diff --git a/src/main/js/Commands.ts b/src/main/js/Commands.ts
index e448094..6f483b2 100644
--- a/src/main/js/Commands.ts
+++ b/src/main/js/Commands.ts
@@ -267,6 +267,7 @@ export class Commands {
 
                 return node.contents;
             })
+            .filter(it => it !== "")
             .join("\n");
     }
 
@@ -367,8 +368,8 @@ export class Commands {
                     return [path, `'${path}' is not a directory.`];
 
                 const dirList = [
-                    new Directory({}).nameString("./", path),
-                    new Directory({}).nameString("../", path.parent)
+                    new Directory().nameString("./", path),
+                    new Directory().nameString("../", path.parent)
                 ];
                 const fileList: string[] = [];
 
@@ -417,6 +418,7 @@ export class Commands {
                     return error.message;
                 }
             })
+            .filter(it => it !== "")
             .join("\n");
     }
 
@@ -479,11 +481,13 @@ export class Commands {
                         input.hasAnyOption(["f", "force"]),
                         input.hasAnyOption(["r", "R", "recursive"]),
                         input.hasOption("no-preserve-root")
-                    )
+                    );
+                    return "";
                 } catch (error) {
                     return error.message;
                 }
             })
+            .filter(it => it !== "")
             .join("\n");
     }
 
@@ -498,6 +502,7 @@ export class Commands {
                     return error.message;
                 }
             })
+            .filter(it => it !== "")
             .join("\n");
     }
 
@@ -524,6 +529,7 @@ export class Commands {
                     return error.message;
                 }
             })
+            .filter(it => it !== "")
             .join("\n");
     }
 
diff --git a/src/main/js/FileSystem.ts b/src/main/js/FileSystem.ts
index 8172b9c..45fe27c 100644
--- a/src/main/js/FileSystem.ts
+++ b/src/main/js/FileSystem.ts
@@ -1,4 +1,4 @@
-import {emptyFunction, getFileExtension, IllegalStateError} from "./Shared";
+import {getFileExtension, IllegalArgumentError, IllegalStateError} from "./Shared";
 
 
 /**
@@ -93,7 +93,7 @@ export class FileSystem {
         if (this.has(destinationPath))
             targetPath = destinationPath.getChild(sourcePath.fileName);
         else
-            targetPath = destinationPath.parent.getChild(destinationPath.fileName);
+            targetPath = destinationPath;
 
         if (!this.has(targetPath.parent))
             throw new Error(`The directory '${targetPath.parent}' does not exist.`);
@@ -102,7 +102,7 @@ export class FileSystem {
         if (this.has(targetPath))
             throw new Error(`The directory or file '${targetPath}' already exists.`);
 
-        this.add(targetPath, source, false);
+        this.add(targetPath, source.copy(), false);
     }
 
     /**
@@ -116,9 +116,7 @@ export class FileSystem {
             return this.root;
 
         const parent = this.get(target.parent);
-        if (!(parent instanceof Directory))
-            return undefined;
-        if (!parent.hasNode(target.fileName))
+        if (!(parent instanceof Directory) || !parent.hasNode(target.fileName))
             return undefined;
 
         return parent.getNode(target.fileName);
@@ -131,7 +129,14 @@ export class FileSystem {
      * @return `true` if and only if there exists a node at the given path
      */
     has(target: Path): boolean {
-        return target.toString() === "/" || this.has(target.parent);
+        if (target.toString() === "/")
+            return true;
+
+        const parent = this.get(target.parent);
+        if (parent === undefined || !(parent instanceof Directory))
+            return false;
+
+        return parent.hasNode(target.fileName);
     }
 
     /**
@@ -172,12 +177,8 @@ export class FileSystem {
         }
 
         const parent = this.get(targetPath.parent);
-        if (!(parent instanceof Directory)) {
-            if (force)
-                return;
-            else
-                throw new IllegalStateError(`'${targetPath.parent}' is not a directory, but its child exists.`);
-        }
+        if (!(parent instanceof Directory))
+            throw new IllegalStateError(`'${targetPath.parent}' is not a directory, but its child exists.`);
 
         if (target instanceof Directory) {
             if (targetPath.toString() === "/" && !noPreserveRoot)
@@ -335,15 +336,6 @@ export abstract class Node {
      */
     abstract nameString(name: string, path: Path): string;
 
-    /**
-     * Recursively visits all nodes contained within this node.
-     *
-     * @param fun the function to apply to each node, including this node
-     * @param pre the function to apply to the current node before applying the first `fun`
-     * @param post the function to apply to the current node after applying the last `fun`
-     */
-    abstract visit(fun: (node: Node) => void, pre: (node: Node) => void, post: (node: Node) => void): void;
-
 
     /**
      * Returns the JSON serialization of this node.
@@ -401,7 +393,7 @@ export class Directory extends Node {
     constructor(nodes: { [name: string]: Node } = {}) {
         super();
 
-        this._nodes = Object.assign({}, nodes);
+        this._nodes = nodes;
     }
 
 
@@ -459,6 +451,9 @@ export class Directory extends Node {
      * @param node the node to add to this directory
      */
     addNode(name: string, node: Node): void {
+        if (new Path(`/${name}`).toString() === "/" || name.indexOf("/") >= 0)
+            throw new IllegalArgumentError(`Cannot add node with name '${name}'.`);
+
         this._nodes[name] = node;
     }
 
@@ -497,17 +492,6 @@ export class Directory extends Node {
         return `<a href="#" class="dirLink" onclick="execute('cd ${path.toString(true)}');execute('ls')">${name}</a>`;
     }
 
-    visit(fun: (node: Node) => void,
-          pre: (node: Node) => void = emptyFunction,
-          post: (node: Node) => void = emptyFunction) {
-        pre(this);
-
-        fun(this);
-        Object.keys(this._nodes).forEach(name => this._nodes[name].visit(fun, pre, post));
-
-        post(this);
-    }
-
 
     /**
      * Parses the given object into a directory.
@@ -568,14 +552,6 @@ export class File extends Node {
         }
     }
 
-    visit(fun: (node: Node) => void,
-          pre: (node: Node) => void = emptyFunction,
-          post: (node: Node) => void = emptyFunction) {
-        pre(this);
-        fun(this);
-        post(this);
-    }
-
 
     /**
      * Parses the given object into a file.
diff --git a/src/main/js/Shared.ts b/src/main/js/Shared.ts
index 51e5b84..53e82f1 100644
--- a/src/main/js/Shared.ts
+++ b/src/main/js/Shared.ts
@@ -114,6 +114,22 @@ export function stripHtmlTags(string: string): string {
 }
 
 
+/**
+ * Indicates that an argument is given to a function that should not have been given.
+ *
+ * The user should not be able to reach this state because user input should have been sanitized.
+ */
+export class IllegalArgumentError extends Error {
+    /**
+     * Constructs a new illegal argument error.
+     *
+     * @param message a message explaining why the error was thrown
+     */
+    constructor(message: string) {
+        super(message);
+    }
+}
+
 /**
  * Indicates that the program has ended up in a state that it should never end up in.
  *
diff --git a/src/test/Directory.spec.ts b/src/test/Directory.spec.ts
new file mode 100644
index 0000000..3f2ec8f
--- /dev/null
+++ b/src/test/Directory.spec.ts
@@ -0,0 +1,143 @@
+import "mocha";
+import {expect} from "chai";
+
+import "../main/js/Extensions"
+import {Directory, File} from "../main/js/FileSystem";
+
+
+describe("directory", () => {
+    let directory: Directory;
+
+
+    beforeEach(() => {
+        directory = new Directory();
+    });
+
+
+    describe("constructor", () => {
+        it("has no nodes by default", () => {
+            expect(directory.nodeCount).to.equal(0);
+        });
+
+        it("has the given nodes", () => {
+            expect(new Directory({node: new File()}).hasNode("node")).to.be.true;
+        });
+    });
+
+    describe("getNode", () => {
+        it("throws an exception if he node does not exist", () => {
+            expect(() => directory.getNode("error")).to.throw;
+        });
+
+        it("does not contain itself", () => {
+            expect(() => directory.getNode(".")).to.throw;
+        });
+
+        it("returns the desired node", () => {
+            const file = new File();
+            directory.addNode("file", file);
+
+            expect(directory.getNode("file")).to.equal(file);
+        });
+    });
+
+    describe("hasNode", () => {
+        it("returns false if the node does not exist", () => {
+            expect(directory.hasNode("error")).to.be.false;
+        });
+
+        it("returns true if the node exists", () => {
+            directory.addNode("file", new File());
+
+            expect(directory.hasNode("file")).to.be.true;
+        });
+
+        it("returns true if the node is the reflexive node", () => {
+            expect(directory.hasNode(".")).to.be.true;
+        });
+
+        it("returns true if the node is the parent node", () => {
+            expect(directory.hasNode("..")).to.be.true;
+        });
+
+        it("returns true if the node refers to itself", () => {
+            expect(directory.hasNode("")).to.be.true;
+        });
+    });
+
+    describe("addNode", () => {
+        it("adds the given node", () => {
+            directory.addNode("file", new File());
+
+            expect(directory.hasNode("file")).to.be.true;
+        });
+
+        it("overwrites an existing node", () => {
+            directory.addNode("file", new File());
+            directory.addNode("file", new Directory());
+
+            expect(directory.getNode("file")).to.be.instanceOf(Directory);
+        });
+
+        it("refuses to add a node at the reflexive path", () => {
+            expect(() => directory.addNode(".", new File())).to.throw;
+        });
+
+        it("refuses to add a node at the parent path", () => {
+            expect(() => directory.addNode("..", new File())).to.throw;
+        });
+
+        it("refuses to add a node that refers to the directory", () => {
+            expect(() => directory.addNode("", new File())).to.throw;
+        });
+
+        it("refuses to add a node with a name containing a slash", () => {
+            expect(() => directory.addNode("a/b", new File())).to.throw;
+        });
+    });
+
+    describe("removeNode", () => {
+        it("removes the desired node", () => {
+            directory.addNode("file", new File());
+
+            directory.removeNode("file");
+
+            expect(directory.nodeCount).to.equal(0);
+        });
+
+        it("empties the directory if the name is empty", () => {
+            directory.addNode("file", new File());
+
+            directory.removeNode("");
+
+            expect(directory.nodeCount).to.equal(0);
+        });
+
+        it("empties the directory if the name is reflexive", () => {
+            directory.addNode("file", new File());
+
+            directory.removeNode(".");
+
+            expect(directory.nodeCount).to.equal(0);
+        });
+    });
+
+    describe("copy", () => {
+        it("returns a deep copy of the directory", () => {
+            directory = new Directory({
+                file: new File("contents"),
+                dir: new Directory()
+            });
+            const copy = directory.copy();
+
+            (<File>directory.getNode("file")).contents = "changed";
+            expect((<File>copy.getNode("file")).contents).to.equal("contents");
+
+            (<Directory>directory.getNode("dir")).addNode("file2", new File());
+            expect((<Directory>copy.getNode("dir")).nodeCount).to.equal(0);
+
+            directory.removeNode("file");
+            expect(copy.nodeCount).to.equal(2);
+        });
+    });
+});
diff --git a/src/test/FileSystem.spec.ts b/src/test/FileSystem.spec.ts
new file mode 100644
index 0000000..1d2d4ca
--- /dev/null
+++ b/src/test/FileSystem.spec.ts
@@ -0,0 +1,299 @@
+import "mocha";
+import {expect} from "chai";
+
+import "../main/js/Extensions"
+import {Directory, File, FileSystem, Path} from "../main/js/FileSystem";
+
+
+describe("input args", () => {
+    let fileSystem: FileSystem;
+
+
+    beforeEach(() => {
+        fileSystem = new FileSystem();
+    });
+
+
+    describe("constructor", () => {
+        it("uses the default root if no directory is given", () => {
+            expect(fileSystem.root).to.not.be.empty;
+        });
+
+        it("uses the given directory as root", () => {
+            const root = new Directory();
+            expect(new FileSystem(root).root).to.equal(root);
+        });
+    });
+
+    describe("add", () => {
+        it("adds the given node", () => {
+            fileSystem.add(new Path("/file"), new File(), false);
+
+            expect(fileSystem.has(new Path("/file"))).to.be.true;
+        });
+
+        it("adds intermediate directories if the option is given", () => {
+            fileSystem.add(new Path("/dir1"), new Directory(), false);
+            fileSystem.add(new Path("/dir1/file1"), new File(), false);
+
+            fileSystem.add(new Path("/dir1/dir2/file2"), new File(), true);
+
+            expect(fileSystem.get(new Path("/dir1/file1"))).to.be.instanceOf(File);
+            expect(fileSystem.get(new Path("/dir1/dir2"))).to.be.instanceOf(Directory);
+            expect(fileSystem.get(new Path("/dir1/dir2/file2"))).to.be.instanceOf(File);
+        });
+
+        it("fails if a node is added as root", () => {
+            expect(() => fileSystem.add(new Path("/"), new File(), false)).to.throw;
+        });
+
+        it("fails if an intermediate directory does not exist", () => {
+            expect(() => fileSystem.add(new Path("/dir1/dir2/file2"), new File(), false)).to.throw;
+        });
+
+        it("fails if an intermediate directory is not a directory", () => {
+            fileSystem.add(new Path("/file1"), new File(), false);
+
+            expect(() => fileSystem.add(new Path("/file1/file2"), new File(), false)).to.throw;
+        });
+
+        it("fails if a node already exists at the path", () => {
+            fileSystem.add(new Path("/file1"), new File(), false);
+
+            expect(() => fileSystem.add(new Path("/file1"), new File(), false)).to.throw;
+        });
+    });
+
+    describe("copy", () => {
+        it("throws an error if the source does not exist", () => {
+            expect(() => fileSystem.copy(new Path("/src"), new Path("/dst"), false)).to.throw;
+        });
+
+        it("throws an error if the source is a directory and the recursive options it not given", () => {
+            fileSystem.add(new Path("/src"), new Directory(), false);
+
+            expect(() => fileSystem.copy(new Path("/src"), new Path("/dst"), false)).to.throw;
+        });
+
+        describe("target is destination", () => {
+            it("throws an error if the target's parent does not exist", () => {
+                fileSystem.add(new Path("/src"), new File(), false);
+
+                expect(() => fileSystem.copy(new Path("/src"), new Path("/parent/dst"), false)).to.throw;
+            });
+
+            it("throws an error if the target's parent is not a directory", () => {
+                fileSystem.add(new Path("/src"), new File(), false);
+                fileSystem.add(new Path("/parent"), new Directory(), false);
+
+                expect(() => fileSystem.copy(new Path("/src"), new Path("/parent/dst"), false)).to.throw;
+            });
+
+            it("throws an error if the target already exists", () => {
+                fileSystem.add(new Path("/src"), new File(), false);
+                fileSystem.add(new Path("/parent"), new Directory(), false);
+                fileSystem.add(new Path("/parent/dst"), new File(), false);
+
+                expect(() => fileSystem.copy(new Path("/src"), new Path("/parent/dst"), false)).to.throw;
+            });
+
+            it("copies the source file to the target", () => {
+                fileSystem.add(new Path("/src"), new File(), false);
+
+                fileSystem.copy(new Path("/src"), new Path("/dst"), false);
+
+                expect(fileSystem.has(new Path("/dst"))).to.be.true;
+            });
+
+            it("copies the source directory to the target", () => {
+                fileSystem.add(new Path("/src"), new Directory(), false);
+                fileSystem.add(new Path("/src/file1"), new File(), false);
+                fileSystem.add(new Path("/src/file2"), new File(), false);
+
+                fileSystem.copy(new Path("/src"), new Path("/dst"), true);
+
+                expect(fileSystem.has(new Path("/dst"))).to.be.true;
+                expect(fileSystem.has(new Path("/dst/file1"))).to.be.true;
+                expect(fileSystem.has(new Path("/dst/file2"))).to.be.true;
+            });
+
+            it("makes a deep copy", () => {
+                const file = new File("old");
+                fileSystem.add(new Path("/src"), file, false);
+
+                fileSystem.copy(new Path("/src"), new Path("/dst"), false);
+                file.contents = "new";
+
+                expect((<File>fileSystem.get(new Path("/dst"))).contents).to.equal("old");
+            });
+        });
+
+        describe("target is child of destination", () => {
+            it("throws an error if the target is not a directory", () => {
+                fileSystem.add(new Path("/src"), new File(), false);
+                fileSystem.add(new Path("/dst"), new File(), false);
+
+                expect(() => fileSystem.copy(new Path("/src"), new Path("/dst"), false)).to.throw;
+            });
+
+            it("throws an error if the target's child already exists", () => {
+                fileSystem.add(new Path("/src"), new File(), false);
+                fileSystem.add(new Path("/dst"), new Directory(), false);
+                fileSystem.add(new Path("/dst/src"), new File(), false);
+
+                expect(() => fileSystem.copy(new Path("/src"), new Path("/dst"), false)).to.throw;
+            });
+
+            it("copies the source file to the target", () => {
+                fileSystem.add(new Path("/src"), new File(), false);
+                fileSystem.add(new Path("/dst"), new Directory(), false);
+
+                fileSystem.copy(new Path("/src"), new Path("/dst"), false);
+
+                expect(fileSystem.has(new Path("/dst/src"))).to.be.true;
+            });
+
+            it("copies the source directory to the target", () => {
+                fileSystem.add(new Path("/src"), new Directory(), false);
+                fileSystem.add(new Path("/src/file1"), new File(), false);
+                fileSystem.add(new Path("/src/file2"), new File(), false);
+                fileSystem.add(new Path("/dst"), new Directory(), false);
+
+                fileSystem.copy(new Path("/src"), new Path("/dst"), true);
+
+                expect(fileSystem.has(new Path("/dst/src"))).to.be.true;
+                expect(fileSystem.has(new Path("/dst/src/file1"))).to.be.true;
+                expect(fileSystem.has(new Path("/dst/src/file2"))).to.be.true;
+            });
+
+            it("makes a deep copy", () => {
+                const file = new File("old");
+                fileSystem.add(new Path("/src"), file, false);
+                fileSystem.add(new Path("/dst"), new Directory(), false);
+
+                fileSystem.copy(new Path("/src"), new Path("/dst"), false);
+                file.contents = "new";
+
+                expect((<File>fileSystem.get(new Path("/dst/src"))).contents).to.equal("old");
+            });
+        });
+    });
+
+    describe("get", () => {
+        it("returns the root node for the root path", () => {
+            expect(fileSystem.get(new Path("/"))).to.equal(fileSystem.root);
+        });
+
+        it("returns undefined if the parent is not a directory", () => {
+            fileSystem.add(new Path("/file1"), new File(), false);
+
+            expect(fileSystem.get(new Path("/file1/file2"))).to.be.undefined;
+        });
+
+        it("returns undefined if the parent does not contain the node", () => {
+            fileSystem.add(new Path("/dir"), new Directory(), false);
+
+            expect(fileSystem.get(new Path("/dir/file"))).to.be.undefined;
+        });
+
+        it("returns the node at the given path", () => {
+            const file = new File();
+            fileSystem.add(new Path("/dir"), new Directory(), false);
+            fileSystem.add(new Path("/dir/file"), file, false);
+
+            expect(fileSystem.get(new Path("/dir/file"))).to.equal(file);
+        });
+    });
+
+    describe("has", () => {
+        it("returns true for the root path", () => {
+            expect(fileSystem.has(new Path("/"))).to.be.true;
+        });
+
+        it("returns false if the node does not exist", () => {
+            expect(fileSystem.has(new Path("/error"))).to.be.false;
+        });
+
+        it("returns true if the node is at the root", () => {
+            fileSystem.add(new Path("/file"), new File(), false);
+
+            expect(fileSystem.has(new Path("/file"))).to.be.true;
+        });
+
+        it("returns false if an intermediate directory does not exist", () => {
+            fileSystem.add(new Path("/file"), new File(), false);
+
+            expect(fileSystem.has(new Path("/dir/file"))).to.be.false;
+        });
+    });
+
+    describe("move", () => {
+        it("moves a file", () => {
+            fileSystem.add(new Path("/src"), new File("old"), false);
+
+            fileSystem.move(new Path("/src"), new Path("/dst"));
+
+            expect(fileSystem.has(new Path("/src"))).to.be.false;
+            expect(fileSystem.has(new Path("/dst"))).to.be.true;
+        });
+
+        it("moves a directory", () => {
+            fileSystem.add(new Path("/src"), new Directory(), false);
+            fileSystem.add(new Path("/src/file1"), new File(), false);
+            fileSystem.add(new Path("/src/file2"), new File(), false);
+
+            fileSystem.move(new Path("/src"), new Path("/dst"));
+
+            expect(fileSystem.has(new Path("/src"))).to.be.false;
+            expect(fileSystem.has(new Path("/src/file1"))).to.be.false;
+            expect(fileSystem.has(new Path("/src/file2"))).to.be.false;
+            expect(fileSystem.has(new Path("/dst"))).to.be.true;
+            expect(fileSystem.has(new Path("/dst/file1"))).to.be.true;
+            expect(fileSystem.has(new Path("/dst/file2"))).to.be.true;
+        });
+    });
+
+    describe("remove", () => {
+        it("throws an error if the node does not exist", () => {
+            expect(() => fileSystem.remove(new Path("/error"), false, false, false)).to.throw;
+        });
+
+        it("does nothing if the node does not exist and the force option is given", () => {
+            expect(() => fileSystem.remove(new Path("/error"), true, false, false)).not.to.throw;
+        });
+
+        it("throws an error if a directory is removed without the recursive option", () => {
+            fileSystem.add(new Path("/dir"), new Directory(), false);
+
+            expect(() => fileSystem.remove(new Path("/dir"), false, false, false)).to.throw;
+        });
+
+        it("throws an error if the root is remove without the recursive or no-preserve-root option", () => {
+            expect(() => fileSystem.remove(new Path("/"), false, false, false)).to.throw;
+            expect(() => fileSystem.remove(new Path("/"), false, true, false)).to.throw;
+            expect(() => fileSystem.remove(new Path("/"), false, false, true)).to.throw;
+        });
+
+        it("removes a file", () => {
+            fileSystem.add(new Path("/file"), new File(), false);
+            fileSystem.remove(new Path("/file"), false, false, false);
+
+            expect(fileSystem.has(new Path("/file"))).to.be.false;
+        });
+
+        it("removes a directory", () => {
+            fileSystem.add(new Path("/dir"), new Directory(), false);
+            fileSystem.remove(new Path("/dir"), false, true, false);
+
+            expect(fileSystem.has(new Path("/dir"))).to.be.false;
+        });
+
+        it("removes the root", () => {
+            fileSystem.add(new Path("/dir"), new Directory(), false);
+            fileSystem.add(new Path("/file"), new File(), false);
+            fileSystem.remove(new Path("/"), false, true, true);
+
+            expect(fileSystem.has(new Path("/dir"))).to.be.false;
+        });
+    });
+});
diff --git a/src/test/InputParser.spec.ts b/src/test/InputParser.spec.ts
index f0fe659..5e5097f 100644
--- a/src/test/InputParser.spec.ts
+++ b/src/test/InputParser.spec.ts
@@ -203,7 +203,6 @@ describe("input args", () => {
         });
 
         it("should ignore the redirect target if inside quotation marks", () => {
-            console.log(parser.parse("command '> file'").args);
             expect(parser.parse("command '> file'").redirectTarget).to.have.members(["default"]);
         });
 
diff --git a/src/test/Path.spec.ts b/src/test/Path.spec.ts
index 8a9b70d..03a4199 100644
--- a/src/test/Path.spec.ts
+++ b/src/test/Path.spec.ts
@@ -50,6 +50,20 @@ describe("paths", () => {
                 expect(new Path("/", "///", "/file").toString()).to.equal("/file");
             });
         });
+
+        describe("interpret", () => {
+            it("uses only the cwd if no paths are given", () => {
+                expect(Path.interpret("/cwd").toString()).to.equal("/cwd");
+            });
+
+            it("ignores the cwd if the first path is absolute", () => {
+                expect(Path.interpret("/cwd", "/dir1", "dir2").toString()).to.equal("/dir1/dir2");
+            });
+
+            it("uses all parts if the first path is relative", () => {
+                expect(Path.interpret("/cwd", "dir1", "/dir2").toString()).to.equal("/cwd/dir1/dir2");
+            });
+        });
     });
 
     describe("fileName", () => {