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Trouble reading input inside bash while loop

Hi all , its been a while since I wrote a bash script but I've run into some trouble in the while clause. The answer reads in fine but it doesn't register that a valid answer has been entered and just assumes that every answer entered is invalid.

I think it has to do with how Im doing on the comparisons , if anyone could provide any insight as to why this is not working I would appreciate it.

Code:

#!/bin/bash



# script to list all the files in the downloads directory and ask the user

# if they wish to move the files and if so , into which directory

# Eg Music Videos Pictures Distros etc







EX_HDD="/media/Expansion\ Drive/"







for f in $(ls "$HOME/Downloads/")



do







        echo "Would you like to move $f to External HDD? Y/N"

        read answer





        while  [ '$answer'  !=  'Y' ]  ||  [  '$answer' !=  'N' ]  # Problem line

        do

                echo "Answer must be of the form Y or N"

                echo "Would you like to move $f to External HDD? Y/N"

                echo "Answer is $answer"

                read answer



        done



        echo "working so far"



done

Sample output:

Would you like to move 119305-GlassMaxX.tar.gz to External HDD? Y/N
Y
Answer must be of the form Y or N
Would you like to move 119305-GlassMaxX.tar.gz to External HDD? Y/N
Answer is Y
N
Answer must be of the form Y or N
Would you like to move 119305-GlassMaxX.tar.gz to External HDD? Y/N
Answer is N
Y
Answer must be of the form Y or N
Would you like to move 119305-GlassMaxX.tar.gz to External HDD? Y/N
Answer is Y
HJHJ
Answer must be of the form Y or N
Would you like to move 119305-GlassMaxX.tar.gz to External HDD? Y/N
Answer is HJHJ

Well, you are looping on the condition that $answer is either unlike Y or unlike N. Since it can't be both Y and N at the same time, the while loop continues indefinitely.

Also, the single quotes prevent shell expansion of the variable, so you're comparing the literal string $answer rather than the contents of the variable.

How about: while [ "$answer" != "Y" ] && [ "$answer" != "N" ]

Thanks Ser Olmy, that did the trick, it works fine now. I appreciate it.

Maybe it can also be phrased as a regex and avoid an external program to be called:

Code:

while ! [[ "$answer" =~ ^[NY]$ ]]; do

Or if looking for the negative maybe use an 'until' loop.

Checking user input is more traditionally done with a case statement, and embedding it in a continuous loop will make it repeat until you get the correct answer and manually break out of it.

Code:

while true; do 

        echo "Would you like to move $f to External HDD? Y/N"

        read answer



        case $answer in



                y|Y*) echo "You answered yes." ; break  ;;



                n|N*) echo "You answered no." ; break  ;;



                *) echo "Answer must be yes or no.  Try again."  ;;



        esac

done

Also consider using a select loop, which is a built-in that provides a pre-formatted variation of the above for simple menus.

Second, parsing ls for filenames is not a good idea, particularly in a for loop. Just use shell globbing.

Code:

for f in "$HOME/Downloads/"* ; do

    ....

done

For more advanced lists, you should switch to using find and a while+read loop.