Linux Bash使用-n nchars读取和处理换行符

Question

Following code:

read -n 3 -r -p "Enter up to 3 characters  -> : "
echo "string"

When typing two characters and pressing Return it outputs:

Enter up to 3 characters  -> : 12
string

However, when entering 3 characters, without Return, it produces:

Enter up to 3 characters  -> : 123string

I would like to see:

Enter up to 3 characters  -> : 123
string

Unfortunately, the newline character \\n is not part of the $REPLY variable, otherwise I could check it and output an echo if a new line was missing.

The reason why I need this is to output an error message if there is invalid input, so using -N or -n 4 is not a solution.

Any ideas? Thanks!

Btw, I was already thinking of the following, which produces the output I want, but unfortunately, I already use "tput sc" for something else and can therefore not use it again.

echo; tput sc; tput cuu1
read -n 3 -r -p "Enter up to 3 characters  -> : "
tput rc; echo "string"

Answer 1

You can check that the length of REPLY matches what you expect. If it matches, then Enter was not pressed, so echo a blank line.

len=3
read -n $len -r -p "Enter up to 3 characters  -> : "
(( ${#REPLY} == $len )) && echo
echo "string"

If the length is less than len , then Enter must have been pressed.

Or, if the user circumvented the input controls, then it probably doesn't really matter, does it?

Answer 2

I've done a few more tests and while checking the length of $REPLY seemed to be the way to go at first, I finally ended up with a different solution.

I noticed that the backspace key when using read -n nchar was not deleting characters, but printed ^? symbols instead. I corrected the issue by adding readline support using the the -e parameter.

-e :on interactive shells: use Bash's readline interface to read the data

For example:

read -e -n 3 -r -p "Enter up to 3 characters  -> : "
or 
read -ern 3 -p Enter up to 3 characters  -> : "

This not only fixed the backspace or delete key, but also added the missing newline as described in the initial post. This makes checking the length of $REPLY no longer necessary, which would actually add an extra newline where none is needed.

Anyway, many thanks for your responses. It did help me to continue with what I'm doing and last not least find the solution in the end, even though it was different than suggested. Best regards!