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Special characters in computing signifying the end of a line of text From Wikipedia, the free encyclopedia
A newline (frequently called line ending, end of line (EOL), next line (NEL) or line break) is a control character or sequence of control characters in character encoding specifications such as ASCII, EBCDIC, Unicode, etc. This character, or a sequence of characters, is used to signify the end of a line of text and the start of a new one.[1]
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In the mid-1800s, long before the advent of teleprinters and teletype machines, Morse code operators or telegraphists invented and used Morse code prosigns to encode white space text formatting in formal written text messages. In particular, the Morse prosign BT (mnemonic break text), represented by the concatenation of literal textual Morse codes "B" and "T" characters, sent without the normal inter-character spacing, is used in Morse code to encode and indicate a new line or new section in a formal text message.
Later, in the age of modern teleprinters, standardized character set control codes were developed to aid in white space text formatting. ASCII was developed simultaneously by the International Organization for Standardization (ISO) and the American Standards Association (ASA), the latter being the predecessor organization to American National Standards Institute (ANSI). During the period of 1963 to 1968, the ISO draft standards supported the use of either CR+LF or LF alone as a newline, while the ASA drafts supported only CR+LF.
The sequence CR+LF was commonly used on many early computer systems that had adopted Teletype machines—typically a Teletype Model 33 ASR—as a console device, because this sequence was required to position those printers at the start of a new line. The separation of newline into two functions concealed the fact that the print head could not return from the far right to the beginning of the next line in time to print the next character. Any character printed after a CR would often print as a smudge in the middle of the page while the print head was still moving the carriage back to the first position. "The solution was to make the newline two characters: CR to move the carriage to column one, and LF to move the paper up."[2] In fact, it was often necessary to send extra padding characters—extraneous CRs or NULs—which are ignored but give the print head time to move to the left margin. Many early video displays also required multiple character times to scroll the display.
On such systems, applications had to talk directly to the Teletype machine and follow its conventions since the concept of device drivers hiding such hardware details from the application was not yet well developed. Therefore, text was routinely composed to satisfy the needs of Teletype machines. Most minicomputer systems from DEC used this convention. CP/M also used it in order to print on the same terminals that minicomputers used. From there MS-DOS (1981) adopted CP/M's CR+LF in order to be compatible, and this convention was inherited by Microsoft's later Windows operating system.
The Multics operating system began development in 1964 and used LF alone as its newline. Multics used a device driver to translate this character to whatever sequence a printer needed (including extra padding characters), and the single byte was more convenient for programming. What seems like a more obvious choice—CR—was not used, as CR provided the useful function of overprinting one line with another to create boldface, underscore and strikethrough effects. Perhaps more importantly, the use of LF alone as a line terminator had already been incorporated into drafts of the eventual ISO/IEC 646 standard. Unix followed the Multics practice, and later Unix-like systems followed Unix. This created conflicts between Windows and Unix-like operating systems, whereby files composed on one operating system could not be properly formatted or interpreted by another operating system (for example a UNIX shell script written in a Windows text editor like Notepad[3][4]).
The concepts of carriage return (CR) and line feed (LF) are closely associated and can be considered either separately or together. In the physical media of typewriters and printers, two axes of motion, "down" and "across", are needed to create a new line on the page. Although the design of a machine (typewriter or printer) must consider them separately, the abstract logic of software can combine them together as one event. This is why a newline in character encoding can be defined as CR and LF combined into one (commonly called CR+LF or CRLF).
Some character sets provide a separate newline character code. EBCDIC, for example, provides an NL character code in addition to the CR and LF codes. Unicode, in addition to providing the ASCII CR and LF control codes, also provides a "next line" (NEL) control code, as well as control codes for "line separator" and "paragraph separator" markers. Unicode also contains printable characters for visually representing line feed ␊, carriage return ␍, and other C0 control codes (as well as a generic newline, ) in the Control Pictures block.
| Operating system | Character encoding | Abbreviation | hex value | dec value | Escape sequence |
|---|---|---|---|---|---|
| Multics POSIX standard oriented systems: Unix and Unix-like systems (Linux, macOS, *BSD, AIX, Xenix, etc.), QNX 4+ Others: BeOS, Amiga, RISC OS, and others[5] |
ASCII | LF | 0A | 10 | \n |
| Windows, MS-DOS compatibles, Atari TOS, DEC TOPS-10, RT-11, CP/M, MP/M, OS/2, Symbian OS, Palm OS, Amstrad CPC, and most other early non-Unix and non-IBM operating systems | CR LF | 0D 0A | 13 10 | \r\n | |
| Commodore 64, Commodore 128, Acorn BBC, ZX Spectrum, TRS-80, Apple II, Oberon, classic Mac OS, HP Series 80, MIT Lisp Machine, and OS-9 | CR | 0D | 13 | \r | |
| Acorn BBC[6] and RISC OS spooled text output[7] | LF CR | 0A 0D | 10 13 | \n\r | |
| QNX pre-POSIX implementation (version < 4) | RS | 1E | 30 | \036 | |
| Atari 8-bit computers | ATASCII | EOL | 9B | 155 | |
| IBM mainframe systems, including z/OS (OS/390) and IBM i (OS/400) | EBCDIC | NL | 15 | 21 | \025 |
| ZX80 and ZX81 (home computers from Sinclair Research Ltd) | ZX80/ZX81 proprietary encoding | 76 | 118 |
0x85) is called NEL (Next Line). EBCDIC also has control characters called CR and LF, but the numerical value of LF (0x25) differs from the one used by ASCII (0x0A). Additionally, some EBCDIC variants also use NL but assign a different numeric code to the character. However, those operating systems use a record-based file system, which stores text files as one record per line. In most file formats, no line terminators are actually stored.# that thus could not be used as the first character in a line. Some early line printers interpreted these characters directly in the records sent to them.Many communications protocols have some sort of new line convention. In particular, protocols published by the Internet Engineering Task Force (IETF) typically use the ASCII CRLF sequence.
In some older protocols, the new line may be followed by a checksum or parity character.
The Unicode standard defines a number of characters that conforming applications should recognize as line terminators:[9]
While it may seem overly complicated compared to an approach such as converting all line terminators to a single character (e.g. LF), because Unicode is designed to preserve all information when converting a text file from any existing encoding to Unicode and back (round-trip integrity), Unicode needs to make the same distinctions between line breaks made by other encodings.
For example: NL is part of EBCDIC, which uses code 0x15; it is normally mapped to Unicode NEL, 0x85, which is a control character in the C1 control set.[10] As such, it is defined by ECMA 48,[11] and recognized by encodings compliant with ISO/IEC 2022 (which is equivalent to ECMA 35).[12] C1 control set is also compatible with ISO-8859-1.[citation needed] The approach taken in the Unicode standard allows round-trip transformation to be information-preserving while still enabling applications to recognize all possible types of line terminators.
Recognizing and using the newline codes greater than 0x7F (NEL, LS and PS) is not often done. They are multiple bytes in UTF-8, and the code for NEL has been used as the ellipsis (…) character in Windows-1252. For instance:
The Unicode special characters U+2424 (SYMBOL FOR NEWLINE, ), U+23CE (RETURN SYMBOL, ⏎), U+240D (SYMBOL FOR CARRIAGE RETURN, ␍) and U+240A (SYMBOL FOR LINE FEED, ␊) are glyphs intended for presenting a user-visible character to the reader of the document, and are thus not recognized themselves as a newline.
To facilitate creating portable programs, programming languages provide some abstractions to deal with the different types of newline sequences used in different environments.
The C language provides the escape sequences '\n' (newline) and '\r' (carriage return). However, these are not required to be equivalent to the ASCII LF and CR control characters. The C standard only guarantees two traits:
On Unix operating system platforms, where C originated, the native newline sequence is ASCII LF (0x0A), so '\n' was simply defined to be that value. With the internal and external representation being identical, the translation performed in text mode is a no-op, and Unix has no notion of text mode or binary mode. This has caused many programmers who developed their software on Unix systems simply to ignore the distinction completely, resulting in code that is not portable to different platforms.
The C library function fgets() is best avoided in binary mode because any file not written with the Unix newline convention will be misread. Also, in text mode, any file not written with the system's native newline sequence (such as a file created on a Unix system, then copied to a Windows system) will be misread as well.
Another common problem is the use of '\n' when communicating using an Internet protocol that mandates the use of ASCII CR+LF for ending lines. Writing '\n' to a text mode stream works correctly on Windows systems, but produces only LF on Unix, and something completely different on more exotic systems. Using "\r\n" in binary mode is slightly better.
Many languages, such as C++, Perl,[20] and Haskell provide the same interpretation of '\n' as C. C++ has an alternative input/output (I/O) model where the manipulator std::endl can be used to output a newline (and flushes the stream buffer).
Java, PHP,[21] and Python[22] provide the '\r\n' sequence (for ASCII CR+LF). In contrast to C, these are guaranteed to represent the values U+000D and U+000A, respectively.
The Java input/output (I/O) libraries do not transparently translate these into platform-dependent newline sequences on input or output. Instead, they provide functions for writing a full line that automatically add the native newline sequence, and functions for reading lines that accept any of CR, LF, or CR+LF as a line terminator (see BufferedReader.readLine()). The System.lineSeparator() method can be used to retrieve the underlying line separator.
Example:
String eol = System.lineSeparator();
String lineColor = "Color: Red" + eol;
Python permits "Universal Newline Support" when opening a file for reading, when importing modules, and when executing a file.[23]
Some languages have created special variables, constants, and subroutines to facilitate newlines during program execution. In some languages such as PHP and Perl, double quotes are required to perform escape substitution for all escape sequences, including '\n' and '\r'. In PHP, to avoid portability problems, newline sequences should be issued using the PHP_EOL constant.[24]
Example in C#:
string eol = Environment.NewLine;
string lineColor = "Color: Red" + eol;
string eol2 = "\n";
string lineColor2 = "Color: Blue" + eol2;
The different newline conventions cause text files that have been transferred between systems of different types to be displayed incorrectly.
Text in files created with programs which are common on Unix-like or classic Mac OS, appear as a single long line on most programs common to MS-DOS and Microsoft Windows because these do not display a single line feed or a single carriage return as a line break.
Conversely, when viewing a file originating from a Windows computer on a Unix-like system, the extra CR may be displayed as a second line break, as ^M, or as <cr> at the end of each line.
Furthermore, programs other than text editors may not accept a file, e.g. some configuration file, encoded using the foreign newline convention, as a valid file.
The problem can be hard to spot because some programs handle the foreign newlines properly while others do not. For example, a compiler may fail with obscure syntax errors even though the source file looks correct when displayed on the console or in an editor. Modern text editors generally recognize all flavours of CR+LF newlines and allow users to convert between the different standards. Web browsers are usually also capable of displaying text files and websites which use different types of newlines.
Even if a program supports different newline conventions, these features are often not sufficiently labeled, described, or documented. Typically a menu or combo-box enumerating different newline conventions will be displayed to users without an indication if the selection will re-interpret, temporarily convert, or permanently convert the newlines. Some programs will implicitly convert on open, copy, paste, or save—often inconsistently.
Most textual Internet protocols (including HTTP, SMTP, FTP, IRC, and many others) mandate the use of ASCII CR+LF ('\r\n', 0x0D 0x0A) on the protocol level, but recommend that tolerant applications recognize lone LF ('\n', 0x0A) as well. Despite the dictated standard, many applications erroneously use the C newline escape sequence '\n' (LF) instead of the correct combination of carriage return escape and newline escape sequences '\r\n' (CR+LF) (see section Newline in programming languages above). This accidental use of the wrong escape sequences leads to problems when trying to communicate with systems adhering to the stricter interpretation of the standards instead of the suggested tolerant interpretation. One such intolerant system is the qmail mail transfer agent that actively refuses to accept messages from systems that send bare LF instead of the required CR+LF.[25]
The standard Internet Message Format[26] for email states: "CR and LF MUST only occur together as CRLF; they MUST NOT appear independently in the body". Differences between SMTP implementations in how they treat bare LF and/or bare CR characters have led to SMTP spoofing attacks referred to as "SMTP smuggling".[27]
The File Transfer Protocol can automatically convert newlines in files being transferred between systems with different newline representations when the transfer is done in "ASCII mode". However, transferring binary files in this mode usually has disastrous results: any occurrence of the newline byte sequence—which does not have line terminator semantics in this context, but is just part of a normal sequence of bytes—will be translated to whatever newline representation the other system uses, effectively corrupting the file. FTP clients often employ some heuristics (for example, inspection of filename extensions) to automatically select either binary or ASCII mode, but in the end it is up to users to make sure their files are transferred in the correct mode. If there is any doubt as to the correct mode, binary mode should be used, as then no files will be altered by FTP, though they may display incorrectly.[28]
Text editors are often used for converting a text file between different newline formats; most modern editors can read and write files using at least the different ASCII CR/LF conventions.
For example, the editor Vim can make a file compatible with the Windows Notepad text editor. Within vim
:set fileformat=dos
:wq
Editors can be unsuitable for converting larger files or bulk conversion of many files. For larger files (on Windows NT) the following command is often used:
D:\>TYPE unix_file | FIND /V "" > dos_file
Special purpose programs to convert files between different newline conventions include unix2dos and dos2unix, mac2unix and unix2mac, mac2dos and dos2mac, and flip.[29] The tr command is available on virtually every Unix-like system and can be used to perform arbitrary replacement operations on single characters. A DOS/Windows text file can be converted to Unix format by simply removing all ASCII CR characters with
$ tr -d '\r' < inputfile > outputfile
or, if the text has only CR newlines, by converting all CR newlines to LF with
$ tr '\r' '\n' < inputfile > outputfile
The same tasks are sometimes performed with awk, sed, or in Perl if the platform has a Perl interpreter:
$ awk '{sub("$","\r\n"); printf("%s",$0);}' inputfile > outputfile # UNIX to DOS (adding CRs on Linux and BSD based OS that haven't GNU extensions)
$ awk '{gsub("\r",""); print;}' inputfile > outputfile # DOS to UNIX (removing CRs on Linux and BSD based OS that haven't GNU extensions)
$ sed -e 's/$/\r/' inputfile > outputfile # UNIX to DOS (adding CRs on Linux based OS that use GNU extensions)
$ sed -e 's/\r$//' inputfile > outputfile # DOS to UNIX (removing CRs on Linux based OS that use GNU extensions)
$ perl -pe 's/\r?\n|\r/\r\n/g' inputfile > outputfile # Convert to DOS
$ perl -pe 's/\r?\n|\r/\n/g' inputfile > outputfile # Convert to UNIX
$ perl -pe 's/\r?\n|\r/\r/g' inputfile > outputfile # Convert to old Mac
The file command can identify the type of line endings:
$ file myfile.txt
myfile.txt: ASCII English text, with CRLF line terminators
The Unix egrep (extended grep) command can be used to print filenames of Unix or DOS files (assuming Unix and DOS-style files only, no classic Mac OS-style files):
$ egrep -L '\r\n' myfile.txt # show UNIX style file (LF terminated)
$ egrep -l '\r\n' myfile.txt # show DOS style file (CRLF terminated)
Other tools permit the user to visualise the EOL characters:
$ od -a myfile.txt
$ cat -e myfile.txt
$ cat -v myfile.txt
$ hexdump -c myfile.txt
Two ways to view newlines, both of which are self-consistent, are that newlines either separate lines or that they terminate lines. If a newline is considered a separator, there will be no newline after the last line of a file. Some programs have problems processing the last line of a file if it is not terminated by a newline. On the other hand, programs that expect newline to be used as a separator will interpret a final newline as starting a new (empty) line. Conversely, if a newline is considered a terminator, all text lines including the last are expected to be terminated by a newline. If the final character sequence in a text file is not a newline, the final line of the file may be considered to be an improper or incomplete text line, or the file may be considered to be improperly truncated.
In text intended primarily to be read by humans using software which implements the word wrap feature, a newline character typically only needs to be stored if a line break is required independent of whether the next word would fit on the same line, such as between paragraphs and in vertical lists. Therefore, in the logic of word processing and most text editors, newline is used as a paragraph break and is known as a "hard return", in contrast to "soft returns" which are dynamically created to implement word wrapping and are changeable with each display instance. In many applications a separate control character called "manual line break" exists for forcing line breaks inside a single paragraph. The glyph for the control character for a hard return is usually a pilcrow (¶), and for the manual line break is usually a carriage return arrow (↵).
RI (U+008D REVERSE LINE FEED,[30] ISO/IEC 6429 8D, decimal 141) is used to move the printing position back one line (by reverse feeding the paper, or by moving a display cursor up one line) so that other characters may be printed over existing text. This may be done to make them bolder, or to add underlines, strike-throughs or other characters such as diacritics.
Similarly, PLD (U+008B PARTIAL LINE FORWARD, decimal 139) and PLU (U+008C PARTIAL LINE BACKWARD, decimal 140) can be used to advance or reverse the text printing position by some fraction of the vertical line spacing (typically, half). These can be used in combination for subscripts (by advancing and then reversing) and superscripts (by reversing and then advancing), and may also be useful for printing diacritics.
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