What Does "\R" Do in the Following Script

What does \r do in the following script?

The '\r' character is the carriage return, and the carriage return-newline pair is both needed for newline in a network virtual terminal session.

From the old telnet specification (RFC 854) (page 11):

The sequence "CR LF", as defined, will cause the NVT to be
positioned at the left margin of the next print line (as would,
for example, the sequence "LF CR").

However, from the latest specification (RFC5198) (page 13):

...
In Net-ASCII, CR MUST NOT appear except when immediately followed
by either NUL or LF, with the latter (CR LF) designating the "new
line" function. Today and as specified above, CR should
generally appear only when followed by LF. Because page layout
is better done in other ways, because NUL has a special
interpretation in some programming languages, and to avoid other
types of confusion, CR NUL should preferably be avoided as
specified above.
LF CR SHOULD NOT appear except as a side-effect of multiple CR LF
sequences (e.g., CR LF CR LF).

So newline in Telnet should always be '\r\n' but most implementations have either not been updated, or keeps the old '\n\r' for backwards compatibility.

how does \r (carriage return) work in Python

Well, it appears that you did move to the left of the line. It just left the rest of the line untouched. Note that return is 6 chars, and I will is also 6.

Difference between \n and \r?

In terms of ascii code, it's 3 -- since they're 10 and 13 respectively;-).

But seriously, there are many:

in Unix and all Unix-like systems, \n is the code for end-of-line, \r means nothing special
as a consequence, in C and most languages that somehow copy it (even remotely), \n is the standard escape sequence for end of line (translated to/from OS-specific sequences as needed)
in old Mac systems (pre-OS X), \r was the code for end-of-line instead
in Windows (and many old OSs), the code for end of line is 2 characters, \r\n, in this order
as a (surprising;-) consequence (harking back to OSs much older than Windows), \r\n is the standard line-termination for text formats on the Internet
for electromechanical teletype-like "terminals", \r commands the carriage to go back leftwards until it hits the leftmost stop (a slow operation), \n commands the roller to roll up one line (a much faster operation) -- that's the reason you always have \r before \n, so that the roller can move while the carriage is still going leftwards!-) Wikipedia has a more detailed explanation.
for character-mode terminals (typically emulating even-older printing ones as above), in raw mode, \r and \n act similarly (except both in terms of the cursor, as there is no carriage or roller;-)

In practice, in the modern context of writing to a text file, you should always use \n (the underlying runtime will translate that if you're on a weird OS, e.g., Windows;-). The only reason to use \r is if you're writing to a character terminal (or more likely a "console window" emulating it) and want the next line you write to overwrite the last one you just wrote (sometimes used for goofy "ascii animation" effects of e.g. progress bars) -- this is getting pretty obsolete in a world of GUIs, though;-).

Difference between \r and \n

\r is "Carriage Return" (CR, ASCII character 13), \n is "Line Feed" (LF, ASCII character 10). Back in the days, you had two ASCII characters at the end of each line to tell a printer what to do - CR would tell the printer to go back to the left edge of the paper, LF would advance to the next line.

Operating systems still have different conventions as to what the end of a line looks like -- some of them have \n\r, some have \n, some have \r\n.

In Javascript, you mostly deal with \n - this is how strings are typically switching to the next line. However, depending on what strings you are working with, you may be encountering \r as well.

Meaning of \r on linux systems

The character '\r' is carriage return. It returns the cursor to the start of the line.

It is often used in Internet protocols conjunction with newline ('\n') to mark the end of a line (most standards specifies it as "\r\n", but some allows the wrong way around). On Windows the carriage-return newline pair is also used as end-of-line. On the old Macintosh operating system (before OSX) a single carriage-return was used instead of newline as end-of-line, while UNIX and UNIX-like systems (like Linux and OSX) uses a single newline.

What's the Use of '\r' escape sequence?

\r is a carriage return character; it tells your terminal emulator to move the cursor at the start of the line.

The cursor is the position where the next characters will be rendered.

So, printing a \r allows to override the current line of the terminal emulator.

Tom Zych figured why the output of your program is o world while the \r is at the end of the line and you don't print anything after that:

When your program exits, the shell prints the command prompt. The terminal renders it where you left the cursor. Your program leaves the cursor at the start of the line, so the command prompt partly overrides the line you printed. This explains why you seen your command prompt followed by o world.

The online compiler you mention just prints the raw output to the browser. The browser ignores control characters, so the \r has no effect.

See https://en.wikipedia.org/wiki/Carriage_return

Here is a usage example of \r:

#include <stdio.h>
#include <unistd.h>

int main()
{
        char chars[] = {'-', '\\', '|', '/'};
        unsigned int i;

        for (i = 0; ; ++i) {
                printf("%c\r", chars[i % sizeof(chars)]);
                fflush(stdout);
                usleep(200000);
        }

        return 0;
}

It repeatedly prints the characters - \ | / at the same position to give the illusion of a rotating | in the terminal.

What's the difference between \n and \r\n?

\r\n is a Windows Style

\n is a POSIX Style

\r is a old pre-OS X Macs Style, Modern Mac's using POSIX Style.

\r is a carriage return and \n is a line feed, in old computers where it not have monitor, have only printer to get out programs result to user, if you want get printing from staring of new line from left, you must get \n for Line Feed, and \r for get Carriage return to the most left position, this is from old computers syntax saved to this time on Windows platform.

What Does "\R" Do in the Following Script