Shell Programming and Scripting

Hi,

Code:

 
echo "yes or no?"
read ans
 
case $ans in
 
[yY]*)
echo "yes"
;;
 
[nN]*)
echo "no"
;;
 
*)
echo "yes or no only"
#here, if the answer is not "Y" or "N", I want to go back to asking "yes or no?"
;;
esac

Thank you.

Code:

while [ 1 ]
do
    echo "yes or no?"
    read ans
    case $ans in
    [yY]es)
        echo "yes"
        # more statements
        break
        ;;
         
    [nN]o)
        echo "no"
        # more statements
        break
        ;;
    *)
        echo "yes or no only"
        ;;
    esac
done

Shell scripting has no concept of GOTO.

Try using functions for your "yes/no" results and run your "case" routine inside a "while"
loop remembering to call you functions...
Example:-

Code:

yes()
{
        echo "Do something."
}
no()
{
        echo "Do something else."
}
while true
do
        # Your case code, remembering to call "yes" or "no" as commands...
        # Remember also to give yourself a getout clause...
done

Off to work...

Perhaps a little background could be useful Goto considered harmful (Edsger Dijkstra)

For endless loops consider

Code:

while :

rather than

Code:

while [ 1 ]

Saves you 4 keystrokes (and a couple of CPU cycles with each iteration).

Sure it does. The break and continue statements are limited gotos. And are always translated to jumps in assembly language.

GOTO style statements are a bad idea. Try to work out the functions of your process. I learnt this on Sinclair BASIC of all things. Using GOTO is easy to understand and GOSUB seemed excessive.

Eventually I was enlightened at school. The overall target of your code should be to perform a process. That process was split into major components, and if necessary a selection made as to which parts actually ran. For each of these major parts, we repeated the approach, and again until we could not refine it further. Each leg/relationship was either a loop, a condition or a single sequence.

Effectively coding up the lowest level respective the relationships would build your application. GOTO was not a recommended even then. We had to make subroutine and then manage the flow. Drawing up a flowchart type design with all sorts of branches quickly becomes unworkable and is a nightmare to decipher, let alone alter, evidenced by plenty of our COBOL code and C-shell scripts that I've had to extract us from in the past.

We have more ksh scripts to pick apart soon with all sorts of mad things like:-

Code:

while true
do
  some-process........
  if it worked
  then
     break        # End the while true loop
  fi
  another-process......
  if it worked
  then
     continue     # Re-start while true loop
  fi
  echo "Failed both things"
done              # Back to while true loop

echo "Finished"

It's just a mess when you end up with a script running to over 500 lines with one main while true loop and a huge case statement to have sections that should or shouldn't run.


Using functions helps organise your thoughts to build more structured code making it easier to understand later (when the documentation is usually long gone)


I have a long project to dismantle it all to more easily maintained code as we move the application to a replacement server. Deep joy fills me soul with the prospect.




Robin