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package org.apache.juneau.svl.vars;

import org.apache.juneau.svl.*;

/**
 * Environment variable variable resolver.
 *
 * <p>
 * The format for this var is <js>"$E{envVar[,defaultValue]}"</js>.
 *
 * <h5 class='section'>Example:</h5>
 * <p class='bjava'>
 * 	<jc>// Create a variable resolver that resolves environment variables (e.g. "$E{PATH}")</jc>
 * 	VarResolver <jv>varResolver</jv> = VarResolver.<jsm>create</jsm>().vars(EnvVariablesVar.<jk>class</jk>).build();
 *
 * 	<jc>// Use it!</jc>
 * 	System.<jsf>out</jsf>.println(<jv>varResolver</jv>.resolve(<js>"Environment variable PATH is set to $E{PATH}"</js>));
 * </p>
 *
 * <p>
 * Since this is a {@link SimpleVar}, any variables contained in the result will be recursively resolved.
 * <br>Likewise, if the arguments contain any variables, those will be resolved before they are passed to this var.
 *
 * <ul class='seealso'>
 * 	<li class='link'>{@doc jm.SimpleVariableLanguage}
 * 	<li class='extlink'>{@source}
 * </ul>
 */
public class EnvVariablesVar extends DefaultingVar {

	/** The name of this variable. */
	public static final String NAME = "E";

	/**
	 * Constructor.
	 */
	public EnvVariablesVar() {
		super(NAME);
	}

	@Override /* Var */
	public String resolve(VarResolverSession session, String varVal) {
		// Note that lookup is case-insensitive on windows.
		return System.getenv(varVal);
	}
}