Anyone who has ever tried to organize complicated topics in a meaningful way understands that the information rarely maps to a simple structure. Ideas overlap, and concepts that fall in one area might easily reappear somewhere else. Outline editors help you unravel this kind of complex information. The outline tools populating the Linux landscape come in several different forms. We decided to compare a couple of popular options: CherryTree and Piggydb. These tools illustrate different approaches to the time-honored task of organizing human thought: CherryTree uses tree structures, while Piggydb relies on mesh graphs.

CherryTree

CherryTree [1] identifies each node in the topic tree (Figure 1, left panel) with a cherry symbol and assigns to it a text file that supports simple formatting, images, and tables. External files can also be embedded.

Figure 1: CherryTree assigns a note to each symbol in the "cherry tree" on the left.

The software packages the whole thing into a SQLite database, with the option of encryption. Alternatively, CherryTree can export to XML, which is only useful for speed if you need access outside of CherryTree.

The program features practical keyboard shortcuts: The Alt+Left Arrow and Alt+Right Arrow shortcuts let you navigate the tree, while the normal arrow keys move the cursor through the text in the usual way. Ctrl+N inserts a node at the same level as the currently selected node; Shift+Ctrl+N inserts a child node.

Shift+Up Arrow and Shift+Down Arrow let you move the node in the tree; Shift+Right Arrow and Shift+Left Arrow change its hierarchy level. Shift+Ctrl+Right Arrow opens a dialog in which you can use the arrow keys to select a new higher-level node for the active element. CherryTree also sorts nodes alphabetically on request. To prevent accidental changes, you can assign write protection to important nodes.

The notes assigned to the nodes support simple formatting such as bold (Ctrl+B), italic (Ctrl+I), underline (Ctrl+U), and strikethrough (Ctrl+E). The Format menu lists all possible formats and their matching keyboard shortcuts. The dark blue text background can be easily changed to Light background, dark text under Settings | Rich Text | Theme.

In addition to embedded images and tables, CherryTree supports enumerations and bullet lists, as well as Task lists with checkboxes. Links can point to web pages, external files, directories, or other nodes in the CherryTree document. If you insert an Anchor in the text, it can be referenced by a link like in HTML code and thus point to a specific place in a CherryTree note.

In addition to the tree structure's fundamental ordering principle, CherryTree also provides a search function that searches either the text of all or certain nodes, as well as the node names. The search can handle regular expressions and, if desired, will also filter by node change or creation date.

CherryTree lets you import numerous file formats. In addition to text and HTML, they also include formats from other notebook programs such as Tomboy, RedNotebook, or TuxCards. The program exports the complete tree structure or parts of it as text, PDF, or HTML. In addition, partial branches can be exported as CherryTree files and inserted into another CherryTree document at any point in the outline.

Piggydb

Piggydb [2] as a Java program does not need any installation in the typical sense. You just need to unpack the piggydb-standalone-7.0.zip [3] zip archive for the desktop version. Then launch the program by typing:

java -jar piggydb-standalone.jar

In the standalone version, an icon appears in the taskbar; it can be used to exit the program. For the server variant, piggydb-all-in-one-7.0.zip, this does not exist. A script named run.sh is used to start the program here; more details can be found in the README.txt file and in Piggydb's documentation [4].

After starting Piggydb for the first time and logging in as owner (which also happens to be the default password), click on the green plus button in the titlebar (Figure 2, item 4) to open the Fragment Editor to create a new text fragment (Figure 3). For already existing fragments, you will also see a plus button below the text content (Figure 2, item 4), which creates index cards linked to this fragment.

Figure 2: 1. Piggydb shows linked tabs in a hierarchical view. 2. The middle column contains the search result list. 3. Keywording is also available. 4. Click on the green plus button to open the Fragment Editor. 5. Create links by dragging the corresponding symbol onto other index cards. 6. You can also create links by selecting multiple elements.

Piggydb has only one element type: the text fragment. You can add outline items such as previously composed text by filling in either the Title field or the larger main text field (Figure 3). Because the Piggydb author believes that using predefined outlines has a negative effect on creativity, he recommends that you first create short, unlinked text fragments in a brainstorming phase and then integrate them at a later stage.

Figure 3: In the Fragment Editor, you can format the text with wiki code. A click on the Preview button shows the formatted result (right).

Piggydb Editor

In the Piggydb editor, you can format the text with wiki code, like used by Wikipedia (Figure 3): '''bold''', ''italic'', and __strikethrough__ provide highlighting; -Element and +Element create enumerations and bullet lists. Piggydb also supports blocks in typewriter font, tables, and embedded external files. You can find out more by clicking on the blue question mark button in the editor.

Formatting switches let you skip the wiki formatting. If you mouse over the switches, you will see the corresponding keyboard shortcuts. You can edit existing fragments by mousing over the gray titlebar and clicking on the pencil icon in the pop-up that appears.

A click on the Preview button shows the results of the wiki formatting on the right. The tool shows your images as thumbnails and other embedded documents as icons that reflect the file type. A click on Register saves the document.