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.1-2024 for information on environment variable usage.
The value of an environment variable is an arbitrary sequence of bytes, except for the null byte. 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 any bytes that have the encoded value of the character '='. For values to be portable across systems conforming to POSIX.1-2024, the value shall be composed of bytes that have the encoded value 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.1-2024 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, and byte
sequences that do not form valid 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.
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.1-2024.
If the application modifies the pointers to which environ points, the behavior of all interfaces described in the System Interfaces volume of POSIX.1-2024 is undefined.
It is unwise to conflict with certain variables that are frequently exported by widely used command interpreters and applications:
ARFLAGS |
IFS |
MAILPATH |
PS1 |
CC |
LANG |
MAILRC |
PS2 |
CDPATH |
LC_ALL |
MAKEFLAGS |
PS3 |
CFLAGS |
LC_COLLATE |
MAKESHELL |
PS4 |
CHARSET |
LC_CTYPE |
MANPATH |
PWD |
COLUMNS |
LC_MESSAGES |
MBOX |
RANDOM |
DATEMSK |
LC_MONETARY |
MORE |
SECONDS |
DEAD |
LC_NUMERIC |
MSGVERB |
SHELL |
EDITOR |
LC_TIME |
NLSPATH |
TERM |
ENV |
LDFLAGS |
NPROC |
TERMCAP |
EXINIT |
LEX |
OLDPWD |
TERMINFO |
FC |
LFLAGS |
OPTARG |
TMPDIR |
FCEDIT |
LINENO |
OPTERR |
TZ |
FFLAGS |
LINES |
OPTIND |
USER |
GET |
LISTER |
PAGER |
VISUAL |
GFLAGS |
LOGNAME |
PATH |
YACC |
HISTFILE |
LPDEST |
PPID |
YFLAGS |
HISTORY |
|
PRINTER |
|
HISTSIZE |
MAILCHECK |
PROCLANG |
|
HOME |
MAILER |
PROJECTDIR |
|
Additionally, a subset of the above variables are manipulated by shell built-in utilities outside of shell assignments. If an attempt is made to mark any of the following variables as readonly, then either the readonly utility shall reject the attempt, or readonly shall succeed but the shell can still modify the variables outside of assignment context, or readonly shall succeed but use of a shell built-in that would otherwise modify such a variable shall fail.
Implementations may provide an implementation-defined set of additional variables which are manipulated by implementation-specific built-in utilities not defined in this standard. The readonly utility shall not reject marking these additional variables as readonly, but when marked readonly, those extension utilities shall either continue to modify the variables, or shall fail because the variable is readonly. None of the variables defined by this standard shall be in this implementation-defined set.LINENO OLDPWD OPTARG OPTIND PWD
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.1-2024 only as specified in POSIX.1-2024. If they are defined in the application's environment, the utilities in the Shell and Utilities volume of POSIX.1-2024 and the functions in the System Interfaces volume of POSIX.1-2024 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 2.13 Shell Execution Environment for methods of accessing these variables.
Implementations may ignore some environment variables at the point of use for security reasons, for example in programs whose real and effective user IDs or real and effective group IDs were not equal at program startup. The behavior shall be as if the implementation obtains the values for these environment variables using secure_getenv() instead of getenv() (see getenv ); they shall not be removed from the environment of affected processes and shall be inherited as required by this standard.
This section describes environment variables that are relevant to the operation of internationalized interfaces described in POSIX.1-2024.
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.1-2024 is described in the ENVIRONMENT VARIABLES section for those utilities in addition to the global effects described in this section.
The locale names in LANGUAGE shall override the locale name associated with the "active category" of the current locale or, in the case of functions with an _l suffix, the provided locale object, and the language-specific part of the default search path for messages objects, unless the locale name that would be overridden is C or POSIX. For the dcgettext(), dcgettext_l(), dcngettext(), and dcngettext_l() functions, the active category is specified by the category argument; for all other gettext family functions and for the gettext and ngettext utilities, the active category is LC_MESSAGES .
For example, if:
then the following pathnames are tried in this order by gettext family functions that have neither a category argument nor an _l suffix until a valid messages object is found:
The conversion specification descriptions below refer to a "currently active text domain". The currently active text domain is, in decreasing order of precedence:
Conversion specifications consist of a '%' symbol, followed by a single-letter keyword. The following conversion specifications are currently defined:
An empty string shall be substituted if the specified value is not currently defined. The separators <underscore> ('_') and <period> ('.') shall not be included in the %t and %c conversion specifications.
Templates defined in NLSPATH are separated by <colon> characters (':'). A leading, trailing, or two adjacent <colon> characters ("::") shall be equivalent to specifying %N.
Since <colon> is a separator in this context, directory names that might be used in NLSPATH should not include a <colon> character.
Example 1, for an application that uses catopen() but does not use the gettext family of functions:
NLSPATH="/system/nlslib/%N.cat"
indicates that catopen() should look for all message catalogs in the directory /system/nlslib, where the catalog name should be constructed from the name argument (replacing %N) passed to catopen(), with the suffix .cat.
Example 2, for an application that uses the gettext family of functions but does not use catopen():
NLSPATH="/usr/lib/locale/fr/LC_MESSAGES/%N.mo"indicates that the gettext family of functions (and the gettext and ngettext utilities) should look for all messages objects in the directory /usr/lib/locale/fr/LC_MESSAGES, where the messages object's name should be constructed from the currently active text domain (replacing %N), with the suffix .mo.
Example 3, for an application that uses catopen() but does not use the gettext family of functions:
NLSPATH=":%N.cat:/nlslib/%L/%N.cat"
indicates that catopen() should look for the requested message catalog in name, name.cat, and /nlslib/localename/name.cat, where localename is the locale name given by the value of the LC_MESSAGES category of the current locale.
Example 4, for an application that uses the gettext family of functions but does not use catopen():
NLSPATH="/usr/lib/locale/%L/%N.mo:/usr/lib/locale/fr/%N.mo"
indicates that the gettext family of functions (and the gettext and ngettext utilities) should look for all messages objects first in /usr/lib/locale/localename/textdomain.mo, and if not found there, then try in /usr/lib/locale/fr/textdomain.mo, where localename is the locale name given by the value of the active category in the current locale or provided locale object.
Example 5, for an application that uses catopen() and the gettext family of functions:
NLSPATH="/usr/lib/locale/%L/%N.mo:/system/nlslib/%L/%N.cat"
indicates that the gettext family of functions (and the gettext and ngettext utilities) should look for all messages objects in /usr/lib/locale/localename/textdomain.mo, where localename is the locale name given by the value of the active category in the current locale or provided locale object. Also, catopen() should look for all message catalogs in the directory /system/nlslib/localename/name.cat, (assuming that /usr/lib/locale/localename/name.mo is not a message catalog). In this scenario, catopen() ignores all files that are not valid message catalogs while traversing NLSPATH . Furthermore, the gettext family of functions and the gettext and ngettext utilities ignore all files that are not valid messages objects found while traversing NLSPATH .
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 may produce undefined results in the standard utilities other than gettext and ngettext, and in applications with appropriate privileges.
Specifying a relative pathname in the NLSPATH environment variable should be avoided without a specific reason, including the use of a leading, trailing, or two adjacent <colon> characters, since it may result in messages objects being searched for in a directory relative to the current working directory of the calling process; if the process calls the chdir() function, the directory searched for may also be changed.
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. See 7.1 General for the consequences of setting these variables to locales with different character sets.
The values of locale categories shall be determined by a precedence order; the first condition met below determines the value:
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 7.2 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.
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.
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.
Since <colon> is a separator in this context, directory names that might be used in PATH should not include a <colon> character. Since <percent-sign> may have an implementation-defined meaning when searching for built-in utilities, directory names in PATH to be used to search for non-built-in utilities should not contain a <percent-sign> character.
:characters
or:
std offset dst offset, rule
or:
A format specifying a geographical timezone or a special timezone.
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 form of the second format (without the inserted spaces) is as follows:
stdoffset[dst[offset][,start[/time],end[/time]]]
Where:
Each of these fields may occur in either of two formats quoted or unquoted:
The interpretation of std and, if present, dst is unspecified if the field is less than three bytes or more than {TZNAME_MAX} bytes, or if it contains characters other than those specified.
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, Daylight Saving 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 '+').
date[/time],date[/time]
where the first date describes when the change from standard time to Daylight Saving Time occurs and the second date describes when it ends; if the second date is specified as earlier in the year than the first, then the year begins and ends in Daylight Saving Time. 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:
The time has the same format as offset except that the hour can range from zero to 167. If preceded by a '-', the time shall count backwards before midnight. For example, "47:30" stands for 23:30 the next day, and "-3:30" stands for 20:30 the previous day. The default, if time is not given, shall be 02:00:00.
Daylight Saving Time is in effect all year if it starts January 1 at 00:00 and ends December 31 at 24:00 plus the difference between Daylight Saving Time and standard time, leaving no room for standard time in the calendar. For example, TZ='EST5EDT,0/0,J365/25' represents a time zone that observes Daylight Saving Time all year, being 4 hours west of UTC with abbreviation "EDT".
If the dst field is specified and the rule field is not, it is implementation-defined when the changes to and from Daylight Saving Time occur.
If TZ is of the third format (that is, if the first character is not a <colon> and the value does not match the syntax for the second format), the value indicates either a geographical timezone or a special timezone from an implementation-defined timezone database. Typically these take the form
Area/Location
as in the IANA timezone database. Examples of geographical timezones that may be supported include Africa/Cairo, America/New_York, America/Indiana/Indianapolis, Asia/Tokyo, and Europe/London. The data for each geographical timezone shall include:
If there are any historical variations, or known future variations, of the above data for a geographical timezone, these variations shall be included in the database, except that historical variations from before the Epoch need not be included.
If the database incorporates an external database such as the one maintained by IANA, the implementation shall provide an implementation-defined method to allow the database to be updated, for example the method specified by RFC 6557.
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