We often think of a folder tree as a set of independent files and folders, where deleting files and folders from that tree has no effect on the contents of the files and folders that remain.
However, to save space, some file systems support symbolic links, which create dependencies between otherwise independent files and folders.
A symbolic link is a file whose only contents is a name or path of another file or folder, called the target for the symbolic link. By default, when asked to open a symbolic link, most file processing tools will recognize the file as a symbolic link, read the target, and open the target file or folder. One advantage of a symbolic link is that the operating system does not have to store two versions of the same file.
However, there is a risk when using symbolic links. The operating system does not change a symbolic link if the target of that link has been renamed, moved, deleted, or never existed in the first place. A symbolic link whose target does not exist is called a broken link.
The target of a symbolic link can be one of the following:
- The absolute path of a file or folder
For example, the target can be “/usr/include/stdio.h” or “/usr/include”.
- The name of a file or folder, relative to the folder containing the symbolic link:
For example, the target can be “stdio.h” or “fldr”, and if the symbolic link is in the folder whose path is, say, “/usr/include”, then target would be “/usr/include/stdio.h” or “/usr/include/fldr”.
- A relative path to a file or folder, relative to the folder containing the symbolic link:
For example, the target can be fldr/stdio.h or fldr/subfldr, and if the symbolic link is in the folder whose path is, say, “/usr/include”, then target would be “/usr/include/fldr/stdio.h” or “/usr/include/fldr/subfldr”.
For a further example, the target can be “../fldr/stdio.h” or “../fldr/subfldr”, and if the symbolic link is in the folder whose path is, say, “/usr/include”, then target would be “/usr/include/../fldr/stdio.h” or “/usr/include/../fldr/subfldr”, which would be the same as “/usr/fldr/stdio.h” and “/usr/fldr/subfldr”, respectively.
Because of the fragile dependencies that exist when symbolic links are used, it is possible for the engineer who assembles a production to unknowingly break one or more symbolic links. This will happen when a symbolic link is included in the production, its target exists on the file system from which the production was copied, but the target is not included in the production and the target does not exist on the review computer to which the production has been copied.
Further, what can be worse is when the target is not included in the production, but the review computer happens to have a file or folder that matches the production, but that file or folder was on the review computer before the production was copied to the review computer, and thus did not come from the production.
Here are some example scenarios:
Scenario A: Say the production contains a symbolic link whose target is a relative path deeper in the folder tree: “include/file.h”. If the engineer who assembled the production pruned the production by removing the folder named “include”, then any attempt to access that symbolic link will fail.
Scenario B: Say the production contains a symbolic link whose target is a relative path that contains one or more references to a parent folder: “../../include/file.h”. If the engineer who assembled the production did not include the grandparent folder in the production, then:
- If the review computer does not have a file whose path is “../../include/stdio.h”, relative to the folder containing the symbolic link, then any attempt to access that symbol link will fail.
- If the review computer does have a file whose path is “../../include/stdio.h”, relative to the folder containing the symbolic link, then any attempt to access that symbolic link will access the file whose path is “../../include/stdio.h”, relative to the folder containing the symbolic link, a file that is probably not part of the production!
Scenario C: Say the production contains a symbolic link whose target is the absolute path: “/usr/include/stdio.h” and the engineer who assembled the production did not include “/usr”, “/usr/include”, or “/usr/include.stdio.h” in the production, then:
- If the review computer does not have a file whose path is “/usr/include/stdio.h”, then any attempt to access that symbol link will fail.
- If the review computer does have a file whose path is “/usr/include/stdio.h”, then any attempt to access that symbolic link will access the file whose path is “/usr/include/stdio.h”, a file that was not part of the production!
Therefore, I created the following script which does the following checks. Given the path of the folder that is the root of the production, for each symbolic link in the production:
- If the target of the symbolic link is an absolute path and the root of the production is not the root of the file system (i.e., “/”), then flag that symbolic link as broken; else determine whether that target exists in the production.
- If the target of the symbolic link is a file name or folder name and that file name or folder name does not exist in the same folder as the symbolic link, then flag that symbolic link as broken.
- If the target of the symbolic link is a relative path deeper in the folder tree and that relative path does not exist relative to folder containing the symbolic link, then flag that symbolic link as broken.
- If the target of the symbolic link is a relative path that contains one or more references to a parent folder and those one or more references to a parent folder reference a folder outside of the production, then flag that symbolic link as broken; else determine whether that target exists in the production.
#!/usr/bin/env python3
"""
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"""
import sys
import os
import shutil
import enum
class Action(enum.Enum):
# Does not use 0 or 1 to avoid matching with False or True if the caller mistakenly passes a Boolean
PRINT = 2
REMOVE = 3
def broken_symbolic_links_tree(root_folder_path: str, action=Action.PRINT):
"""
Recursively traverses the specified folder(s) and takes action on symbolic links which either refer to targets that
do not refer to a file or folder under their specified root folder path or are broken symbolic links
:param root_folder_path: Path of a folder
:param action: Prints (default) or removes broken symbolic links
:return: None
"""
for folder, __, files in os.walk(root_folder_path):
for file in files:
source_path = os.path.join(folder, file)
if os.path.islink(source_path):
target = os.readlink(source_path)
if os.path.isabs(target):
target_path = target
else:
target_path = os.path.join(os.path.dirname(source_path), target)
if not target_path_under_root_path(target_path, root_folder_path) or not os.path.exists(target_path):
if action == Action.REMOVE:
os.remove(source_path)
else:
print(source_path, '->', target)
def target_path_under_root_path(target_path, root_path):
target_path_normalized = os.path.normpath(target_path)
root_path_normalized = os.path.normpath(root_path)
if target_path_normalized.startswith(root_path_normalized):
return True
return False
def _broken_symbolic_links_main():
"""
Recursively traverses the specified folder(s) and takes action on symbolic links which either refer to targets that
do not refer to a file or folder under their specified root folder path or are broken symbolic links
:usage: broken_symbolic_links [-r] folder_path ...
-r Remove files which are broken symbolic links
default Print files which are broken symbolic links
"""
start_of_path_args = 1
action = Action.PRINT
if len(sys.argv) >= 3:
if sys.argv[1] == '-r':
start_of_path_args = 2
action = Action.REMOVE
for root_folder_path in sys.argv[start_of_path_args:]:
broken_symbolic_links_tree(root_folder_path, action)
if __name__ == '__main__':
_broken_symbolic_links_main()