How to read the user input line by line until Ctrl+D and include the line where Ctrl+D was typed

To do that, you'd have to read character by character, not line by line.

Why? The shell very likely uses the standard C library function read() to read the data that the user is typing in, and that function returns the number of bytes actually read. If it returns zero, that means it has encountered EOF (see the read(2) manual; man 2 read). Note that EOF isn't a character but a condition, i.e. the condition "there is nothing more to be read", end-of-file.

Ctrl+D sends an end-of-transmission character (EOT, ASCII character code 4, $'\04' in bash) to the terminal driver. This has the effect of sending whatever there is to send to the waiting read() call of the shell.

When you press Ctrl+D halfway through entering the text on a line, whatever you have typed so far is sent to the shell¹. This means that if you enter Ctrl+D twice after having typed something on a line, the first one will send some data, and the second one will send nothing, and the read() call will return zero and the shell interpret that as EOF. Likewise, if you press Enter followed by Ctrl+D, the shell gets EOF at once as there wasn't any data to send.

So how to avoid having to type Ctrl+D twice?

As I said, read single characters. When you use the read shell built-in command, it probably has an input buffer and asks read() to read a maximum of that many characters from the input stream (maybe 16 kb or so). This means that the shell will get a bunch of 16 kb chunks of input, followed by a chunk that may be less than 16 kb, followed by zero bytes (EOF). Once encountering the end of input (or a newline, or a specified delimiter), control is returned to the script.

If you use read -n 1 to read a single character, the shell will use a buffer of a single byte in its call to read(), i.e. it will sit in a tight loop reading character by character, returning control to the shell script after each one.

The only issue with read -n is that it sets the terminal to "raw mode", which means that characters are sent as they are without any interpretation. For example, if you press Ctrl+D, you'll get a literal EOT character in your string. So we have to check for that. This also has the side-effect that the user will be unable to edit the line before submitting it to the script, for example by pressing Backspace, or by using Ctrl+W (to delete the previous word) or Ctrl+U (to delete to the beginning of the line).

To make a long story short: The following is the final loop that your bash script needs to do to read a line of input, while at the same time allowing the user to interrupt the input at any time by pressing Ctrl+D:

while true; do
    line=''

    while IFS= read -r -N 1 ch; do
        case "$ch" in
            $'\04') got_eot=1   ;&
            $'\n')  break       ;;
            *)      line="$line$ch" ;;
        esac
    done

    printf 'line: "%s"\n' "$line"

    if (( got_eot )); then
        break
    fi
done

Without going into too much detail about this:

• IFS= clears the IFS variable. Without this, we would not be able to read spaces. I use read -N instead of read -n, otherwise we wouldn't be able to detect newlines. The -r option to read enables us to read backslashes properly.

• The case statement acts on each read character ($ch). If an EOT ($'\04') is detected, it sets got_eot to 1 and then falls through to the break statement which gets it out of the inner loop. If a newline ($'\n') is detected, it just breaks out of the inner loop. Otherwise it adds the character to the end of the line variable.

• After the loop, the line is printed to standard output. This would be where you call your script or function that uses "$line". If we got here by detecting an EOT, we exit the outermost loop.

¹ You may test this by running cat >file in one terminal and tail -f file in another, and then enter a partial line into the cat and press Ctrl+D to see what happens in the output of tail.

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For ksh93 users: The loop above will read a carriage return character rather than a newline character in ksh93, which means that the test for $'\n' will need to change to a test for $'\r'. The shell will also display these as ^M.

To work around this:

stty_saved="$( stty -g )"
stty -echoctl

# the loop goes here, with $'\n' replaced by $'\r'

stty "$stty_saved"

You might also want to output a newline explicitly just before the break to get exactly the same behaviour as in bash.