Command to trim file names to given amount of letters in bash - bash

Question

Im trying to make a simple script to trim all files names in directory where the script is opened to given amount of letters in bash.
Example of script
To how many letters do you want to trim files?
User: 3
Adsbgnyr.txt > Ads.txt
echo "What do you want to do? Changes will be done to all files in current catalog"
echo "1. Change files names to uppercase"
echo "2. Change files names to lowercase"
echo "3. Trim files names to give amount of letters"
echo "4. Exit"
echo "option"
read option
if [ $option = "1" ]
then
for i in $( ls | grep [a-z] )
do
mv -i $i `echo $i | tr 'a-z' 'A-Z'`
done
fi
if [ $option = "2" ]
then
for g in $( ls | grep [A-Z] )
do
mv -i $g `echo $g | tr 'A-Z' 'a-z'`
done
fi
if [ $option = "3" ]
then
echo "o how many letters do you want to trim files?"
read howmany
>>THIS IS THE PLACE WHERE I WANT TO PUT THE COMMAND<<
fi
if [ $option = "4" ]
then
exit
fi

Answer 1

Try in this way
shopt -s nullglob
for h in *
do
echo ${h:0:$howmany}
done

Answer 2

some code review suggestions:
indentation is important
quote your variables
don't parse ls
use select for menus
The answer to your question can be found in the bash manual: Shell Parameter Expansion
choices=(
"Change files names to uppercase"
"Change files names to lowercase"
"Trim files names to give amount of letters"
"Exit"
)
echo "Changes will be done to all files in current catalog:"
PS3="What do you want to do? "
select choice in "${choices[#]}"; do
case $REPLY in
1) for file in *[a-z]*; do
mv -i "$file" "${file^^}"
done
;;
2) for file in *[A-Z]*; do
mv -i "$file" "${file,,}"
done
;;
3) read -p "To how many letters do you want to trim files?" howmany
for file in *; do
mv -i "$file" "${file:0:howmany}"
done
;;
4) exit ;;
esac
done
Note there's no $ for "howmany" in ${file:0:howmany} -- bash lets you omit the $ for variables in arithmetic expressions, and both the offset and length are arithmetic expressions in the ${var:offset:length} expansion.
What are you going to do if the user trims the files too small: if filename1 and filename2 exist, and $howmany = 8 then filename1 will be lost after mv filename2 filename