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<h4 class="subsection">25.4.2 Standard Environment Variables</h4>

<p><a name="index-standard-environment-variables-3109"></a>
These environment variables have standard meanings.  This doesn't mean
that they are always present in the environment; but if these variables
<em>are</em> present, they have these meanings.  You shouldn't try to use
these environment variable names for some other purpose.

<!-- Extra blank lines make it look better. -->
     <dl>
<dt><code>HOME</code><dd><a name="index-g_t_0040code_007bHOME_007d-environment-variable-3110"></a><a name="index-home-directory-3111"></a>
This is a string representing the user's <dfn>home directory</dfn>, or
initial default working directory.

     <p>The user can set <code>HOME</code> to any value. 
If you need to make sure to obtain the proper home directory
for a particular user, you should not use <code>HOME</code>; instead,
look up the user's name in the user database (see <a href="User-Database.html#User-Database">User Database</a>).

     <p>For most purposes, it is better to use <code>HOME</code>, precisely because
this lets the user specify the value.

     <!-- !!! also USER -->
     <br><dt><code>LOGNAME</code><dd><a name="index-g_t_0040code_007bLOGNAME_007d-environment-variable-3112"></a>
This is the name that the user used to log in.  Since the value in the
environment can be tweaked arbitrarily, this is not a reliable way to
identify the user who is running a program; a function like
<code>getlogin</code> (see <a href="Who-Logged-In.html#Who-Logged-In">Who Logged In</a>) is better for that purpose.

     <p>For most purposes, it is better to use <code>LOGNAME</code>, precisely because
this lets the user specify the value.

     <br><dt><code>PATH</code><dd><a name="index-g_t_0040code_007bPATH_007d-environment-variable-3113"></a>
A <dfn>path</dfn> is a sequence of directory names which is used for
searching for a file.  The variable <code>PATH</code> holds a path used
for searching for programs to be run.

     <p>The <code>execlp</code> and <code>execvp</code> functions (see <a href="Executing-a-File.html#Executing-a-File">Executing a File</a>)
use this environment variable, as do many shells and other utilities
which are implemented in terms of those functions.

     <p>The syntax of a path is a sequence of directory names separated by
colons.  An empty string instead of a directory name stands for the
current directory (see <a href="Working-Directory.html#Working-Directory">Working Directory</a>).

     <p>A typical value for this environment variable might be a string like:

     <pre class="smallexample">          :/bin:/etc:/usr/bin:/usr/new/X11:/usr/new:/usr/local/bin
</pre>
     <p>This means that if the user tries to execute a program named <code>foo</code>,
the system will look for files named <samp><span class="file">foo</span></samp>, <samp><span class="file">/bin/foo</span></samp>,
<samp><span class="file">/etc/foo</span></samp>, and so on.  The first of these files that exists is
the one that is executed.

     <!-- !!! also TERMCAP -->
     <br><dt><code>TERM</code><dd><a name="index-g_t_0040code_007bTERM_007d-environment-variable-3114"></a>
This specifies the kind of terminal that is receiving program output. 
Some programs can make use of this information to take advantage of
special escape sequences or terminal modes supported by particular kinds
of terminals.  Many programs which use the termcap library
(see <a href="../termcap/Finding-a-Terminal-Description.html#Finding-a-Terminal-Description">Find</a>) use the <code>TERM</code> environment variable, for example.

     <br><dt><code>TZ</code><dd><a name="index-g_t_0040code_007bTZ_007d-environment-variable-3115"></a>
This specifies the time zone.  See <a href="TZ-Variable.html#TZ-Variable">TZ Variable</a>, for information about
the format of this string and how it is used.

     <br><dt><code>LANG</code><dd><a name="index-g_t_0040code_007bLANG_007d-environment-variable-3116"></a>
This specifies the default locale to use for attribute categories where
neither <code>LC_ALL</code> nor the specific environment variable for that
category is set.  See <a href="Locales.html#Locales">Locales</a>, for more information about
locales.

     <br><dt><code>LC_ALL</code><dd><a name="index-g_t_0040code_007bLC_005fALL_007d-environment-variable-3117"></a>
If this environment variable is set it overrides the selection for all
the locales done using the other <code>LC_*</code> environment variables.  The
value of the other <code>LC_*</code> environment variables is simply ignored
in this case.

     <br><dt><code>LC_COLLATE</code><dd><a name="index-g_t_0040code_007bLC_005fCOLLATE_007d-environment-variable-3118"></a>
This specifies what locale to use for string sorting.

     <br><dt><code>LC_CTYPE</code><dd><a name="index-g_t_0040code_007bLC_005fCTYPE_007d-environment-variable-3119"></a>
This specifies what locale to use for character sets and character
classification.

     <br><dt><code>LC_MESSAGES</code><dd><a name="index-g_t_0040code_007bLC_005fMESSAGES_007d-environment-variable-3120"></a>
This specifies what locale to use for printing messages and to parse
responses.

     <br><dt><code>LC_MONETARY</code><dd><a name="index-g_t_0040code_007bLC_005fMONETARY_007d-environment-variable-3121"></a>
This specifies what locale to use for formatting monetary values.

     <br><dt><code>LC_NUMERIC</code><dd><a name="index-g_t_0040code_007bLC_005fNUMERIC_007d-environment-variable-3122"></a>
This specifies what locale to use for formatting numbers.

     <br><dt><code>LC_TIME</code><dd><a name="index-g_t_0040code_007bLC_005fTIME_007d-environment-variable-3123"></a>
This specifies what locale to use for formatting date/time values.

     <br><dt><code>NLSPATH</code><dd><a name="index-g_t_0040code_007bNLSPATH_007d-environment-variable-3124"></a>
This specifies the directories in which the <code>catopen</code> function
looks for message translation catalogs.

     <br><dt><code>_POSIX_OPTION_ORDER</code><dd><a name="index-g_t_0040code_007b_005fPOSIX_005fOPTION_005fORDER_007d-environment-variable_002e-3125"></a>
If this environment variable is defined, it suppresses the usual
reordering of command line arguments by <code>getopt</code> and
<code>argp_parse</code>.  See <a href="Argument-Syntax.html#Argument-Syntax">Argument Syntax</a>.

     <!-- !!! GNU also has COREFILE, CORESERVER, EXECSERVERS -->
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