Chapter 21 - Hardlinks
In chapter 21. LINUX FILESYSTEMS AND THE VFS
I have a small problem with understanding the part with the hard links. (the bold marked part)
On the page for "Hard and Soft Links" it´s explained as:
Hard links point to an inode. They are made by using ln without an option. Two or more files can point to the same inode (hard link). All hard linked files have to be on the same filesystem. Changing the content of a hard linked file in one place may not change it in other places.
i create a file on my desktop and hardlink it to a file in my home folder.
when i change the file in one place the other file (hardlink) is also changed and vice-versa
lars@Lars:~/Desktop$ echo Test > Test lars@Lars:~/Desktop$ cat Test Test lars@Lars:~$ ln Desktop/Test linktest lars@Lars:~$ cat linktest Test lars@Lars:~$ echo test123 >> linktest lars@Lars:~$ cat Desktop/Test Test test123 lars@Lars:~$ echo test123 >> Desktop/Test lars@Lars:~$ cat linktest Test test123 test123 lars@Lars:~$ cat Desktop/Test Test test123 test123
Comments
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I think one of the cases they may be talking about is copying files. If you have the following files and links for example:
ln link1 file1 #hard link ln -s link2 file2 #soft link
If you try to copy a file onto link1, file1 does not change. If you copy a file onto link2, file2 does change.
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Thank you for your reply. i checked it again.
I don´t get it the way as its explained in the chapter. regardless of what im doing, content is changing.
Where is the mistake???Example #1
Copy a file on the sourcefile, is changing source and hardlink content.
lars@Lars:~/testme$ echo source > source lars@Lars:~/testme$ cat source source lars@Lars:~/testme$ ln source hardlink lars@Lars:~/testme$ cat source hardlink source source lars@Lars:~/testme$ cp test source lars@Lars:~/testme$ cat source hardlink test test
Example #2
Copy a file on the hardlink is changing source and hardlink content, toolars@Lars:~/testme$ echo source > source lars@Lars:~/testme$ ln source hardlink lars@Lars:~/testme$ cat source hardlink source source lars@Lars:~/testme$ cp test hardlink lars@Lars:~/testme$ cat source hardlink test test
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This is correct behaviour, this is often misunderstood. most programs should modify both instances when something is hardlinked. However, there are cases where an update is done by a different procedure; the file is copied, the update is on the copy, and the copy is then overwritten to the original location, and the link is broken. It will not happen if you just edit a file in place. For example, there is a script called
ready-for.sh
you may have run in preparation for this course (you can see it at https://training.linuxfoundation.org/cm/prep/ready-for.sh). If you run./ready-for.sh --update
, it does exactly what I am saying -- any hard links are broken.One upon a time many programs thoughtlessly broke the links when doing updates, but people tend to be more careful now and do it only by design -- we hope
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Yes, i know the ready-for.sh script, used it for the ubuntu vm i use for the exercises.
If its correct behaviour than i think i have understand the topic. At the beginning i had a problem with how its explained in the chapter.
thank you
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