible to combine your Makefiles? Then there is not chance of losing your different configurations by not storing them in version control.
For example, you could add a conditional statement based on the username. My username is ryan and this code echos my name, but if it is run on your computer, it probably will echo "not ryan."
all:
if [ `whoami` = "ryan" ]; then echo "ryan"; else echo "not ryan"; fi
I often have the following scenario: in order to reproduce a bug for reporting, I create a small sample project, sometimes a maven multi module project. So there may be a hierarchy of directories and it will usually contain a few small text files. Standard procedure would of course be to create a zip file and send that. But on some mailing lists attachments are not allowed, and so I am looking for a way to automatically create an installation script, that I can post to such mailing lists.
Basically I would be happy with a Unix flavor only that creates mkdir statements to create directories and >> statements to write the file contents. (Actually, apart from the relative path delimiters, the windows and unix versions can probably be identical)
Does such a tool exist somewhere? If not, I'll probably write one in java, but I'm happy to accept solutions in all kinds of languages.
(The tool could run under windows or unix, but the target platform for the generated scripts should be either unix or configurable)
I think you're looking for shar, which creates a shell archive (shell script that when run produces a given directory hierarchy). It is available on most systems; you can use GNU sharutils if you don't already have it.
Normal usage for packing up a directory tree would be something like:
shar `find somedirectory -print` > archive.sh
If you're using GNU sharutils, and want to create "vanilla" archives which use only the most portable of shell builtins, mkdir, and sed, then you should invoke it as shar -V. You can remove some more extra baggage from the scripts by using -xQ; -x to remove checks for existing files, and -Q to remove verbose output from the archive.
shar -VxQ `find somedir -print` > archive.sh
If you really want something even simpler, here's a dirt-simple version of shar as a shell script. It takes filenames on standard input instead of arguments for simplicity and to be a little more robust.
#!/bin/sh
while read filename
do
if test -d $filename
then
echo "mkdir -p '$filename'"
else
echo "sed 's/^X//' <<EOF > '$filename'"
sed 's/^/X/' < "$filename"
echo 'EOF'
fi
done
Invoke as:
find somedir -print | simpleshar > archive.sh
You still need to invoke sed, as you need some way of ensuring that no lines in the here document begin with the delimiter, which would close the document and cause later lines to be interpreted as part of the script. I can't think of any really good way to solve the quoting problem using only shell builtins, so you will have to rely on sed (which is standard on any Unix-like system, and has been practically forever).
if your problem are non-text-file-hating filters:
in times long forgotten, we used uuencode to get past 8-bit eating relays -
is that a way to get past attachment eating mail boxes these days ?
So why not zip and uuencode ?
(or base64, which is its younger cousin)
I am trying to write a script that will parse a local file and upload its contents to a MySQL database. Right now, I am thinking that a batch script that runs a Perl script would work, but am not sure if this is the best method of accomplishing this.
In addition, I would like this script to run immediately when the data file is added to a certain directory. Is this possible in Windows?
Thoughts? Feedback? I'm fairly new to Perl and Windows batch scripts, so any guidance would be appreciated.
You can use Win32::ChangeNotify. Your script will be notified when a file is added to the target directory.
Checking a folder for newly created files can be implemented using the WMI functionality. Namely, you can create a Perl script that subscribes to the __InstanceCreationEvent WMI event that traces the creation of the CIM_DirectoryContainsFile class instances. Once that kind of event is fired, you know a new file has been added to the folder and can process it as you need.
These articles provide more information on the subject and contain VBScript code samples (hope it won't be hard for you to convert them to Perl):
How Can I Automatically Run a Script Any Time a File is Added to a Folder?
WMI and File System Monitoring
The function you want is ReadDirectoryChangesW. A quick search for a perl wrapper yields this Win32::ReadDirectoryChanges module.
Your script would look something like this:
use Win32::ReadDirectoryChanges;
$rdc = new Win32::ReadDirectoryChanges(path => $path,
subtree => 1,
filter => $filter);
while(1) {
#results = $rdc->read_changes;
while (scalar #results) {
my ($action, $filename) = splice(#results, 0, 2);
... run script ...
}
}
You can easily achieve this in Perl using File::ChangeNotify. This module is to be found on CPAN: http://search.cpan.org/dist/File-ChangeNotify/lib/File/ChangeNotify.pm
You can run the code as a daemon or as a service, make it watch one or more directories and then automatically execute some code (or start up a script) if some condition matches.
Best of all, it's cross-platform, so should you want to switch to a Linux machine or a Mac, it would still work.
It wouldn't be too hard to put together a small C# application that uses the FileSystemWatcher class to detect files being added to a folder and then spawn the required script. It would certainly use less CPU / system resources / hard disk bandwidth than polling the folder at regular intervals.
You need to consider what is a sufficient heuristic for determining "modified".
In increasing order of cost and accuracy:
file size (file content can still be changed as long as size is maintained)
file timestamp (If you aren't running ntpd time is not monotonic)
file sha1sum (bulletproof but expensive)
I would run ntpd, and then loop over the timestamps, and then compare the checksum if the timestamp changes. This can cover a lot of ground in little time.
These methods are not appropriate for a computer security application, they are for file management on a sane system.