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Appendix A
Perl CGI Programming FAQ |
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The server is generally configured so that it executes
CGI scripts that are located in the cgi-bin
directory. However, the server administrator can set up aliases
in the server configuration files, so that scripts with certain
extensions (i.e., .cgi, .pl)
can also be executed.
File permissions allow read, write, and execute
access to users based on their user identification (also known as
UID), and their membership in certain groups. You can use the command
chmod to change a file's permissions. Here
is an example:
% ls -ls form.cgi
1 -rwx------ 1 shishir 974 Oct 31 22:15 form.cgi*
This has a
permission of
0700 (octal), which means that no one (besides the owner) can read
to, write from, or execute this file. Let's use the chmod
command to change the permissions:
% chmod 755 form.cgi
% ls -ls form.cgi
1 -rwxr-xr-x 1 shishir 974 Oct 31 22:15 form.cgi*
This changes the permissions so that users in the same group
as "shishir," as well as all other users, have
the permission to read from and execute this file.
See the manpages for the chmod command
for a full explanation of the various octal codes.
Perl can be installed anywhere on the system! The
only thing you have to ensure is that the server is not running
in a chroot-ed environment, and that it can
access the interpreter. In other words, system administrators can
change the root directory, so that "/" does not point to the actual
root ("/"), but to another directory.
You can get a server error for the following reasons:
- If the script does not contain the "#!/usr/local/bin/perl"
header line that points to the Perl interpreter, or if the path
to the interpreter is invalid.
- If the first line output from the script is not
a valid HTTP header (i.e., "Content-type: text/html"),
or if there is no blank line after the header
data.
Generally, the HTTP server will
be running as user "nobody," or "www,"
or some other user ID that has minimal privileges.
As a result, the directory (where you intend to create the file)
must be writeable by this process ID.
To be on the safe side, always check the return status from
the open ( ) command to see if it was a success:
open (FILE, "/abc/data.txt") ||
&error ("Could not open file /abc/data.txt");
.
.
.
sub error {
local ($message) = @_;
print "Content-type: text/html", "\n";
print "Status: 500 CGI Error", "\n\n";
print "<TITLE>CGI Error </TITLE>", "\n";
print "< H1>Oops! Error </H1>", "\n";
print "< HR>", $message, "< HR>", "\n";
}
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