Dead End

During a restore process, nothing is more disappointing than discovering that some of the links in your previously backed up data no longer work. Although the link is still there, the target no longer exists, resulting in the link pointing nowhere. These broken data structures can also cause problems when you are developing software and publishing it in the form of an archive, or if you need to install different versions of an application.

Finding and fixing broken links manually takes a lot of effort. You can avoid this scenario by using scripts and Unix/Linux tools to check for broken links in directory structures. In this article, I'll look at several ways to check the consistency of these data structures and detect broken links. Read on to avoid hiccups for you and your users.

Sample Data

As an example, I will use the directory structure shown in Figure 1, which is similar to a piece of software or a project directory that you might encounter in the wild. You can easily create a tree such as Figure 1 with the tree command [1].

Figure 1: The example project directory structure.

The directory tree in Figure 1 contains two versions of the software. There are three links: One points to the old version (named old), one to the current version (named current), and the third to a data file named dataset3 (which is missing).

Options

A small, manageable project structure like Figure 1 can be tested and checked manually. With larger projects, however, this quickly leads to errors because you are bound to overlook something. To automate the testing procedure, I rummaged around in my Unix/Linux toolbox and came up with four options that are suitable for everyday use: a shell script as a combination of a recursive function and a for loop over all the files and directories, a special call to find, a Python script, and the tools symlinks, FSlint, rmlint, and chase.

Shell Script

The shell script (Listing 1) uses a recursive function named check(). check() only expects one parameter: the directory you want it to check for broken links (line 16). In the function, a for loop iterates across all entries (lines 2 to 14).

01 function check {
02   for entry in $1/*; do
03     # echo "check $entry ... "
04     if [ -d "$entry" ]; then
05       check $entry
06     else
07       if [ -L "$entry" ]; then
08         target=$(readlink "$entry")
09         if [ ! -e "$target" ]; then
10           echo "broken link: from $entry to $target"
11         fi
12       fi
13     fi
14   done
15 }
16 check $1

For each entry, check() first checks whether the entry actually is a directory (line 4). If so, the function is called again with this directory as a parameter (line 5). Otherwise, two more tests are made: Is it a link (line 7), and, if so, where does it point to (lines 8 and 9)? In line 8, the readlink command returns the target to which the link points and stores the result in the local variable target.

In line 9, the script checks if the link target exists. If not, the function sends an error message to that effect to stdout (line 10). The routine ignores entries in the directory that are not links. Once the entire list has been processed, check() returns to the call point. After processing the entire original directory, the script exits.

If you now call the script, you will see output similar to that in Listing 2. I made the call using a period (.) for the current directory as the starting point. The output includes two lines because current points to version2 and my function follows the link.

$ ./find-broken-links.sh .
broken link: from ./project/current/data/dataset3 to project/version1/data/dataset3
broken link: from ./project/version2/data/dataset3 to project/version1/data/dataset3