To Err Is Human - Perl

When it comes to dealing with errors, there are a number of variables you
should know about. The first of them is the $? variable, which stores the
error code returned by a call to an external binary, or to the system()
function.

#!/usr/bin/perl

# add a user who already exists

`/usr/sbin/useradd root 2>/dev/null`;
# if error code, return it

if ($?)

{

print "Error code ", $? >> 8;

}

Here's the output:

Error code 9

In case you're wondering about the bitwise operation in the program above -
the value stored in the $? variable is a 16-bit integer, of which the first
8 bits represent the error code returned by the invoked command.

You can also use the

$? & 127

operation to obtain information on the termination signal of the command, and

$? & 128

operation to get a Boolean value indicating whether or not the program
dumped core.

As you may (or may not) know, Perl also allows you to trap errors in a
syntax similar to Java's try-catch() blocks, by enclosing your code in an
eval() block. In case the code within the eval() block produces an error,
Perl stores the error in the $@ system variable without escalating it to
the main program, from whence it may be retrieved for exception-handling
purposes. The following example illustrates:

#!/usr/bin/perl

# attempt to use a file which does not exist

eval( "use Timezone;" );
# check for error

if ($@ ne "")

{

print "The following error occurred: ", $@;

}

In this case, since the call to use() is within an eval() block, the error
returned when Perl is unable to locate the Timezone package will be trapped
by the special $@ variable and will not be escalated upwards to the main
program. You can then write your own exception-handling routine to inspect
$@ and resolve the error appropriately.

Here's the output, with the $@ error-trapping above in action:

The following error occurred: Can't locate Timezone.pm in @INC (@INC
contains: /usr/lib/perl5/5.8.0/i386-linux-thread-multi .) at (eval 1) line 1.
BEGIN failed--compilation aborted at (eval 1) line 1.

You can also catch error messages returned by die() within an eval() block
with the $@ variable - as illustrated below:

#!/usr/bin/perl

# open file

eval( "open(FILE, '/tmp/dummy.txt') or die ('Could not open file');");
# check for error

if ($@ ne "")

{

print "The following error occurred: ", $@;

}

Here's the output:

The following error occurred: Could not open file at (eval 1) line 1.

In the case of Perl functions that use C library calls, you can also access
the error returned by the underlying C library with the special $!
variable. In order to illustrate, consider the Perl open() function, which
uses the C open() call, in a variant of the example above:

#!/usr/bin/perl

# open file

eval( "open(FILE, '/tmp/dummy.txt') or die ('Could not open file');");
# check for error

print "The following error occurred: $!";

Note how, in this case, the error message displayed is the one returned by
the C library, not Perl (compare it with the previous example to see the
difference):

The following error occurred: No such file or directory