Shell Programming and Scripting

Thank you both so very much!! Works like it's supposed to and how I imagined it would!

I have one more question about the following lines of code:

#!/bin/ksh

if [ "$1" ]; then
num=$1
while [ $num -gt 0 ]; do
echo $num
((num=num-1))
done
elif [ "$1" ]; then
num=$1
while [ $num -le 0 ]; do
echo Command line argument must be a positive number and not zero
done
else
echo You must enter one number on the command line
fi

I don't receive any error messages when I run this, but when I enter a negative number on the command line, nothing happens. Any suggestions?

If you are checking for positive numbers there are better ways to do it. But you could just change your if statement to:

if [ "$1" -gt 0 ]; then
just like you have in the while loop

Then make your echo statement read must be a postive number, and drop the elif.

what the script is supposed to do is that I want it to accept an integer and do the following:
1. If it's positive and greater than zero, count down to one
2. If no argument is entered at the command line, then to display a message.
3. If a negative number is entered at the command line, I want a message to display saying to enter a positive number. I just can't get this step to work.

I'm just trying to learn loops and how to nest them inside of each other.

That will accomplish everything in that list.


if [ "$1 -gt 0 " ]; then
num=$1
while [ $num -gt 0 ]; do
echo $num
((num=num-1))
done
else
echo Please Enter a Positive Integer
fi

Try this out, scott:

Code:

if [ "$1" ]; then
  num=$1
  if [ "$1 -gt 0" ]; then
    while [ $num -gt 0 ]; do
      echo $num
      ((num=num-1))
    done 
  else
    echo "Enter a positive number!"
  fi
else 
  echo "Enter a number!"
fi

This: if [ "$1 -gt 0" ]; then
is not going to do what you think it might do.

But also, why are there two errors messages? First we reject input because it's not an integer, then we reject input because it's not a positive integer. User's don't like it when the requirements seem to change like that. Error messages really need to go to stderr as a rule anyway. How about:

Code:

#! /usr/bin/ksh

if [[ $1 = ?(+)+([0-9]) ]] ; then
      ((num=$1))
      while ((num)) ; do
            echo $num
            ((num=num+1))
      done

else

      echo enter a positive integer  >&2
fi
exit 0

Now as much as I love structured programming, I would never code like that in real life. I think that an early exit is fine. The real goal of structured programming is easily readable code. I only want to nest one structure inside another when I must. So I would actually go with

Code:

#! /usr/bin/ksh

if [[ $1 != ?(+)+([0-9]) ]] ; then
        echo enter a positive number >&2
        exit 1
fi

((num=$1))
while ((num)) ; do
       echo $num
       ((num=num+1))
done

exit 0

And to answer the question before it's asked, ?(+)+([0-9]) is a ksh pattern for a positive integer. ?() optionally matches any of the patterns inside the parentheses, so ?(+) will optionally match a plus sign. +() matches one or more of the patterns inside the parentheses, so +([0-9]) matches one or more numbers.