Shell Script reading Enter key as an Input - bash

Question

I want to use the enter key as an input for a certain command.
But when there is no input I want to just loop.
If I write script like this then as the read command execution ends after 3 seconds with no input. it does not loop, but executes my desired output for enter key input command.
How can I fix this?
while true;
do
read -s -t 3 -n 3 key
if [ "${key}" == $'\0A' ]; then
#do something
elif [ "$key" == ""]; then
continue
fi
done

Answer 1

You have two problems, when using a character-literal in bash (it's bash-only), you need to use the actual-character, e.g.
if [ "${key}" == $'\n' ]; then
Next the equality comparison with [ ... ] is = not == (though bash will accept the latter). However, you must have a space before the closing ] in:
elif [ "$key" = "" ]; then
Making those changes the loop will loop continually with a 3-second timeout waiting for 3-characters of input in the manner you want, e.g.
#!/bin/bash
while true;
do
echo "looping"
read -s -t 3 -n 3 key
if [ "${key}" == $'\n' ]; then
echo "enter key"
elif [ "$key" = "" ]; then
continue
else
echo "key $key"
fi
done
Give it a go and let me know if you have further questions.
Edit Per Comment
If you want to execute a function on timeout, then you need to check the return of read. When a timeout occurs, the return will be greater than or equal to 128. If you then want to catch the Enter key, use an else and then check for an empty key, that will show that enter alone was pressed. Otherwise, you hae 3-chars in key, e.g.
#!/bin/bash
while true;
do
echo "looping"
read -s -t 3 -n 3 key
if [ $? -ge 128 ]; then
echo "timeout - execute function"
else
if [ "$key" = "" ]; then
echo "enter key"
continue
else
echo "key $key"
fi
fi
done
Example Use/Output
$ bash test.sh
looping
timeout - execute function
looping
timeout - execute function
looping
enter key
looping
timeout - execute function
looping
key foo
looping
enter key
looping
If I have the timeout/empty reversed, just switch the logic. I'm still not 100% clear on your comment, but I think this is what you indicated.

Answer 2

Since you want to re-ask the input if the user did not input an enter, I'd suggest to change the while true to something like while [[ "${key}" != $'\0A' ]]. THis way you can re-ask the input until it's an enter, and run your command after the while.
Would look something like:
# Read first time
read -s -t 3 -n 3 key
# If this wasn't an enter, keep asking until it is
while [[ "${key}" != $'\0A' ]]; do
read -s -t 3 -n 3 key
done
# Done
echo 'Running special command'