Weird Shell Script Bug/Puzzle 3

[SamBones]

OK, I have a little bit of Korn shell script that is exhibiting some VERY weird behavior. I figured out what was happening, so I don't need someone to tell me what's going on. But, I thought it was kind of funny so I thought I'd share it as a little programming puzzle! Here it is.

I have a little test script that loops a number of times. In each loop, it sleeps for several seconds just to slow down the loop. Here's the code...

#!/bin/ksh

SECONDS=2
ITERATIONS=5

while (( ITERATIONS ))
do
        print "Waiting for ${SECONDS} seconds"
        sleep ${SECONDS}

        (( ITERATIONS -= 1 ))
done

print "Done!"

The expected output is that it will print out "Waiting for 2 seconds", five times, with a delay of two seconds after each, then print "Done!" and exit.

The actual output is very different...

Waiting for 2 seconds
Waiting for 4 seconds
Waiting for 8 seconds
Waiting for 16 seconds
Waiting for 32 seconds
Done!

There is nothing in the script that increments the variable SECONDS, but it keeps doubling each time! It takes a total time of 62 seconds (1 minute 2 seconds) to run.

Do you know why?

A hearty attaboy/attagirl (plus a star) to anyone that answers it without peeking at other peoples' answers (use the spoiler tag to hide your answer).

 

[p5wizard]

Spoiler

man ksh
use another variable name instead of SECONDS

HTH,

p5wizard

 

[stefanwagner]

Spoiler

man ksh:
SECONDS
Each time this variable is referenced, the number of seconds since shell invocation is returned. If this variable is assigned a value, then the value returned upon reference will be the value that was assigned plus the number of seconds since the assignment.

don't visit my homepage:

http://home.arcor.de/hirnstrom/minis/index.html

 

[SamBones]

Heh, heh, yeah.

I'll try not to give anything away with this comment, but when you're writing a script quickly, you chose names that make sense, and are obvious as to what their purpose or use is. That name seemed like a good choice. This "problem" was actually coded into a much larger script that ran for days, and was exhibiting some very weird symptoms as you can imagine.

Spoiler

I also had a coworker try to use the variable name "PATH" for something other than PATH. He was really confused when almost everything in his script stopped working.

 

[jmanj]

I believe if you change ITERATIONS to something else that will solve the behavior

 

[jmanj]

I also think that SECONDS is a reserved word and if there is a value in it then that value is multiplied by that value for every encounter or iteration. It seems to me that way but I maybe wrong..

 

[jmanj]

Okay I'm close but no cigar!! But here's a better explanation:

If a value is assigned to SECONDS, the sum of that value and the number of seconds since its assignment is displayed when the variable is referenced:

Courtesy of the INTERNET..

 

[SamBones]

Here's a big hint. Just log in and type this...

echo $SECONDS
echo $SECONDS
echo $SECONDS

ITERATIONS is fine.

 

[Annihilannic]

I guess that's why when Larry wrote Perl he choose built-in variable names that no-one else would be crazy enough to use. $_, $`, $<, $', etc...

Annihilannic.

 

[p5wizard]

I mostly use FunnyCapitalizedNames for my shell variables to avoid such weird behaviour...

HTH,

p5wizard