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Sometimes shell scripts just don't work the way you think they should, or you get strange error messages when running your script. Just remember: The computer is always right. It's easy to omit a significant blank, quotation mark, or bracket, or to mistakenly use a single quotation mark when you should have used double quotation marks or a backtick.
count=1
until [ "$*" = "" ]
do
echo "Arg $count : $1 "
shift
count=$count+1
done
At first glance, it looks golden, but for some reason the counter is not incrementing properly. Try running it with the command
% bash -x listarg abc def
and look at the trace output as the shell executes. The lines prefixed with a plus sign show the progress of the running script, and the lines without the plus sign are the script's normal output.
+ count=1
+ [ abc def = ]
+ echo Arg 1 : abc
Arg 1 : abc
+ shift
+ count=1+1 Hmmm . . .
+ [ def = ]
+ echo Arg 1+1 : def
Arg 1+1 : def Not Good!
+ shift
+ count=1+1+1
+ [ = ]
Instead of printing Arg 2 : def, we got Arg 1+1 : def. But the trace output line reading count=1+1 nails the problem. You forgot to use the expr command, so the shell is treating this as a string concatenate instead of a mathematical calculation.
Note: You can always press Ctrl-C to stop a running shell script. This is handy if you accidentally create a script with an infinite loop (one that will never end by itself).
Previous Lesson: Shell Script Looping
Next Lesson: Perl Basics
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