I'd have thought one was enough. But what's the point of doing CRLF (0x0D0A), when you can simply use CR (0D)? Normally, whenever I'm using strings (C++), I do this:
myString = "Test\nThis should be a new line!\nAnother linefeed.";
NOTE: For non-C++ programmers reading this, "\n" is a linefeed (0x0A).
But should I really be doing this:
myString = "Test\r\nThis should be a new line!\r\nAnother carriage return/linefeed pair.";
NOTE: "\r" means carriage return (0x0D).
EDIT: Should this be on Programmers.SE?
Remember that these codes all came from old Teletype machines. These were effectively typewriters: it was necessary both to advance the paper by a line (line-feed), but also to return the print head (on the carriage) to the left side of the paper (carriage-return).
Windows / Unix / old Mac systems have each different way of writing new lines in text files (not binary ones). If you're programming under windows, then in binary mode, you will read (and you probably want to write) CRLF endings. Under unix-like systems it would be just LF.
If you deal with your own data formats... it shouldn't really matter which way you choose. It all really depends only on what you want to do with the string and where did you get it from.
Some systems like UNIX and OSX just use linefeed, DOS used an additional carriage return in order to be compatible with teletype machines and Windows inherited the architecture.
You use both on Windows because that's the custom on Windows. It's that simple. But you only write both for files destined for Windows.
Related
I'd like to know the difference (with examples if possible) between
CR LF (Windows), LF (Unix) and CR (Macintosh) line break types.
CR and LF are control characters, respectively coded 0x0D (13 decimal) and 0x0A (10 decimal).
They are used to mark a line break in a text file. As you indicated, Windows uses two characters the CR LF sequence; Unix only uses LF and the old MacOS ( pre-OSX MacIntosh) used CR.
An apocryphal historical perspective:
As indicated by Peter, CR = Carriage Return and LF = Line Feed, two expressions have their roots in the old typewriters / TTY. LF moved the paper up (but kept the horizontal position identical) and CR brought back the "carriage" so that the next character typed would be at the leftmost position on the paper (but on the same line). CR+LF was doing both, i.e. preparing to type a new line. As time went by the physical semantics of the codes were not applicable, and as memory and floppy disk space were at a premium, some OS designers decided to only use one of the characters, they just didn't communicate very well with one another ;-)
Most modern text editors and text-oriented applications offer options/settings etc. that allow the automatic detection of the file's end-of-line convention and to display it accordingly.
This is a good summary I found:
The Carriage Return (CR) character (0x0D, \r) moves the cursor to the beginning of the line without advancing to the next line. This character is used as a new line character in Commodore and early Macintosh operating systems (Mac OS 9 and earlier).
The Line Feed (LF) character (0x0A, \n) moves the cursor down to the next line without returning to the beginning of the line. This character is used as a new line character in Unix-based systems (Linux, Mac OS X, etc.)
The End of Line (EOL) sequence (0x0D 0x0A, \r\n) is actually two ASCII characters, a combination of the CR and LF characters. It moves the cursor both down to the next line and to the beginning of that line. This character is used as a new line character in most other non-Unix operating systems including Microsoft Windows, Symbian and others.
Source
It's really just about which bytes are stored in a file. CR is a bytecode for carriage return (from the days of typewriters) and LF similarly, for line feed. It just refers to the bytes that are placed as end-of-line markers.
Way more information, as always, on wikipedia.
Summarized succinctly:
Carriage Return (Mac pre-OS X)
CR
\r
ASCII code 13
Line Feed (Linux, Mac OS X)
LF
\n
ASCII code 10
Carriage Return and Line Feed (Windows)
CRLF
\r\n
ASCII code 13 and then ASCII code 10
If you see ASCII code in a strange format, they are merely the number 13 and 10 in a different radix/base, usually base 8 (octal) or base 16 (hexadecimal).
ASCII chart
Jeff Atwood has a recent blog post about this: The Great Newline Schism
Here is the essence from Wikipedia:
The sequence CR+LF was in common use
on many early computer systems that
had adopted teletype machines,
typically an ASR33, as a console
device, because this sequence was
required to position those printers at
the start of a new line. On these
systems, text was often routinely
composed to be compatible with these
printers, since the concept of device
drivers hiding such hardware details
from the application was not yet well
developed; applications had to talk
directly to the teletype machine and
follow its conventions. The separation
of the two functions concealed the
fact that the print head could not
return from the far right to the
beginning of the next line in
one-character time. That is why the
sequence was always sent with the CR
first. In fact, it was often necessary
to send extra characters (extraneous
CRs or NULs, which are ignored) to
give the print head time to move to
the left margin. Even after teletypes
were replaced by computer terminals
with higher baud rates, many operating
systems still supported automatic
sending of these fill characters, for
compatibility with cheaper terminals
that required multiple character times
to scroll the display.
CR - ASCII code 13
LF - ASCII code 10.
Theoretically, CR returns the cursor to the first position (on the left). LF feeds one line, moving the cursor one line down. This is how in the old days you controlled printers and text-mode monitors.
These characters are usually used to mark end of lines in text files.
Different operating systems used different conventions. As you pointed out, Windows uses the CR/LF combination while pre-OS X Macs use just CR and so on.
CR and LF are a special set of characters that help us format our code.
CR(/r) stands for CARRIAGE RETURN. It puts the cursor at the beginning of a line, but it doesn't create a new line. This is how MAC OS works.
LF(/n) stands for LINE FEED. It creates a new line, but it doesn't put the cursor at the beginning of that line. The cursor stays back at the end of the last line. This is how Unix and Linux work.
CRLF (/r/f) creates a new line as well as puts the cursor at the beginning of the new line. This is how we see it in Windows OS.
Git uses LF by default. So when we use Git on Windows it throws a warning like "CRLF will be replaced by LF" and automatically converts all CRLF into LF, so that code becomes compatible.
NB: Don't worry...see this less as a warning and more as a notice thing.
Systems based on ASCII or a
compatible character set use either LF
(Line feed, 0x0A, 10
in decimal) or CR (Carriage return, 0x0D, 13 in decimal)
individually, or CR followed by
LF (CR+LF, 0x0D 0x0A);
These characters are based on printer commands: The line feed
indicated that one line of
paper should feed out of the printer, and a carriage return
indicated that the printer
carriage should return to the beginning of the current line.
Here is the details.
The sad state of "record separators" or "line terminators" is a legacy of the dark ages of computing.
Now, we take it for granted that anything we want to represent is in some way structured data and conforms to various abstractions that define lines, files, protocols, messages, markup, whatever.
But once upon a time this wasn't exactly true. Applications built-in control characters and device-specific processing. The brain-dead systems that required both CR and LF simply had no abstraction for record separators or line terminators. The CR was necessary in order to get the teletype or video display to return to column one and the LF (today, NL, same code) was necessary to get it to advance to the next line. I guess the idea of doing something other than dumping the raw data to the device was too complex.
Unix and Mac actually specified an abstraction for the line end, imagine that. Sadly, they specified different ones. (Unix, ahem, came first.) And naturally, they used a control code that was already "close" to S.O.P.
Since almost all of our operating software today is a descendent of Unix, Mac, or Microsoft operating software, we are stuck with the line ending confusion.
NL is derived from EBCDIC NL = 0x15 which would logically compare to CRLF 0x0D 0x0A ASCII... This becomes evident when physically moving data from mainframes to midrange. Colloquially (as only arcane folks use EBCDIC), NL has been equated with either CR or LF or CRLF.
I'm trying to send file using curl to an endpoint and save the file to the machine.
Sending curl from Linux and saving it on the machine works well,
but doing the same curl from Windows is adding ^M character to every end of line.
I'm printing the file before saving it and can't see ^M. Only viewing the file on the remote machine after saving it shows me ^M.
A simple string replacement doesn't seem to work.
Why is ^M being added? How can I prevent this?
Quick Answer: That's a carriage return. They're a harmless but mildly irritating artifact of how Windows encodes text files. You can strip them out of your files with dos2unix. You can configure most text editors to use "Unix Line Endings" or "LF Line Endings" to prevent them from appearing in new files that you create from Windows PCs in the future.
Long Answer (with some historical trivia):
In a plain text file, when you create a new line (by pressing enter/return), a "line break" is embedded in the file. On Unix/Linux, this is a single character, '\n', the "line feed". On Windows, this is two sequential characters, '\r\n', the "carriage return" followed by the "line feed".
When physical teletype terminals, which behaved much like typewriters, were still in use, the "line feed" character meant "move the paper up to the next line" and the "carriage return" character meant "slide the carriage all the way over so the typing head is on the far left". From the very beginning, nearly all teletype terminals supported implicit carriage return; i.e., triggering a line feed would automatically trigger a carriage return. The developers working on what later evolved into Windows decided that it would be best to include explicit carriage returns, just in case (for some reason) the teletype does not perform one implicitly. The Unix developers, on the other hand, chose to work with the assumption of implicit carriage return.
The carriage return and line feed are ASCII Control Characters which means they do not have a visible representation as standalone printable characters, instead they affect the output cursor itself (in this case, the position of the output cursor).
The "^M" you see is a stand-in representation for the carriage return character, used by programs that don't fully "cook" their output (i.e., don't apply the effects of some ASCII Control Characters). (Other control characters have other representations starting with "^", and the "^" character is also used to represent the "ctrl" keyboard key in some Unix programs like nano.)
You can use dos2unix to convert the line endings from Windows-style to Unix-style.
$ curl https://example.com/file_with_crlf.txt | dos2unix > file.txt
On some distros, this tool is included by default, on others it can be installed via the package manager (e.g., on Ubuntu, sudo apt install dos2unix). There also exists a package, unix2dos, for the inverse.
Most "smart" text editors for coding (Sublime, Atom, VS Code, Notepad++, etc.) will happily read and write with either Windows-style or Unix-style line endings (this might require changing some configuration options). Often, the line-endings are auto-detected by scanning the contents of a file, and usually new files are created with the Operating System's native line endings (by default). Even the new version of Notepad supports Unix-style line endings. On the other hand, some Unix tools will produce strange results in the presence of Windows-style line breaks. If your codebase will be used by people on both Unix and Windows operating systems, the nice thing to do is to use Unix-style line endings everywhere.
Git on Windows also has an optional mode that checks out all files with Windows-style line breaks, but checks them back in with Unix-style line breaks.
Side Notes (interesting, but not directly related to your question):
What the carriage return actually does (on a modern virtual terminal, be it Windows or Unix) is move the output cursor to the beginning of the line. If you use the carriage return without a line feed, you can "overwrite" part of a string that has already been printed.
$ printf "dogdog" ; printf "\rcat\n"
catdog
Some Unix programs use this to asynchronously update part of the last line of output, to implement things like a live-updating progress indicator. For example, curl, which shows download progress on stdout if the file contents are piped elsewhere.
Also: If you had a tool that interpreted Windows-style line endings as literally as possible, and you fed it a string with Unix-style line endings such as "hello\nworld", you would get output like this:
hello
world
Fortunately, such implementations are extremely rare and, in general, the vast majority of Windows tools can render Unix-style line-endings identically to Windows-style line endings without any problem.
I'm looking for a way to get the platform end of line character sequence (CRLF for Windows, LF for Linux/macOS) at runtime.
I don't believe there is any feature that does this specifically. Even the line-aware features of the standard library don't: BufRead::read_line is documented to only recognize \n, and BufRead::lines (source), which strips end-of-line characters, only does so for \n and \r\n, indiscriminate of what platform it's invoked on.
"Platform line ending" is really a category error, though. Files are sent across networks and copied from one computer to another. If your program writes files that need to be opened on Windows in Notepad, it doesn't matter whether the program that generates them is running on Windows or Linux; it needs to emit \r\n. Similarly if the program is writing a specific file format or implementing some network protocol; the format or protocol should tell you what line separator to use. If the format allows either and there is no convention, pick the one you prefer; just use it consistently.
If you're reading line endings, you should probably tolerate either one, like BufRead::lines does.
However, should you really need to, like if your output will be read by a poorly written program that expects different line endings on different platforms, you can use conditional compilation attributes to achieve this effect:
#[cfg(windows)]
const LINE_ENDING: &'static str = "\r\n";
#[cfg(not(windows))]
const LINE_ENDING: &'static str = "\n";
I develop and use Git on both Windows and Linux. When using IDEs or Git on Windows, I'm frequently prompted on whether to save files with CR LF or not.
I am doing mainly C# and JavaScript ES6 development which involves code that contains multi-line strings.
What reasons are there to save files with CR LF on Windows? Are CR-LFs mostly of historical significance? I have not yet noticed a drawback to working with UNIX \n line endings on Windows.
Windows batch files can malfunction when saved with newline endings because the goto command works by jumping to the appropriate offset in the script — but it is not computed correctly unless the lines end with carriage-return/line-feed.
Some Windows programs don't properly handle '\n' without '\r' but any decent editor or for that matter any decent program should handle them identically. But CRLF is traditionally the sanctioned way to do line endings on Windows and you might have compatibility issues if you don't.
The current project I'm working requires me to follow certain procedures to eliminate whitespace in my code. Apparently this has got something to do with line endings since one requirement explicitly tells me to "end all lines with a Unix line ending (\n)".
I code in VIM from the terminal, and I press enter for a new line to write on. Am I missing something here?
What is the reason to keep the code clean from trailing whitespace and using specific types of line breaks?
On a side note, what standard VI/VIM settings do you guys use to adhere to common coding standards?
Sincerely,
Why
Different operating systems have different line break conventions. Unix-like systems prefer \n (LF); Windows prefers \r\n (CR LF); pre-OSX Mac OS used \r (CR). Maintaining one convention across a project is usually a good idea.
As for trailing whitespace, AFAIK it's just sloppy (may indicate "quick and dirty" reformatting). In some environments trailing whitespace might also be significant.
Perhaps not everyone on the project codes in VIM, they might be using a windows based IDE which would insert \r\n for a new line.
They would have to ensure that their line-endings are correct before committing code, whereas you shouldn't have this problem as vim will use \n as its natural line ending.
To enforce this in vim, you can use the fileformat option. Setting it to unix will make your newlines use \n.