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| <h3 class="section">25.4 Environment Variables</h3> |
| |
| <p><a name="index-environment-variable-3098"></a>When a program is executed, it receives information about the context in |
| which it was invoked in two ways. The first mechanism uses the |
| <var>argv</var> and <var>argc</var> arguments to its <code>main</code> function, and is |
| discussed in <a href="Program-Arguments.html#Program-Arguments">Program Arguments</a>. The second mechanism uses |
| <dfn>environment variables</dfn> and is discussed in this section. |
| |
| <p>The <var>argv</var> mechanism is typically used to pass command-line |
| arguments specific to the particular program being invoked. The |
| environment, on the other hand, keeps track of information that is |
| shared by many programs, changes infrequently, and that is less |
| frequently used. |
| |
| <p>The environment variables discussed in this section are the same |
| environment variables that you set using assignments and the |
| <code>export</code> command in the shell. Programs executed from the shell |
| inherit all of the environment variables from the shell. |
| <!-- !!! xref to right part of bash manual when it exists --> |
| |
| <p><a name="index-environment-3099"></a>Standard environment variables are used for information about the user's |
| home directory, terminal type, current locale, and so on; you can define |
| additional variables for other purposes. The set of all environment |
| variables that have values is collectively known as the |
| <dfn>environment</dfn>. |
| |
| <p>Names of environment variables are case-sensitive and must not contain |
| the character ‘<samp><span class="samp">=</span></samp>’. System-defined environment variables are |
| invariably uppercase. |
| |
| <p>The values of environment variables can be anything that can be |
| represented as a string. A value must not contain an embedded null |
| character, since this is assumed to terminate the string. |
| |
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| <li><a accesskey="1" href="Environment-Access.html#Environment-Access">Environment Access</a>: How to get and set the values of |
| environment variables. |
| <li><a accesskey="2" href="Standard-Environment.html#Standard-Environment">Standard Environment</a>: These environment variables have |
| standard interpretations. |
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