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When a page refuses to show up on the screen
Try the following in the Unix window. Let's say the script's name is test.pl.
Issue the command
test.pl
and see what happens.
If the machine responds that there is no such command, then you know that test.pl does
not have execute permission.
Issue the command
chmod a+x test.pl
and see if when one repeats the command
test.pl
and this second time one sees something wierd, i.e.,
(offline mode: enter name=value pairs on standard input)
then you know that the permissions were your problem.
By the way, to see what happens next, hold down the control key and press the `d' key
and you will see your script execute.
chmod is an instruction which changes the permissions on a file.
a+x says change the user, group, and other's permission to include execute
as a possibility.
This makes test.pl executable (runable as a program) rather than a text file.
To see this issue the command
ls -al test.pl
and you will see something funny on the left hand side:
-rwxr-xr-x etc.,
where r means read, w means write, and x means execute.
The three groups are indicated as triples.
The order is first user, then group, then other (world).
When a page shows up empty on the screen, but there is no Net-based Error Message
Try the following.
Change the first character of every line but the first 4 and the last to comments
by putting a `#' sign at position 1 of each line.
Then a program would read:
#!/usr/local/bin/perl -- -*- C -*-
use CGI;
$query = new CGI;
print $query->header;
print "This is a Title";
#print <<EOF;
#this is text which has been commented out
#EOF
print $query->end_html;
and check that the title appears.
Then try
#!/usr/local/bin/perl -- -*- C -*-
use CGI;
$query = new CGI;
print $query->header;
print "This is a Title";
print "this is a line of text";
#print <<EOF;
#this is text which has been commented out
#EOF
print $query->end_html;
and see if the line `this is a line of text' appears.
Once this is done, start logically removing the comments `#'.
- Another possibility is to set buffering off by inserting as the second line
in your file, the special $|=1 instruction
setting the $OUTPUT_AUTOFLUSH variable, which
makes the output write immediately (to the browser's screen) rather than buffering it:
#!/usr/local/bin/perl -- -*- C -*-
$| = 1;
use CGI;
$query = new CGI;
print $query->header;
print "This is a Title";
print "this is a line of text";
#print <<EOF;
#this is text which has been commented out
#EOF
print $query->end_html;
- Yet another possibility is to route the stderr output to the screen
i.e., open(&STDERR,>>STDOUT) as in:
#!/usr/local/bin/perl -- -*- C -*-
open(&STDERR,>>STDOUT);
use CGI;
$query = new CGI;
print $query->header;
print "This is a Title";
print "this is a line of text";
#print <<EOF;
#this is text which has been commented out
#EOF
print $query->end_html;
- There is yet one more way to get errors reported.
Insert the lines
use CGI::Carp qw(fatalsToBrowser);
use diagnostics;
after "use CGI".
This will result in ouput showing up on your screen.
Of course, once the program is outputting something to the screen,
judicious use of the print statement should lead to enlightenment.
Remember to save the current version of test.pl, then reload it in your
browser.
