diff --git a/common/src/main/java/org/implab/gradle/common/dsl/ProcessSpec.java b/common/src/main/java/org/implab/gradle/common/dsl/ProcessSpec.java
--- a/common/src/main/java/org/implab/gradle/common/dsl/ProcessSpec.java
+++ b/common/src/main/java/org/implab/gradle/common/dsl/ProcessSpec.java
@@ -5,7 +5,7 @@ import java.util.ArrayList;
 import java.util.Collection;
 import java.util.List;
 
-import org.implab.gradle.common.exec.Executor;
+import org.implab.gradle.common.exec.ExecBuilder;
 import org.implab.gradle.common.exec.RedirectFrom;
 import org.implab.gradle.common.exec.RedirectTo;
 
@@ -63,7 +63,7 @@ public class ProcessSpec {
         return this;
     }
 
-    public void accept(Executor executor) {
+    public void accept(ExecBuilder executor) {
         command.forEach(executor::argument);
         executor.directory(directory);
         if (inputRedirect != null)
diff --git a/common/src/main/java/org/implab/gradle/common/exec/Executor.java b/common/src/main/java/org/implab/gradle/common/exec/ExecBuilder.java
rename from common/src/main/java/org/implab/gradle/common/exec/Executor.java
rename to common/src/main/java/org/implab/gradle/common/exec/ExecBuilder.java
--- a/common/src/main/java/org/implab/gradle/common/exec/Executor.java
+++ b/common/src/main/java/org/implab/gradle/common/exec/ExecBuilder.java
@@ -2,24 +2,60 @@ package org.implab.gradle.common.exec;
 
 import java.util.concurrent.CompletableFuture;
 
+import org.eclipse.jdt.annotation.NonNullByDefault;
+
 import java.io.File;
 import java.io.IOException;
 
-/** Interface for a command line execution. */
-public interface Executor {
+/** Command line builder */
+@NonNullByDefault
+public interface ExecBuilder {
+
+    /** Sets command line, clears previous one if any */
+    void command(String cmd, String... args);
+
+    /** Adds an argument to the command line */
     void argument(String arg);
 
+    /** Sets the working directory */
     void directory(File directory);
 
+    /**
+     * Sets the environment value. The value cannot be null.
+     * 
+     * @param envVar The name of the environment variable
+     * @param value The value to set, 
+     */
+    void setEnvironment(String envVar, String value);
+
+    void unsetEnvironment(String envVar);
+
+    /**
+     * Sets redirection for the stdin, {@link RedirectFrom} will be applied
+     * every time the process is started.
+     * 
+     * <p>If redirection 
+     */
     void stdin(RedirectFrom from);
 
+    /**
+     * Sets redirection for the stdout, {@link RedirectTo} will be applied
+     * every time the process is started.
+     */
     void stdout(RedirectTo out);
 
+    /**
+     * Sets redirection for the stderr, {@link RedirectTo} will be applied
+     * every time the process is started.
+     */
     void stderr(RedirectTo err);
-    
+
+    /**
+     * Creates and starts new process and returns {@link CompletableFuture}. The
+     * process may be a remote process, or a shell process or etc.
+     * @return
+     * @throws IOException
+     */
     CompletableFuture<Integer> start() throws IOException;
 
-    public static Executor system() {
-        return new SystemExec();
-    }
 }
diff --git a/common/src/main/java/org/implab/gradle/common/exec/ExecShell.java b/common/src/main/java/org/implab/gradle/common/exec/ExecShell.java
new file mode 100644
--- /dev/null
+++ b/common/src/main/java/org/implab/gradle/common/exec/ExecShell.java
@@ -0,0 +1,16 @@
+package org.implab.gradle.common.exec;
+
+/** Shell used to start processes */
+public interface ExecShell {
+
+    /** Creates new process builder {@link ExecBuilder} */
+    ExecBuilder builder();
+
+    /**
+     * Creates default process execution "shell" which will execute processes
+     * directly.
+     */
+    static ExecShell system() {
+        return () -> new SystemExecBuilder();
+    }
+}
diff --git a/common/src/main/java/org/implab/gradle/common/exec/RedirectFrom.java b/common/src/main/java/org/implab/gradle/common/exec/RedirectFrom.java
--- a/common/src/main/java/org/implab/gradle/common/exec/RedirectFrom.java
+++ b/common/src/main/java/org/implab/gradle/common/exec/RedirectFrom.java
@@ -6,6 +6,10 @@ import java.io.InputStream;
 import java.io.OutputStream;
 import java.util.concurrent.CompletableFuture;
 
+/** Describes how to redirect input streams. This interface is used to configure
+ * lazy redirection. {@link #redirect(OutputStream)} is called when the process
+ * is started.
+ */
 public interface RedirectFrom {
     CompletableFuture<Void> redirect(OutputStream to);
 
diff --git a/common/src/main/java/org/implab/gradle/common/exec/SystemExec.java b/common/src/main/java/org/implab/gradle/common/exec/SystemExecBuilder.java
rename from common/src/main/java/org/implab/gradle/common/exec/SystemExec.java
rename to common/src/main/java/org/implab/gradle/common/exec/SystemExecBuilder.java
--- a/common/src/main/java/org/implab/gradle/common/exec/SystemExec.java
+++ b/common/src/main/java/org/implab/gradle/common/exec/SystemExecBuilder.java
@@ -6,7 +6,7 @@ import java.lang.ProcessBuilder.Redirect
 import java.util.ArrayList;
 import java.util.concurrent.CompletableFuture;
 
-class SystemExec implements Executor {
+class SystemExecBuilder implements ExecBuilder {
 
     private final ProcessBuilder builder = new ProcessBuilder();
 
@@ -48,22 +48,22 @@ class SystemExec implements Executor {
                 .thenApply(t -> proc.exitValue());
     }
 
-    public void command(String... args) {
-        builder.command(args);
-    }
-
+    @Override
     public void directory(File workingDir) {
         builder.directory(workingDir);
     }
 
+    @Override
     public void stderr(RedirectTo to) {
         errorRedirect = to;
     }
 
+    @Override
     public void stdin(RedirectFrom from) {
         inputRedirect = from;
     }
 
+    @Override
     public void stdout(RedirectTo to) {
         outputRedirect = to;
     }
@@ -73,4 +73,26 @@ class SystemExec implements Executor {
         builder.command().add(arg);
     }
 
+    @Override
+    public void setEnvironment(String envVar, String value) {
+        builder.environment().put(envVar, value);
+        
+    }
+
+    @Override
+    public void unsetEnvironment(String envVar) {
+        builder.environment().remove(envVar);
+    }
+
+    @Override
+    public void command(String cmd, String... args) {
+        var commandLine = new ArrayList<String>();
+
+        commandLine.add(cmd);
+        for (var arg : args) 
+            commandLine.add(arg);
+
+        builder.command(commandLine);
+    }
+
 }
diff --git a/common/src/main/java/org/implab/gradle/common/tasks/ExternalTask.java b/common/src/main/java/org/implab/gradle/common/tasks/ExternalTask.java
--- a/common/src/main/java/org/implab/gradle/common/tasks/ExternalTask.java
+++ b/common/src/main/java/org/implab/gradle/common/tasks/ExternalTask.java
@@ -6,7 +6,7 @@ import java.util.concurrent.ExecutionExc
 
 import org.gradle.api.DefaultTask;
 import org.implab.gradle.common.dsl.ProcessSpec;
-import org.implab.gradle.common.exec.Executor;
+import org.implab.gradle.common.exec.ExecBuilder;
 import org.implab.gradle.common.exec.RedirectFrom;
 import org.implab.gradle.common.exec.RedirectTo;
 
@@ -100,5 +100,5 @@ public abstract class ExternalTask exten
         return executor.start().get() == code;
     }
 
-    protected abstract Executor exec();
+    protected abstract ExecBuilder exec();
 }