Newline

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For other uses, see New Line (disambiguation).

In computing, a newline (also known as a line break or end-of-line / EOL character) is a special character or sequence of characters signifying the end of a line of text. The name comes from the fact that the next character after the newline will appear on a new line—that is, on the next line below the text, immediately proceeding the newline. The actual codes representing a newline vary across hardware platforms and operating systems, which can be a problem when exchanging data between systems with different representations.

There is also some confusion as to whether newlines terminate or separate lines. If a newline is considered a separator, there will be no newline after the last line of a file. The general convention on most systems is to add a newline even after the last line, i.e., to treat newline as a line terminator. Some programs have problems processing the last line of a file if it isn't newline terminated. Conversely, programs that expect newline to be used as a separator will interpret a final newline as starting a new (empty) line. This can result in a different line count being reported for the file, but is otherwise generally harmless.

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. See hard return and soft return.

Contents

Representations

Software applications and operating systems usually represent a newline with one or two control characters:

  • Systems based on ASCII or a compatible character set use either LF (Line feed, 0Ah) or CR (Carriage Return, 0Dh) individually, or CR followed by LF (CR+LF, 0Dh 0Ah); see below for the historical reason for the CR+LF convention. 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.
  • EBCDIC systems—mainly IBM mainframe systems, including z/OS (OS/390) and i5/OS (OS/400)—use NEL (Next Line, 0x15) as the newline character. Note that EBCDIC also has control characters called CR and LF, but the numerical value of LF (25) differs from the one used by ASCII (0A). Additionally, there are some EBCDIC variants that also use NEL but assign a different numeric code to the character.
  • Operating systems for the CDC 6000 series defined a newline as two or more zero-valued six-bit characters at the end of a 60-bit word. Some configurations also defined a zero-valued character as a colon character, with the result that multiple colons could be interpreted as a newline depending on position.
  • OpenVMS uses a record-based file system, which stores text files as one record per line. In most file formats, no line terminators are actually stored, but the Record Management Services facility can transparently add a terminator to each line when it is retrieved by an application.
  • Fixed line length was used by some early mainframe operating systems. In such a system, an implicit end-of-line was assumed every, say, 80 characters. No newline character was stored. If a file was imported from the outside world, lines shorter than the line length had to be padded with spaces, while lines longer than the line length had to be truncated. This mimicked the use of punched cards, on which each line was stored on a separate card, usually with 80 columns on each card.

Most textual Internet protocols (including HTTP, SMTP, FTP, IRC and many others) mandate the use of ASCII CR+LF (0Dh 0Ah) on the protocol level, but recommend that tolerant applications recognize lone LF as well. In practice, there are many applications that erroneously use the C newline character '\n' instead (see section Newline in programming languages below). This leads to problems when trying to communicate with systems adhering to a stricter interpretation of the standards; one such system is the qmail MTA that actively refuses to accept messages from systems that send bare LF instead of the required CR+LF.[1]

Unicode

The Unicode standard addresses the problem by defining a large number of characters that conforming applications should recognize as line terminators:

 LF:    Line Feed, U+000A
 CR:    Carriage Return, U+000D
 CR+LF: CR followed by LF, U+000D followed by U+000A
 NEL:   Next Line, U+0085
 FF:    Form Feed, U+000C
 LS:    Line Separator, U+2028
 PS:    Paragraph Separator, U+2029

This may seem overly complicated compared to an approach such as converting all line terminators to a single character, for example LF. The simple approach breaks down, however, when trying to convert a text file from an encoding like EBCDIC to Unicode and back. When converting to Unicode, NEL would have to be replaced by LF, but when converting back it would be impossible to decide if an LF should be mapped to an EBCDIC LF or NEL. 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.

History

ASCII was developed simultaneously by the ISO and the ASA, the predecessor organization to ANSI. During the period of 1963–1968, 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 Multics operating system began development in 1964 and used LF alone as its newline. Unix followed the Multics practice, and later systems followed Unix.

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.

MS-DOS, built upon a CP/M clone called 86-DOS (which Microsoft purchased and renamed), adopted CP/M's CR+LF; CP/M's use of CR+LF made sense for using computer terminals via serial lines. This convention was inherited by Microsoft's later Windows operating system.

Newline in programming languages

To facilitate the creation of portable programs, programming languages provide some abstractions to deal with the different types of newline sequences used in different environments.

The C programming language provides the escape sequences '\n' (newline) and '\r' (carriage return). However, contrary to popular belief, these are in fact not required to be equivalent to the ASCII LF and CR control characters. The C standard only guarantees two things:

  1. Each of these escape sequences maps to a unique implementation-defined number that can be stored in a single char value.
  2. When writing a file in text mode, '\n' is transparently translated to the native newline sequence used by the system, which may be longer than one character. (Note that a C implementation is allowed to not store newline characters in files. For example, the lines of a text file could be stored as rows of a SQL table or as fixed-length records.) When reading in text mode, the native newline sequence is translated back to '\n'. In binary mode, the second mode of I/O supported by the C library, no translation is performed, and the internal representation of any escape sequence is output directly.

On Unix 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 effectively turns into a no-op, making text mode and binary mode behave the same. This has caused many programmers who developed their software on Unix systems to simply ignore the distinction completely, resulting in code that is not portable to different platforms.

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, as it works on many ASCII-compatible systems, but still fails in the general case. One approach is to use binary mode and specify the numeric values of the control sequence directly, "\x0D\x0A".

Many languages, such as C++, Perl[2] and Python provide the same interpretation of '\n' as C.

Java also provides '\n' and '\r' escape sequences. In contrast to C, these are guaranteed to represent the values U+000A and U+000D, respectively. The Java 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.getProperties method can be used to retrieve the underlying line separator.

Some languages have created special variables, constants and subroutines to facilitate newlines during program execution. One example is the PHP constant PHP_EOL, which will produce either '\r\n' or '\n' appropriate to the operating system the program is executed on.[3] Though special newline handling facilities can aid execution during runtime, they do not ensure the validity of newlines for the source code itself.

Common problems

The different newline conventions often cause text files that have been transferred between systems of different types to be displayed incorrectly. For example, files originating on Unix or Apple Macintosh systems may appear as a single long line on a Windows system. Conversely, when viewing a file from a Windows computer on a Unix system, the extra CR may be displayed as ^M at the end of each line or as a second line break.

The problem can be hard to spot if some programs handle the foreign newlines properly while others don't. 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 the user to convert between the different standards. Web browsers are usually also capable of displaying text files of different types.

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 the user to make sure his or her 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.

Conversion utilities

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Generally, using a text editor is the simplest and most convenient way of 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. Unfortunately, the standard Windows editor Notepad is not one of them (though Wordpad is).

On Windows systems without a better editor, the old MS-DOS editor EDIT that still ships with modern Windows versions can be used to convert a Unix text file to DOS/Windows newlines. A simple way of doing this is by creating a shortcut to EDIT on the desktop (context menu / New / Shortcut / "edit" / Next / Finish), dragging the text file in question onto it, and then saving the file again (File / Save).

Editors are often unsuitable for converting larger files. For larger files (on Windows NT/2000/XP) you can use the following command:

TYPE unix_file | FIND "" /V > dos_file

On many Unix systems, the dos2unix (sometimes named fromdos or d2u) and unix2dos (sometimes named todos or u2d) utilities can be used to translate between ASCII CR+LF (DOS/Windows) and LF (Unix) newlines. Different versions of these commands vary slightly in their syntax. However, 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 CRs, by converting CRs to LFs with

tr '\r' '\n' < inputfile > outputfile

The same tasks can be performed with sed, or in Perl if the platform has a Perl interpreter:

sed -e 's/$/\r/' inputfile > outputfile                # UNIX to DOS  (adding CRs)
sed -e 's/\r$//' inputfile > outputfile                # DOS  to UNIX (removing CRs)
perl -pe 's/\r\n|\n|\r/\r\n/g' inputfile > outputfile  # Convert to DOS
perl -pe 's/\r\n|\n|\r/\n/g'   inputfile > outputfile  # Convert to UNIX
perl -pe 's/\r\n|\n|\r/\r/g'   inputfile > outputfile  # Convert to old Mac

To identify what type of line breaks a text file contains one can use grep. The following grep commands will echo the filename (in this case myfile.txt) to the command line if the file is of the specified style:

grep -PL $'\r\n' myfile.txt # show UNIX style file (LF terminated)
grep -Pl $'\r\n' myfile.txt # show DOS style file (CRLF terminated)

For Debian-based systems, you can use these commands:

egrep -L $'\r\n' myfile.txt # show UNIX style file (LF terminated)
egrep -l $'\r\n' myfile.txt # show DOS style file (CRLF terminated)

The above grep commands will work under Unix systems or in Cygwin under Windows. Note that these commands make some assumptions about the kinds of files that exist on the system (specifically it's assuming only UNIX and DOS-style files—no Mac OS 9-style files). Check the -P, -L, and -l options to understand how it works.

This technique can be further combined with find to list files recursively. For instance, the following will check all "regular files" (e.g. it will exclude directories, symbolic links, etc.) to find all UNIX-style files in a directory tree, starting from the current directory (.), and save the results in file unix_files.txt, overwriting it if the file already exists:

find . -type f -exec grep -PL '\r\n' {} \; > unix_files.txt

The file command should also detect the type of EOL used:

file myfile.txt
> myfile.txt: ASCII text, with CRLF line terminators

Other tools permit to visualise the EOL characters like the following commands:

od -a myfile.txt
cat -e myfile.txt
hexdump -c myfile.txt

dos2unix, unix2dos, mac2unix, unix2mac, mac2dos, dos2mac can perform conversions. The flip [1] command may also be useful.

See also

References

  1. ^ http://cr.yp.to/docs/smtplf.html
  2. ^ http://perldoc.perl.org/functions/binmode.html
  3. ^ http://www.php.net/reserved.constants

External links

Retrieved from "http://en.wikipedia.org/wiki/Newline"

This article is from Wikipedia. All text is available under the terms of the GNU Free Documentation License.

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