Recently I came across the os library in Python and found out about the existence of symbolic links. I would like to know what a symbolic link is, why it exists, and what are various uses of it?
What is the purpose of creating a symbolic link between files?
Very briefly, if you're familiar with Windows, symbolic links in Linux act like shortcuts in Windows –
Folkestone
There is non such thing as a 'symbolic link between files'. It is a link between names. There is only one file. 'In Unix, names have files': D.M. Ritchie. –
Valise
I will answer this from a perspective of an *nix user (specifically Linux). If you're interested in how this relates to Windows I suggest you look for tutorials like this one. This will be a bit of a roundabout, but I find it that symbolic links or symlinks are best explained together with hard links and generic properties of a filesystem on Linux.
Links and files on Linux
As a rule of thumb, in Linux everything is treated as a file. Directories are files that contain mappings from names (paths) to inodes, which are just unique identifiers of different objects residing on your system. Basically, if I give you a name like /home/gst/mydog.png the accessing process will first look into the / directory (the root directory) where it will find information on where to find home, then opening that file it will look into it to see where gst is and finally in that file it will try to find the location of mydog.png, and if successful try do whatever it set out to do with it. Going back to directory files, the mappings they contain are called links. Which brings us to hard and symbolic links.
Hard vs Symbolic links
A hard link is just a mapping like the one we discussed previously. It points directly to a certain object. A symlink on the other hand does not point directly to an object. Rather it just saves a path to an object. For example, say that I created a symbolic link to /home/gst/mydog.png at /home/gst/Desktop/mycat.png with os.symlink("/home/gst/mydog.png", "/home/gst/Desktop/mycat.png"). When I try to open it, the name /home/gst/Desktop/mycat.png is usually resolved to /home/gst/mydog.png. By following the symlink located at /home/gst/Desktop/mycat.png I actually (try to) access an object pointed to by /home/gst/mydog.png.
If I create a hard link (for example by calling os.link) I just add entries to the relevant directory files, such that the specific name can be followed to the linked object. When I create a symbolic link I create a file that contains a path to another file (which might be another symbolic link).
More specific to your question, if I pass /home/gst/Desktop/mycat.png to os.readlink it will return /home/gst/mydog.png. This name resolution also happens when calling functions in os with an (optional) parameter follow_symlinks set to True, however, if it's set to False the name does not get resolved (for instance you'd set it to false when you want to manipulate the symlink itself not the object it points to). From the module documentation:
not following symlinks: If follow_symlinks is
False, and the last element of the path to operate on is a symbolic link, the function will operate on the symbolic link itself instead of the file the link points to. (For POSIX systems, Python will call thel...version of the function.)You can check whether or not follow_symlinks is supported on your platform using os.supports_follow_symlinks. If it is unavailable, using it will raise a NotImplementedError.
Why use hard links?
This question has already been answered here, quoting from the accepted answer:
The main advantage of hard links is that, compared to soft links, there is no size or speed penalty. Soft links are an extra layer of indirection on top of normal file access; the kernel has to dereference the link when you open the file, and this takes a small amount of time. The link also takes a small amount of space on the disk, to hold the text of the link. These penalties do not exist with hard links because they are built into the very structure of the filesystem.
I'd like to add that hard links allow for an easy method of file backup. For every file the system keeps a count of its hard links. Once this count reaches 0 the memory segment on which the file is located is marked as free, meaning that the system will eventually overwrite it with another data (effectively deleting the previous file - which doesn't happen for at least as long as a running process has an opened stream associated with the file, but that's another story). Why would that matter?
Let's say you have a huge directory full of files you'd like to manipulate somehow (rename some, delete others, etc.) and you write a script to do this for you. However, you're not completely sure that the script will work as intended and you fear it might delete some wrong files. You also don't want to copy all the files, as this would take up too much space and time. One solution is to just create a hard link for each file at some other point in the filesystem. If you delete a file in the target directory, the associated object is still available because there's another hard link associated with it. Creating that many hard links will consume much less time and space than copying all the file, yet it will give you a reasonable backup strategy.
This is not the case with symbolic links. Remember, symlinks point to other links (possibly another symlink as well) not to actual files. Hence, I might create a symlink to a file, but that it will only save the link. If the (eventual) hard link that the symlink is pointing to gets removed from the system, trying to resolve the symlink won't lead you to a file. Such symlinks are said to be "broken" or "dangling". Thus you cannot rely on symlinks to preserve access to a certain file. (Conversely, deleting a symlink does not affect the link count associated with a target file.) So what's their use?
Why use symbolic links?
You can operate on symlinks as if they were the actual files to which they pointing somewhere down the line (except deleting them). This allows you to have multiple "access points" to a file, without having excess copies (that remain up to date, since they always access the same file). If you want to replace the file that is being accessed you only need to change it once and all of the symlinks will point to it (as long as the path saved by them is not changed). However, if you have hard links to a certain file and you then replace that file with another one, you also need to replace the hard links as otherwise they'll still be pointing to the old file.
Lastly, it is not uncommon to have different filesystems mounted on the same Linux machine. That is to say, that the way data is organized and interpreted at some point in the file hierarchy (say /home/gst/fs1) can be different to how it is organized and interpreted at another point (say /home/gst/Desktop/fs2). A hard link can only reside on the same filesystem as the file it's pointing to. Whereas, a symlink can be created on one filesystem but effectively pointing to a file on another filesystem (see answers to this question).
Excellent explanation. Thanks for the wonderful explanation. –
Mischiefmaker
Symbolic links, also known as soft links, are special types of files that point to other files, much like shortcuts in Windows and Macintosh aliases. The data in the target file does not appear in a symbolic link, unlike a hard link. Instead, it points to another file system entry.
Read more here: https://kb.iu.edu/d/abbe#:~:text=A%20symbolic%20link%2C%20also%20termed,somewhere%20in%20the%20file%20system.
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