Read line by line from a text file and print how I want in shell scripting - shell

Question

I want to read below file line by line from a text file and print how I want in shell scripting
Text file content:
zero#123456
one#123
two#12345678
I want to print this as:
zero#1-6
one#1-3
two#1-8
I tried the following:
file="readFile.txt"
while IFS= read -r line
do echo "$line"
done <printf '%s\n' "$file"

Answer 1

Create a script like below: my_print.sh
file="readFile.txt"
while IFS= read -r line
do
one=$(echo $line| awk -F'#' '{print $1}') ## This splits the line based on '#' and picks the 1st value. So, we get zero from 'zero#123456 '
len=$(echo $line| awk -F'#' '{print $2}'|wc -c) ## This takes the 2nd value which is 123456 and counts the number of characters
two=$(echo $line| awk -F'#' '{print $2}'| cut -c 1) ## This picks the 1st character from '123456' which is 1
three=$(echo $line| awk -F'#' '{print $2}'| cut -c $((len-1))) ## This picks the last character from '123456' which is 6
echo $one#$two-$three ## This is basically printing the output in the format you wanted 'zero#1-6'
done <"$file"
Run it like:
mayankp#mayank:~/$ sh my_print.sh
mayankp#mayank:~/$ cat output.txt
zero#1-6
one#1-3
two#1-8
Let me know of this helps.

Answer 2

It's no shell scripting (missed that first, sorry) but using perl with combined lookahead and lookbehind for a number:
$ perl -pe 's/(?<=[0-9]).*(?=[0-9])/-/' file
Text file content:
zero#1-6
one#1-3
two#1-8
Explained some:
s//-/ replace with a -
(?<=[0-9]) positive lookbehind, if preceeded by a number
(?=[0-9]) positive lookahead, if followed by a number

Answer 3

With sed:
sed -r 's/^(.+)#([0-9])[0-9]*([0-9])\s*$/\1#\2-\3/' readFile.txt
-r: using extented regular expressions (just to write some stuff without escaping them by a backslash)
s/expr1/expr2/: substitute expr1 by expr2
epxr1 is described by a regular expression, relevant matching patterns are caught by 3 capturing groups (parenthesized ones).
epxr2 retrieves captured strings (\1, \2, \3) and insert them in a formatted output (the one you wanted).
Regular-Expressions.info seems to be interesting to start with them. Also you can check your own regexp with Regx101.com.
Update: Also you could do that with awk:
awk -F'#' '{ \
gsub(/\s*/,"", $2) ; \
print $1 "#" substr($2, 1, 1) "-" substr($2, length($2), 1) \
}' < test.txt
I added a gsub() call because your file seems to have trailing blank characters.