8. Environment Variables

8.1 Environment Variable Definition

Environment variables defined in this chapter affect the operation of multiple utilities, functions, and applications. There are other environment variables that are of interest only to specific utilities. Environment variables that apply to a single utility only are defined as part of the utility description. See the ENVIRONMENT VARIABLES section of the utility descriptions in the Shell and Utilities volume of POSIX.1-2017 for information on environment variable usage.

The value of an environment variable is a string of characters. For a C-language program, an array of strings called the environment shall be made available when a process begins. The array is pointed to by the external variable environ, which is defined as:

extern char **environ;

These strings have the form name=value; names shall not contain the character '='. For values to be portable across systems conforming to POSIX.1-2017, the value shall be composed of characters from the portable character set (except NUL and as indicated below). There is no meaning associated with the order of strings in the environment. If more than one string in an environment of a process has the same name, the consequences are undefined.

Environment variable names used by the utilities in the Shell and Utilities volume of POSIX.1-2017 consist solely of uppercase letters, digits, and the <underscore> ( '_' ) from the characters defined in Portable Character Set and do not begin with a digit. Other characters may be permitted by an implementation; applications shall tolerate the presence of such names. Uppercase and lowercase letters shall retain their unique identities and shall not be folded together. The name space of environment variable names containing lowercase letters is reserved for applications. Applications can define any environment variables with names from this name space without modifying the behavior of the standard utilities.

Note:

Other applications may have difficulty dealing with environment variable names that start with a digit. For this reason, use of such names is not recommended anywhere.

The values that the environment variables may be assigned are not restricted except that they are considered to end with a null byte and the total space used to store the environment and the arguments to the process is limited to {ARG_MAX} bytes.

Other name= value pairs may be placed in the environment by, for example, calling any of the setenv(), unsetenv(), ^[XSI]  or putenv() functions, assigning a new value to the environ variable, or by using envp arguments when creating a process; see exec in the System Interfaces volume of POSIX.1-2017.

If the application modifies the pointers to which environ points, the behavior of all interfaces described in the System Interfaces volume of POSIX.1-2017 is undefined.

It is unwise to conflict with certain variables that are frequently exported by widely used command interpreters and applications:

If the variables in the following two sections are present in the environment during the execution of an application or utility, they shall be given the meaning described below. Some are placed into the environment by the implementation at the time the user logs in; all can be added or changed by the user or any ancestor of the current process. The implementation adds or changes environment variables named in POSIX.1-2017 only as specified in POSIX.1-2017. If they are defined in the application's environment, the utilities in the Shell and Utilities volume of POSIX.1-2017 and the functions in the System Interfaces volume of POSIX.1-2017 assume they have the specified meaning. Conforming applications shall not set these environment variables to have meanings other than as described. See getenv and XCU Shell Execution Environment for methods of accessing these variables.

8.2 Internationalization Variables

This section describes environment variables that are relevant to the operation of internationalized interfaces described in POSIX.1-2017.

Users may use the following environment variables to announce specific localization requirements to applications. Applications can retrieve this information using the setlocale() function to initialize the correct behavior of the internationalized interfaces. The descriptions of the internationalization environment variables describe the resulting behavior only when the application locale is initialized in this way. The use of the internationalization variables by utilities described in the Shell and Utilities volume of POSIX.1-2017 is described in the ENVIRONMENT VARIABLES section for those utilities in addition to the global effects described in this section.

LANG

This variable shall determine the locale category for native language, local customs, and coded character set in the absence of the LC_ALL and other LC_* (LC_COLLATE, LC_CTYPE, LC_MESSAGES, LC_MONETARY, LC_NUMERIC, LC_TIME) environment variables. This can be used by applications to determine the language to use for error messages and instructions, collating sequences, date formats, and so on.

LC_ALL

This variable shall determine the values for all locale categories. The value of the LC_ALL environment variable has precedence over any of the other environment variables starting with LC_ (LC_COLLATE, LC_CTYPE, LC_MESSAGES, LC_MONETARY, LC_NUMERIC, LC_TIME) and the LANG environment variable.

LC_COLLATE

This variable shall determine the locale category for character collation. It determines collation information for regular expressions and sorting, including equivalence classes and multi-character collating elements, in various utilities and the strcoll() and strxfrm() functions. Additional semantics of this variable, if any, are implementation-defined.

LC_CTYPE

This variable shall determine the locale category for character handling functions, such as tolower(), toupper(), and isalpha(). This environment variable determines the interpretation of sequences of bytes of text data as characters (for example, single as opposed to multi-byte characters), the classification of characters (for example, alpha, digit, graph), and the behavior of character classes. Additional semantics of this variable, if any, are implementation-defined.

LC_MESSAGES

This variable shall determine the locale category for processing affirmative and negative responses and the language and cultural conventions in which messages should be written. It also affects the behavior of the catopen() function in determining the message catalog. Additional semantics of this variable, if any, are implementation-defined. The language and cultural conventions of diagnostic and informative messages whose format is unspecified by POSIX.1-2017 should be affected by the setting of LC_MESSAGES.

LC_MONETARY

This variable shall determine the locale category for monetary-related numeric formatting information. Additional semantics of this variable, if any, are implementation-defined.

LC_NUMERIC

This variable shall determine the locale category for numeric formatting (for example, thousands separator and radix character) information in various utilities as well as the formatted I/O operations in printf() and scanf() and the string conversion functions in strtod(). Additional semantics of this variable, if any, are implementation-defined.

LC_TIME

This variable shall determine the locale category for date and time formatting information. It affects the behavior of the time functions in strftime(). Additional semantics of this variable, if any, are implementation-defined.

NLSPATH

This variable shall contain a sequence of templates that the catopen() function uses when attempting to locate message catalogs. Each template consists of an optional prefix, one or more conversion specifications, a pathname, and an optional suffix.

For example:

NLSPATH="/system/nlslib/%N.cat"

defines that catopen() should look for all message catalogs in the directory /system/nlslib, where the catalog name should be constructed from the name parameter passed to catopen() ( %N ), with the suffix .cat.

Conversion specifications consist of a '%' symbol, followed by a single-letter keyword. The following keywords are currently defined:

%N

The value of the name parameter passed to catopen().

%L

The value of the LC_MESSAGES category.

%l

The language element from the LC_MESSAGES category.

%t

The territory element from the LC_MESSAGES category.

%c

The codeset element from the LC_MESSAGES category.

%%

A single '%' character.

An empty string is substituted if the specified value is not currently defined. The separators <underscore> ( '_' ) and <period> ( '.' ) are not included in the %t and %c conversion specifications.

Templates defined in NLSPATH are separated by <colon> characters ( ':' ). A leading or two adjacent <colon> characters ( "::" ) is equivalent to specifying %N. For example:

NLSPATH=":%N.cat:/nlslib/%L/%N.cat"

indicates to catopen() that it should look for the requested message catalog in name, name.cat, and /nlslib/category/name.cat, where category is the value of the LC_MESSAGES category of the current locale.

Users should not set the NLSPATH variable unless they have a specific reason to override the default system path. Setting NLSPATH to override the default system path produces undefined results in the standard utilities and in applications with appropriate privileges.

The environment variables LANG, LC_ALL, LC_COLLATE, LC_CTYPE, LC_MESSAGES, LC_MONETARY, LC_NUMERIC, LC_TIME, and NLSPATH provide for the support of internationalized applications. The standard utilities shall make use of these environment variables as described in this section and the individual ENVIRONMENT VARIABLES sections for the utilities. If these variables specify locale categories that are not based upon the same underlying codeset, the results are unspecified.

The values of locale categories shall be determined by a precedence order; the first condition met below determines the value:

1. If the LC_ALL environment variable is defined and is not null, the value of LC_ALL shall be used.

2. If the LC_* environment variable (LC_COLLATE, LC_CTYPE, LC_MESSAGES, LC_MONETARY, LC_NUMERIC, LC_TIME) is defined and is not null, the value of the environment variable shall be used to initialize the category that corresponds to the environment variable.

3. If the LANG environment variable is defined and is not null, the value of the LANG environment variable shall be used.

4. If the LANG environment variable is not set or is set to the empty string, the implementation-defined default locale shall be used.

If the locale value is "C" or "POSIX", the POSIX locale shall be used and the standard utilities behave in accordance with the rules in POSIX Locale for the associated category.

If the locale value begins with a <slash>, it shall be interpreted as the pathname of a file that was created in the output format used by the localedef utility; see OUTPUT FILES under localedef. Referencing such a pathname shall result in that locale being used for the indicated category.

^[XSI] If the locale value has the form:

language[_territory][.codeset]

it refers to an implementation-provided locale, where settings of language, territory, and codeset are implementation-defined.

LC_COLLATE, LC_CTYPE, LC_MESSAGES, LC_MONETARY, LC_NUMERIC, and LC_TIME are defined to accept an additional field @ modifier, which allows the user to select a specific instance of localization data within a single category (for example, for selecting the dictionary as opposed to the character ordering of data). The syntax for these environment variables is thus defined as:

[language[_territory][.codeset][@modifier]]

For example, if a user wanted to interact with the system in French, but required to sort German text files, LANG and LC_COLLATE could be defined as:

LANG=Fr_FR
LC_COLLATE=De_DE

This could be extended to select dictionary collation (say) by use of the @ modifier field; for example:

LC_COLLATE=De_DE@dict

An implementation may support other formats.

If the locale value is not recognized by the implementation, the behavior is unspecified.

These environment variables are used by the newlocale() and setlocale() functions, and by the standard utilities.

Additional criteria for determining a valid locale name are implementation-defined.

8.3 Other Environment Variables

COLUMNS

This variable shall represent a decimal integer >0 used to indicate the user's preferred width in column positions for the terminal screen or window; see Column Position. If this variable is unset or null, the implementation determines the number of columns, appropriate for the terminal or window, in an unspecified manner. When COLUMNS is set, any terminal-width information implied by TERM is overridden. Users and conforming applications should not set COLUMNS unless they wish to override the system selection and produce output unrelated to the terminal characteristics.

Users should not need to set this variable in the environment unless there is a specific reason to override the implementation's default behavior, such as to display data in an area arbitrarily smaller than the terminal or window.

DATEMSK

^[XSI] Indicates the pathname of the template file used by getdate().

HOME

The system shall initialize this variable at the time of login to be a pathname of the user's home directory. See <pwd.h>.

LINES

This variable shall represent a decimal integer >0 used to indicate the user's preferred number of lines on a page or the vertical screen or window size in lines. A line in this case is a vertical measure large enough to hold the tallest character in the character set being displayed. If this variable is unset or null, the implementation determines the number of lines, appropriate for the terminal or window (size, terminal baud rate, and so on), in an unspecified manner. When LINES is set, any terminal-height information implied by TERM is overridden. Users and conforming applications should not set LINES unless they wish to override the system selection and produce output unrelated to the terminal characteristics.

Users should not need to set this variable in the environment unless there is a specific reason to override the implementation's default behavior, such as to display data in an area arbitrarily smaller than the terminal or window.

LOGNAME

The system shall initialize this variable at the time of login to be the user's login name. See <pwd.h>. For a value of LOGNAME to be portable across implementations of POSIX.1-2017, the value should be composed of characters from the portable filename character set.

MSGVERB

^[XSI] Describes which message components shall be used in writing messages by fmtmsg().

PATH

This variable shall represent the sequence of path prefixes that certain functions and utilities apply in searching for an executable file known only by a filename. The prefixes shall be separated by a <colon> ( ':' ). When a non-zero-length prefix is applied to this filename, a <slash> shall be inserted between the prefix and the filename if the prefix did not end in <slash>. A zero-length prefix is a legacy feature that indicates the current working directory. It appears as two adjacent <colon> characters ( "::" ), as an initial <colon> preceding the rest of the list, or as a trailing <colon> following the rest of the list. A strictly conforming application shall use an actual pathname (such as .) to represent the current working directory in PATH. The list shall be searched from beginning to end, applying the filename to each prefix, until an executable file with the specified name and appropriate execution permissions is found. If the pathname being sought contains a <slash>, the search through the path prefixes shall not be performed. If the pathname begins with a <slash>, the specified path is resolved (see Pathname Resolution). If PATH is unset or is set to null, the path search is implementation-defined.

Since <colon> is a separator in this context, directory names that might be used in PATH should not include a <colon> character.

PWD

This variable shall represent an absolute pathname of the current working directory. It shall not contain any components that are dot or dot-dot. The value is set by the cd utility, and by the sh utility during initialization.

SHELL

This variable shall represent a pathname of the user's preferred command language interpreter. If this interpreter does not conform to the Shell Command Language in XCU Shell Command Language, utilities may behave differently from those described in POSIX.1-2017.

TMPDIR

This variable shall represent a pathname of a directory made available for programs that need a place to create temporary files.

TERM

This variable shall represent the terminal type for which output is to be prepared. This information is used by utilities and application programs wishing to exploit special capabilities specific to a terminal. The format and allowable values of this environment variable are unspecified.

TZ

This variable shall represent timezone information. The contents of the environment variable named TZ shall be used by the ctime(), ctime_r(), localtime(), localtime_r() strftime(), mktime(), functions, and by various utilities, to override the default timezone. The value of TZ has one of the two forms (spaces inserted for clarity):

:characters

or:

std offset dst offset, rule

If TZ is of the first format (that is, if the first character is a <colon>), the characters following the <colon> are handled in an implementation-defined manner.

The expanded format (for all TZ s whose value does not have a <colon> as the first character) is as follows:

stdoffset[dst[offset][,start[/time],end[/time]]]

Where:

std and dst

Indicate no less than three, nor more than {TZNAME_MAX}, bytes that are the designation for the standard (std) or the alternative (dst -such as Daylight Savings Time) timezone. Only std is required; if dst is missing, then the alternative time does not apply in this locale.

Each of these fields may occur in either of two formats quoted or unquoted:

• In the quoted form, the first character shall be the <less-than-sign> ( '<' ) character and the last character shall be the <greater-than-sign> ( '>' ) character. All characters between these quoting characters shall be alphanumeric characters from the portable character set in the current locale, the <plus-sign> ( '+' ) character, or the <hyphen-minus> ( '-' ) character. The std and dst fields in this case shall not include the quoting characters.

• In the unquoted form, all characters in these fields shall be alphabetic characters from the portable character set in the current locale.

The interpretation of these fields is unspecified if either field is less than three bytes (except for the case when dst is missing), more than {TZNAME_MAX} bytes, or if they contain characters other than those specified.

offset

Indicates the value added to the local time to arrive at Coordinated Universal Time. The offset has the form:

hh[:mm[:ss]]

The minutes (mm) and seconds (ss) are optional. The hour (hh) shall be required and may be a single digit. The offset following std shall be required. If no offset follows dst, the alternative time is assumed to be one hour ahead of standard time. One or more digits may be used; the value is always interpreted as a decimal number. The hour shall be between zero and 24, and the minutes (and seconds)-if present-between zero and 59. The result of using values outside of this range is unspecified. If preceded by a '-', the timezone shall be east of the Prime Meridian; otherwise, it shall be west (which may be indicated by an optional preceding '+' ).

rule

Indicates when to change to and back from the alternative time. The rule has the form:

date[/time],date[/time]

where the first date describes when the change from standard to alternative time occurs and the second date describes when the change back happens. Each time field describes when, in current local time, the change to the other time is made.

The format of date is one of the following:

Jn

The Julian day n (1 <= n <= 365). Leap days shall not be counted. That is, in all years-including leap years-February 28 is day 59 and March 1 is day 60. It is impossible to refer explicitly to the occasional February 29.

n

The zero-based Julian day (0 <= n <= 365). Leap days shall be counted, and it is possible to refer to February 29.

Mm.n.d

The d'th day (0 <= d <= 6) of week n of month m of the year (1 <= n <= 5, 1 <= m <= 12, where week 5 means "the last d day in month m" which may occur in either the fourth or the fifth week). Week 1 is the first week in which the d'th day occurs. Day zero is Sunday.

The time has the same format as offset except that no leading sign ( '-' or '+' ) is allowed. The default, if time is not given, shall be 02:00:00.