Increment value in for loop while reading a file - shell

Question

My code looks something like this:
for line in `cat fileName`
do
if [[ $line == "Marker 1" ]]
then
while [[ $line != "---" ]]
do
#basically I want to read all the data below "Marker 1" till "---"
echo $line
((line++)) #this is wrong
done
elif [[ $line == "Marker 2" ]]
then
while [[ $line != "---" ]]
do
echo $line
((line++))
done
fi
done
How do I increment the value of $line when in the while loop? ((line++)) doesn't work

Answer 1

Using sed
If the goal is to echo all the lines from a line with Marker 1 or Marker 2 to a line with ---, then that entire shell loop can be replaced with this simple sed command:
sed -n '/^Marker [12]$/,/^---$/p' File
Example
Consider this test file:
$ cat File
beginning
Marker 1
one
---
more
Marker 2
two
Two
---
end
Now, let's run our command:
$ sed -n '/^Marker [12]$/,/^---$/p' File
Marker 1
one
---
Marker 2
two
Two
---
Using awk
With the same test file:
$ awk '/^Marker [12]$/,/^---$/' File
Marker 1
one
---
Marker 2
two
Two
---

Answer 2

((line++)) is for incrementing an integer value.
But the value of line in your example is a string.
In fact what you seem to want is get the next line from the file.
You need a different approach for that, and use while read instead of a for-loop.
#!/usr/bin/env bash
read_and_print_until_dashes() {
while read -r line; do
[[ $line = '---' ]] && break
echo "$line"
done
}
while read -r line; do
if [[ $line = "Marker 1" ]]; then
echo "$line"
read_and_print_until_dashes
elif [[ $line = "Marker 2" ]]; then
echo "$line"
read_and_print_until_dashes
fi
done < file.txt