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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"><html><head><meta name="generator" content="HTML Tidy, see www.w3.org"><meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1"><link type="text/css" rel="stylesheet" href="style.css"><!-- Generated by The Open Group's rhtm tool v1.2.1 --><!-- Copyright (c) 2001-2003 The Open Group, All Rights Reserved --><title>Rationale</title></head><body><basefont size="3"> <center><font size="2">The Open Group Base Specifications Issue 6<br>IEEE Std 1003.1, 2003 Edition<br>Copyright © 2001-2003 The IEEE and The Open Group</font></center><hr size="2" noshade><h3><a name="tag_01_08"></a>Environment Variables</h3><h4><a name="tag_01_08_01"></a>Environment Variable Definition</h4><p>The variable <i>environ</i> is not intended to be declared in any header, but rather to be declared by the user for accessingthe array of strings that is the environment. This is the traditional usage of the symbol. Putting it into a header could breaksome programs that use the symbol for their own purposes.</p><p>The decision to restrict conforming systems to the use of digits, uppercase letters, and underscores for environment variablenames allows applications to use lowercase letters in their environment variable names without conflicting with any conformingsystem.</p><p>In addition to the obvious conflict with the shell syntax for positional parameter substitution, some historical applications(including some shells) exclude names with leading digits from the environment.</p><h4><a name="tag_01_08_02"></a>Internationalization Variables</h4><p>The text about locale implies that any utilities written in standard C and conforming to IEEE Std 1003.1-2001 mustissue the following call:</p><blockquote><pre><tt>setlocale(LC_ALL, "")</tt></pre></blockquote><p>If this were omitted, the ISO C standard specifies that the C locale would be used.</p><p>If any of the environment variables are invalid, it makes sense to default to an implementation-defined, consistent localeenvironment. It is more confusing for a user to have partial settings occur in case of a mistake. All utilities would then behavein one language/cultural environment. Furthermore, it provides a way of forcing the whole environment to be theimplementation-defined default. Disastrous results could occur if a pipeline of utilities partially uses the environment variablesin different ways. In this case, it would be appropriate for utilities that use <i>LANG</i> and related variables to exit with anerror if any of the variables are invalid. For example, users typing individual commands at a terminal might want <a href="../utilities/date.html"><i>date</i></a> to work if <i>LC_MONETARY</i> is invalid as long as <i>LC_TIME</i> is valid. Since theseare conflicting reasonable alternatives, IEEE Std 1003.1-2001 leaves the results unspecified if the locale environmentvariables would not produce a complete locale matching the specification of the user.</p><p>The locale settings of individual categories cannot be truly independent and still guarantee correct results. For example, whencollating two strings, characters must first be extracted from each string (governed by <i>LC_CTYPE )</i> before being mapped tocollating elements (governed by <i>LC_COLLATE )</i> for comparison. That is, if <i>LC_CTYPE</i> is causing parsing according to therules of a large, multi-byte code set (potentially returning 20000 or more distinct character codeset values), but<i>LC_COLLATE</i> is set to handle only an 8-bit codeset with 256 distinct characters, meaningful results are obviouslyimpossible.</p><p>The <i>LC_MESSAGES</i> variable affects the language of messages generated by the standard utilities.</p><p>The description of the environment variable names starting with the characters "LC_" acknowledges the fact that the interfacespresented may be extended as new international functionality is required. In the ISO C standard, names preceded by "LC_" arereserved in the name space for future categories.</p><p>To avoid name clashes, new categories and environment variables are divided into two classifications:"implementation-independent" and "implementation-defined".</p><p>Implementation-independent names will have the following format:</p><blockquote><pre><tt>LC_</tt><i>NAME</i></pre></blockquote><p>where <i>NAME</i> is the name of the new category and environment variable. Capital letters must be used forimplementation-independent names.</p><p>Implementation-defined names must be in lowercase letters, as below:</p><blockquote><pre><tt>LC_</tt><i>name</i></pre></blockquote><h4><a name="tag_01_08_03"></a>Other Environment Variables</h4><h5><a name="tag_01_08_03_01"></a>COLUMNS, LINES</h5><p>The default values for the number of column positions, <i>COLUMNS ,</i> and screen height, <i>LINES ,</i> are unspecifiedbecause historical implementations use different methods to determine values corresponding to the size of the screen in which theutility is run. This size is typically known to the implementation through the value of <i>TERM ,</i> or by more elaborate methodssuch as extensions to the <a href="../utilities/stty.html"><i>stty</i></a> utility or knowledge of how the user is dynamicallyresizing windows on a bit-mapped display terminal. Users should not need to set these variables in the environment unless there isa specific reason to override the default behavior of the implementation, such as to display data in an area arbitrarily smallerthan the terminal or window. Values for these variables that are not decimal integers greater than zero are implicitly undefinedvalues; it is unnecessary to enumerate all of the possible values outside of the acceptable set.</p><h5><a name="tag_01_08_03_02"></a>LOGNAME</h5><p>In most implementations, the value of such a variable is easily forged, so security-critical applications should rely on othermeans of determining user identity. <i>LOGNAME</i> is required to be constructed from the portable filename character set forreasons of interchange. No diagnostic condition is specified for violating this rule, and no requirement for enforcement exists.The intent of the requirement is that if extended characters are used, the "guarantee" of portability implied by a standard isvoid.</p><h5><a name="tag_01_08_03_03"></a>PATH</h5><p>Many historical implementations of the Bourne shell do not interpret a trailing colon to represent the current working directoryand are thus non-conforming. The C Shell and the KornShell conform to IEEE Std 1003.1-2001 on this point. The usual nameof dot may also be used to refer to the current working directory.</p><p>Many implementations historically have used a default value of <b>/bin</b> and <b>/usr/bin</b> for the <i>PATH</i> variable.IEEE Std 1003.1-2001 does not mandate this default path be identical to that retrieved from <i>getconf</i> _CS_PATHbecause it is likely that the standardized utilities may be provided in another directory separate from the directories used bysome historical applications.</p><h5><a name="tag_01_08_03_04"></a>SHELL</h5><p>The <i>SHELL</i> variable names the preferred shell of the user; it is a guide to applications. There is no direct requirementthat that shell conform to IEEE Std 1003.1-2001; that decision should rest with the user. It is the intention of thestandard developers that alternative shells be permitted, if the user chooses to develop or acquire one. An operating system thatbuilds its shell into the "kernel" in such a manner that alternative shells would be impossible does not conform to the spirit ofIEEE Std 1003.1-2001.</p><h5><a name="tag_01_08_03_05"></a>TZ</h5><p>The quoted form of the timezone variable allows timezone names of the form UTC+1 (or any name that contains the character plus (<tt>'+'</tt> ), the character minus ( <tt>'-'</tt> ), or digits), which may be appropriate for countries that do not have anofficial timezone name. It would be coded as <UTC+1>+1<UTC+2>, which would cause <i>std</i> to have a value of UTC+1and <i>dst</i> a value of UTC+2, each with a length of 5 characters. This does not appear to conflict with any existing usage. Thecharacters <tt>'<'</tt> and <tt>'>'</tt> were chosen for quoting because they are easier to parse visually than a quotingcharacter that does not provide some sense of bracketing (and in a string like this, such bracketing is helpful). They were alsochosen because they do not need special treatment when assigning to the <i>TZ</i> variable. Users are often confused by embeddingquotes in a string. Because <tt>'<'</tt> and <tt>'>'</tt> are meaningful to the shell, the whole string would have to bequoted, but that is easily explained. (Parentheses would have presented the same problems.) Although the <tt>'>'</tt> symbolcould have been permitted in the string by either escaping it or doubling it, it seemed of little value to require that. This couldbe provided as an extension if there was a need. Timezone names of this new form lead to a requirement that the value of{_POSIX_TZNAME_MAX} change from 3 to 6.</p><p>Since the <i>TZ</i> environment variable is usually inherited by all applications started by a user after the value of the<i>TZ</i> environment variable is changed and since many applications run using the C or POSIX locale, using characters that arenot in the portable character set in the <i>std</i> and <i>dst</i> fields could cause unexpected results.</p><p>The format of the <i>TZ</i> environment variable is changed in Issue 6 to allow for the quoted form, as defined in previousversions of the ISO POSIX-1 standard.</p><p>IEEE Std 1003.1-2001/Cor 1-2002, item XBD/TC1/D6/7 is applied, adding the <a href="../functions/ctime_r.html"><i>ctime_r</i>()</a> and <a href="../functions/localtime_r.html"><i>localtime_r</i>()</a> functions tothe list of functions that use the <i>TZ</i> environment variable.</p><hr size="2" noshade><center><font size="2"><!--footer start-->UNIX ® is a registered Trademark of The Open Group.<br>POSIX ® is a registered Trademark of The IEEE.<br>[ <a href="../mindex.html">Main Index</a> | <a href="../basedefs/contents.html">XBD</a> | <a href="../utilities/contents.html">XCU</a> | <a href="../functions/contents.html">XSH</a> | <a href="../xrat/contents.html">XRAT</a>]</font></center><!--footer end--><hr size="2" noshade></body></html>⌨️ 快捷键说明
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