[Melbourne-pm] programming tutorials: Perl
jarich at perltraining.com.au
Tue Nov 16 04:25:55 PST 2010
john thornton wrote:
> a bad tutorial. I don't give up on any language easily. Except maybe
> bash with its 0 and 1 the wrong way around for truth tables.
Even this does actually make sense if someone explains why. Consider the
You are running a process. Two broad categories of things can happen:
1. runs to completion and finishes successfully
2. it fails to run to completion or finish successfully for some reason.
In many ways, 1. is boring. Either it succeeds or it doesn't. True or false.
However 2. is really interesting. Did it get to completion? If not, why not?
Why didn't it succeed?
If we flag success with 0, we can then leave open *all* of the positive integers
as answers to what went wrong to cause the second situation.
For example; grep returns 0 to say it ran to completion and found the thing you
were looking for. It returns 1 to say it ran to completion but did not find the
thing you were looking for, and 2 for all other situations (errors). Thus if
you're running grep and you only care whether the string exists in the file, you
check it against 0.
On the other hand return values for mount are much more interesting:
1 incorrect invocation or permissions
2 system error (out of memory, cannot fork, no more loop devices)
4 internal mount bug or missing nfs support in mount
8 user interrupt
16 problems writing or locking /etc/mtab
32 mount failure
64 some mount succeeded
So, as you can see, success = 0 is logical if you want to make it possible to
signal further information with your exit values. Once you have this extra
information you can write more robust scripts which can handle some possible
errors rather than just giving up on failure because the reason for failure was
All the best,
More information about the Melbourne-pm