Comma-separated values

In computing, a comma-separated values (CSV) file stores tabular data (numbers and text) in plain text. Each line of the file is a data record. Each record consists of one or more fields, separated by commas. The use of the comma as a field separator is the source of the name for this file format.

An official standard for the CSV file format does not exist, but RFC 4180 provides a de facto standard for many aspects of it. In popular usage, however, the term "CSV" may denote some closely related delimiter-separated formats, which use a variety of different field-delimiters. These include tab-separated values and space-separated values, both of which are popular. Such files are often even given a .csv extension, despite the use of a different field separator (not a comma). This loose use of terminology can cause problems in data exchange.

Data exchange

CSV is a common data exchange format that is widely supported by consumer, business, and scientific applications. Among its most common uses is moving tabular data^[3] between programs that natively operate on incompatible (often proprietary and/or undocumented) formats.^[1] This works despite lack of adherence to RFC 4180 (or any other standard), because so many programs support variations on the CSV format for data import.

For example, a user may need to transfer information from a database program that stores data in a proprietary format, to a spreadsheet that uses a completely different format. The database program most likely can export its data as "CSV"; the exported CSV file can then be imported by the spreadsheet program.

Specification

RFC 4180 proposes a specification for the CSV format, and this is the definition commonly used. However, in popular usage "CSV" is not a single, well-defined format. As a result, in practice the term "CSV" might refer to any file that:^[2][4]

1. is plain text using a character set such as ASCII, various Unicode character sets (e.g. UTF-8), EBCDIC, or Shift JIS,
2. consists of records (typically one record per line),
3. with the records divided into fields separated by delimiters (typically a single reserved character such as comma, semicolon, or tab; sometimes the delimiter may include optional spaces),
4. where every record has the same sequence of fields.

Within these general constraints, many variations are in use. Therefore, without additional information (such as whether RFC 4180 is honored), a file claimed simply to be in "CSV" format is not fully specified. As a result, many applications supporting CSV files allow users to preview the first few lines of the file and then specify the delimiter character(s), quoting rules, etc. If a particular CSV file's variations fall outside what a particular receiving program supports, it is often feasible to examine and edit the file by hand (i.e., with a text editor) or write a script or program to produce a conforming format.

History

Comma-separated values is a data format that pre-dates personal computers by more than a decade: the IBM Fortran (level H extended) compiler under OS/360 supported them in 1972.^[5] List-directed ("free form") input/output was defined in FORTRAN 77, approved in 1978. List-directed input used commas and/or spaces for delimiters, so unquoted character strings could not contain commas or spaces.^[6]

Comma-separated value lists are easier to type (for example into punched cards) than fixed-column-aligned data, and were less prone to producing incorrect results if a value was punched one column off from its intended location.

Comma separated files are used for the interchange of database information between machines of two different architectures. The plain-text character of CSV files largely avoids incompatibilities such as byte-order and word size. The files are largely human-readable, so it is easier to deal with them in the absence of perfect documentation or communication.^[7]

The main standardization initiative – transforming "de facto fuzzy definition" into a more precise and de jure one – was in 2005, with RFC4180, defining CSV as a MIME Content Type. Later, in 2013, some of RFC4180's lacks were covered by a W3C recommendation.^[8]

In 2014 IETF published RFC7111 describing application of URI fragments to CSV documents. RFC7111 specifies how row, column, and cell ranges can be selected from a CSV document using position indexes.

In 2015 W3C, in an attempt to enhance CSV with formal semantics, publicized the first drafts of recommendations for CSV-metadata standards.

General functionality

CSV formats are best used to represent sets or sequences of records in which each record has an identical list of fields. This corresponds to a single relation in a relational database, or to data (though not calculations) in a typical spreadsheet.

The format dates back to the early days of business computing and is widely used to pass data between computers with different internal word sizes, data formatting needs, and so forth. For this reason, CSV files are common on all computer platforms.

CSV is a delimited text file that uses a comma to separate values (many implementations of CSV import/export tools allow other separators to be used). Simple CSV implementations may prohibit field values that contain a comma or other special characters such as newlines. More sophisticated CSV implementations permit them, often by requiring " (double quote) characters around values that contain reserved characters (such as commas, double quotes, or less commonly, newlines). Embedded double quote characters may then be represented by a pair of consecutive double quotes,^[9] or by prefixing an escape character such as a backslash (for example in Sybase Central).

CSV formats are not limited to a particular character set.^[1] They work just as well with Unicode character sets (such as UTF-8 or UTF-16) as with ASCII (although particular programs that support CSV may have their own limitations). CSV files normally will even survive naive translation from one character set to another (unlike nearly all proprietary data formats). CSV does not, however, provide any way to indicate what character set is in use, so that must be communicated separately, or determined at the receiving end (if possible).

Databases that include multiple relations cannot be exported as a single CSV file.

Similarly, CSV cannot naturally represent hierarchical or object-oriented database or other data. This is because every CSV record is expected to have the same structure. CSV is therefore rarely appropriate for documents such as those created with HTML, XML, or other markup or word-processing technologies.

Statistical databases in various fields often have a generally relation-like structure, but with some repeatable groups of fields. For example, health databases such as the Demographic and Health Survey typically repeat some questions for each child of a given parent (perhaps up to a fixed maximum number of children). Statistical analysis systems often include utilities that can "rotate" such data; for example, a "parent" record that includes information about five children can be split into five separate records, each containing (a) the information on one child, and (b) a copy of all the non-child-specific information. CSV can represent either the "vertical" or "horizontal" form of such data.

In a relational database, similar issues are readily handled by creating a separate relation for each such group, and connecting "child" records to the related "parent" records using a foreign key (such as an ID number or name for the parent). In markup languages such as XML, such groups are typically enclosed within a parent element and repeated as necessary (for example, multiple <child> nodes within a single <parent> node). With CSV there is no widely accepted single-file solution.

Standardization

The name "CSV" indicates the use of the comma to separate data fields. Nevertheless, the term "CSV" is widely used to refer a large family of formats, which differ in many ways. Some implementations allow or require single or double quotation marks around some or all fields; and some reserve the very first record as a header containing a list of field names. The character set being used is undefined: some applications require a Unicode byte order mark (BOM) to enforce Unicode interpretation (sometimes even a UTF-8 BOM.)^[1] Files that use the tab character instead of comma can be more precisely referred to as "TSV" for tab separated values.

Other implementation differences include handling of more commonplace field separators (such as space or semicolon) and newline characters inside text fields. One more subtlety is the interpretation of a blank line: it can equally be the result of writing a record of zero fields, or a record of one field of zero length; thus decoding it is ambiguous.

Reliance on the standard documented by RFC 4180 can simplify CSV exchange. However, this standard only specifies handling of text-based fields. Interpretation of the text of each field is still application-specific.

RFC 4180 formalized CSV. It defines the MIME type "text/csv", and CSV files that follow its rules should be very widely portable. Among its requirements:

• MS-DOS-style lines that end with (CR/LF) characters (optional for the last line)
• An optional header record (there is no sure way to detect whether it is present, so care is required when importing).
• Each record "should" contain the same number of comma-separated fields.
• Any field may be quoted (with double quotes).
• Fields containing a line-break, double-quote, and/or commas should be quoted. (If they are not, the file will likely be impossible to process correctly).
• A (double) quote character in a field must be represented by two (double) quote characters.

The format can be processed by most programs that claim to read CSV files. The exceptions are: (a) programs may not support line-breaks within quoted fields, and (b) programs may confuse the optional header with data or interpret the first data line as an optional header.

In 2013 the W3C "CSV on the Web" working group began to specify technologies providing a higher interoperability for web applications using CSV or similar formats.^[10] They are drafting recommendations for modeling "Tabular Data",^[11] and enhancing CSV with metadata and semantics.

Basic rules

Many informal documents exist that describe "CSV" formats. IETF RFC 4180 (summarized above) defines the format for the "text/csv" MIME type registered with the IANA.

Rules typical of these and other "CSV" specifications and implementations are as follows:

• CSV is a delimited data format that has fields/columns separated by the comma character and records/rows terminated by newlines.

• A CSV file does not require a specific character encoding, byte order, or line terminator format (some software does not support all line-end variations).

• A record ends at a line terminator. However, line-terminators can be embedded as data within fields, so software must recognize quoted line-separators (see below) in order to correctly assemble an entire record from perhaps multiple lines.

• All records should have the same number of fields, in the same order.

• Data within fields is interpreted as a sequence of characters, not as a sequence of bits or bytes (see RFC 2046, section 4.1). For example, the numeric quantity 65535 may be represented as the 5 ASCII characters "65535" (or perhaps other forms such as "0xFFFF", "000065535.000E+00", etc.); but not as a sequence of 2 bytes intended to be treated as a single binary integer rather than as two characters (e.g. the numbers 11264-11307 have a comma as their high order byte: ord(',')*256..ord(',')*257-1). If this "plain text" convention is not followed, then the CSV file no longer contains sufficient information to interpret it correctly, the CSV file will not likely survive transmission across differing computer architectures, and will not conform to the text/csv MIME type.

• Adjacent fields must be separated by a single comma. However, "CSV" formats vary greatly in this choice of separator character. In particular, in locales where the comma is used as a decimal separator, semicolon, TAB, or other characters are used instead.

1997,Ford,E350

• Any field may be quoted (that is, enclosed within double-quote characters). Some fields must be quoted, as specified in following rules.

"1997","Ford","E350"

• Fields with embedded commas or double-quote characters must be quoted.

1997,Ford,E350,"Super, luxurious truck"

• Each of the embedded double-quote characters must be represented by a pair of double-quote characters.

1997,Ford,E350,"Super, ""luxurious"" truck"

• Fields with embedded line breaks must be quoted (however, many CSV implementations do not support embedded line breaks).

1997,Ford,E350,"Go get one now
they are going fast"

• In some CSV implementations, leading and trailing spaces and tabs are trimmed (ignored). Such trimming is forbidden by RFC 4180, which states "Spaces are considered part of a field and should not be ignored."

1997, Ford, E350
not same as
1997,Ford,E350

• According to RFC 4180, spaces outside quotes in a field are not allowed; however, the RFC also says that "Spaces are considered part of a field and should not be ignored." and "Implementors should 'be conservative in what you do, be liberal in what you accept from others' (RFC 793 [8]) when processing CSV files."

1997, "Ford" ,E350

• In CSV implementations that do trim leading or trailing spaces, fields with such spaces as meaningful data must be quoted.

1997,Ford,E350," Super luxurious truck "

• Double quote processing need only apply if the field starts with a double quote. Note, however, that double quotes are not allowed in unquoted fields according to RFC 4180.

Los Angeles,34°03′N,118°15′W
New York City,40°42′46″N,74°00′21″W
Paris,48°51′24″N,2°21′03″E

• The first record may be a "header", which contains column names in each of the fields (there is no reliable way to tell whether a file does this or not; however, it is uncommon to use characters other than letters, digits, and underscores in such column names).

Year,Make,Model
1997,Ford,E350
2000,Mercury,Cougar

Example

The above table of data may be represented in CSV format as follows:

Year,Make,Model,Description,Price
1997,Ford,E350,"ac, abs, moon",3000.00
1999,Chevy,"Venture ""Extended Edition""","",4900.00
1999,Chevy,"Venture ""Extended Edition, Very Large""",,5000.00
1996,Jeep,Grand Cherokee,"MUST SELL!
air, moon roof, loaded",4799.00

Example of a USA/UK CSV file (where the decimal separator is a period/full stop and the value separator is a comma):

Year,Make,Model,Length
1997,Ford,E350,2.34
2000,Mercury,Cougar,2.38

Example of an analogous European CSV/DSV file (where the decimal separator is a comma and the value separator is a semicolon):

Year;Make;Model;Length
1997;Ford;E350;2,34
2000;Mercury;Cougar;2,38

The latter format is not RFC 4180 compliant.^[12] Compliance could be achieved by the use of a comma instead of a semicolon as a separator and either the international notation for the representation of the decimal mark or the practice of quoting all numbers that have a decimal mark.

Application support

The CSV file format is supported by almost all spreadsheets and database management systems. Many programming languages have libraries available that support CSV files. Many implementations support changing the field-separator character and some quoting conventions, although it is safest to use the simplest conventions, to maximize the recipients' chances of handling the data.

Microsoft Excel will open .csv files, but depending on the system's regional settings, it may expect a semicolon as a separator instead of a comma, since in some languages the comma is used as the decimal separator. It also applies some magic, such as reformatting what looks like numbers, eliminating leading + or 0, which breaks phone numbers, or a leading = makes the cell a formula, where function names must be in the opener's local language. Also, many regional versions of Excel will not be able to deal with Unicode in CSV. One simple solution when encountering such difficulties is to change the filename extension from .csv to .txt; then opening the file from an already running Excel instance with the "Open" command, where the user can manually specify the delimiters, encoding, format of columns, etc.

Apache OpenOffice Calc and LibreOffice Calc handle CSV files and pasted text with a Text Import dialog asking the user to manually specify the delimiters, encoding, format of columns, etc.

Numbers (spreadsheet), the Apple equivalent of Microsoft Excel, supports import and export of CSV files as well. In fact, this feature is one that can be expected on any spreadsheet editing program.

There are many utility programs on Unix-style systems that can deal with at least some CSV files. Many such utilities have a way to change the delimiter character, but lack support for any other variations (or for Unicode). Some of the useful programs are:

• column (-s to change the delimiter character(s))
• cut (-d to change the delimiter character)
• paste (-d to change the delimiter character(s))
• join (-t to change the delimiter character)
• sort (-t to change the delimiter character)
• uniq (-f to skip comparing the first N fields)
• emacs (using csv-nav mode)^[13]
• awk (-F to change the delimiter character)

See also

• Comparison of data serialization formats
• Delimiter-separated values

References