Why cd .., typed at root folder, does not warn or fails with an error?
I would expect:
/$ cd ..
-bash: cd: ..: No such file or directory
Instead, I'm left at /. Of course, this is since .. does exist in /, and is simply /, just like .. I just wonder why it is like that.
According to the Open Group (responsible for the POSIX standard):
Each directory has exactly one parent directory which is represented by the name dot-dot in the first directory. [...] What the filename dot-dot refers to relative to the root directory is implementation-defined. In Version 7 it refers to the root directory itself; this is the behavior mentioned in POSIX.1-2008. In some networked systems the construction /../hostname/ is used to refer to the root directory of another host, and POSIX.1 permits this behavior.
The dot-dot entry in the root directory is interpreted to mean the root directory itself. Thus, dot-dot cannot be used to access files outside the subtree rooted at the root directory.
You don't get an error because even the / directory actually has a valid directory entry for .., but unlike with other directories it points to the directory itself and thus behaves identical to .:
$ ls -lid / /. /..
128 drwxr-xr-x 22 root root 4096 Apr 15 11:26 /
128 drwxr-xr-x 22 root root 4096 Apr 15 11:26 /.
128 drwxr-xr-x 22 root root 4096 Apr 15 11:26 /..
$
As the first column tells you, ., .., and / all have the same inode-number and thus are the same filesystem entries.
So even if you cd .. inside of / you just stay in /.
.. directory entry is there.
/ just like any other folder, including a valid entry for ... Otherwise you would have to treat / differently compared to all other folders in the system.
It's there because removing it would require creating special-case handling code in the kernel and the C libraries. Right now you can assume that there will always be a . and .. in any directory you go to.
The only special-case code required right now is in filesystem mounting code, where the code overrides the inode value of .. to point to the directory containing the mount point, since root directories aren't always root directories.
mount command and its associated special file /etc/fstab.
Another reason '..' is in / is that if it wasn't there it would create another special case: root directory would have one hard link fewer than all the other directories (all directory nodes have n+2 links, where n is the number of direct subdirectories inside). This would break various programs that rely on in for optimization of directory scans.
Another way to check what . and .. in / really is:
$ readlink -f ..
/home
$ readlink -f /.
/
$ readlink -f /..
/
As you can see, . and .. is pointed to /.
It's nice that it's there, because otherwise I wouldn't be able to spam ../../../../ to get to the root from a folder. If it didn't allow the .. at the root then I'd be stuck counting the number of folders down the working dir is.
../ is better. I also do a lot of file operations in PHP, where I can't do / for several reasons.
../s and operate on (or remove) a file you didn't intend to without error.
../ more times than needed and wishing it to work as intended looks like a (dormant) logic error to me, even if the filesystem apparently allows it.
.., you could just do while [ -d .. ]; do cd ..; done