The du (for "disk usage") utility is a useful tool for estimating file size and cleaning up a hard drive. du summarizes the disk usage for each file in the directory and even looks down into subdirectories.

du worked well in the old days of printed output, but in today's interactive environments, it has some limitations. Like many old-school command-line tools, du prints the output on the screen by default, and the terminal screen is nowhere near large enough to see all the files in a large directory system. Of course, you could print to a file and examine the file with a text editor, or you could filter and refine the du results using standard command-line piping and sorting techniques. But sometimes what you really want to do is move around in the output as if perusing a printed page, progressing interactively through subdirectories as you would with a GUI-based file manager.

The Ncdu utility [1] is a souped-up version of du that has some GUI-like features. Ncdu uses the ncurses API [2], which is a common tool for building GUI features into command-line programs. (See the box entitled "Why Work in the Terminal?")

Ncdu, which has been under development since 2007, is written in C and works on all POSIX-compliant systems. Packages are available for many Linux distributions, as well as for BSD, Solaris, Cygwin, and Mac OS.

Ncdu comes preinstalled on many Linux distributions, and if you can't find it on your system now, you can easily install it using your local package manager. The Ncdu utility is only a few kilobytes in size, and it is released under an MIT license. The easiest way to control Ncdu is to use the arrow keys, the Enter key, and a few letters.

Easy to Use

To start Ncdu, type ncdu. If you only want to examine hard disk usage, you can simply start the application as a user. If you want to delete data outside of your home directory, you will need root privileges. The ncdu command supports a number of command-line options (see the box entitled "Command Line Options.")

Command-Line Options

The ncdu command supports some common command-line options, such as -h (Figure 1) for help and -v for displaying the version number. If you start Ncdu in quiet mode with the -q option, the screen will update less frequently, which will save bandwidth if you are working through an SSH connection.

The -x option tells Ncdu not to analyze all directories in the path but only those with the same filesystem as the source directory. This means that if the entire system is scanned, other mounted filesystems are not included. -X lets you exclude files and directories from the scan.

Figure 1: Use the -h option to display an incomplete summary of command-line options. View the manpage by entering man ncdu to see a complete list.

Ncdu sorts directories and files in descending order by default (Figure 2). The letter S reverses the sort order and shows the smallest folders and files at the start. Pressing G lets you toggle through the different modes for displaying the relative size. We found figures in percent format the most meaningful (Figure 3). All told, the application offers a handful of options and a good dozen keyboard shortcuts to manipulate the output. The most common operating options are listed in Table 1.

Figure 2: In the standard output, Ncdu shows the relative size of elements with hashtags.

Figure 3: The G key lets you toggle to a mode where the relative sizes are expressed in percent format.

The four arrow keys are all you actually need to navigate Ncdu. Use up and down arrow keys to move through the directory tree. The right arrow key takes you down into the selected directory, and the left arrow key back up again in the directory tree. Instead of the right arrow key, the Enter key also works in both directions. Alternatively, you can move up and down the directory tree with the K and J keys. Alternatively, you can open a directory with the L; the < key closes it again.

Customizing the Default Output

The default output format for Ncdu shows the size of the largest directory or file in the first row on the left. On the right is the file or directory name, where a forward slash marks a directory. In the middle column, the relative size of the element is shown with a graph of hashtags. Toggle the size display by pressing the G key. It shows additional information, or only percentages, or no information at all on the relative size.

If you want to scan the whole filesystem, enter ncdu /. To exclude other filesystems from the search, add -x.

Delete with Care

Once you have found the element you are looking for, and assuming you want to remove disk hogs that you no longer need, press the D key to delete the selected element. Be careful, because the process is irreversible; Ncdu uses the rm command in the background. To protect yourself from accidental deletion, start Ncdu with the -r option, which disables the delete feature (Figure 4).

Figure 4: To prevent accidental deletion, start Ncdu with the -r option for read only.