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How do Unix / Linux "hard links" work?A reader writes:
So, I'm partway through your book, and I want to check on hard links. When you say it creates another door to the same body of data, is that literally true? For example, if all I have in my ./Documents directory is 001.doc (size 512K), and I hard link as hardlink.doc then run ls, do I see A or B?
An interesting question! The main point of hardlinks is to allow a single data entity to be shared by multiple entries in the file system. If I saved this posting as "001.html" on my server, then created a hard link to the same file from "002.html", then I could subsequently delete "001.html" and "002.html" would be unaffected, with the same contents within. It's only when all links to a data entity are removed that the data gets hooked into the free list and is essentially recycled for later reuse.
In terms of your specific question, the system will view the two files as having separate content. This can be demonstrated thusly: $ du 249 . $ ls -l total 248 -rw-r--r-- 1 dtint vuser 247034 Aug 24 03:24 some.data Now let's create some hard links. Notice "ls" reports 248 blocks in this directory $ ln some.data link2 $ ln some.data link3 $ ln some.data link4 $ ls -l total 992 -rw-r--r-- 4 dtint vuser 247034 Aug 24 03:24 link2 -rw-r--r-- 4 dtint vuser 247034 Aug 24 03:24 link3 -rw-r--r-- 4 dtint vuser 247034 Aug 24 03:24 link4 -rw-r--r-- 4 dtint vuser 247034 Aug 24 03:24 some.data As expected, "ls" now reports 992 blocks in use, seeing each of the new hard linked files as new data. But... notice what "du" reports, since it actually looks deeper into the file system: $ du 249 .As you can see, some elements of the file system see hard links as new and separate data entities (e.g. "ls"), while others dig further into the file system to ascertain actual data blocks used and associated with the current location (e.g., "du"). This makes sense because otherwise if you had lots of hard links you could theoretically end up in a situation where you have greater than 100% disk space utilization, which would clearly be a bit confusing at best!
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Categorized:
Unix and Linux Help
(Article 3733,
Written by Dave Taylor)
Tagged: Previous: Extracting the correct column with "ps" and "awk" Next: How not to build traffic: respond to email solicitations of Link Exchanges Reader Comments To Date: 3anjan said, on May 22, 2009 5:28 AM:
the information was very use full. please post some on cron jobs. Thanks in advance Azeem said, on June 24, 2009 9:23 PM:
I just tested hardlinks on an NFS share, and it worked! .. i knew that symbolic links didn't work on NFS so i tried hard links and it worked that leads me to believe you can also hard link between different harddrives or can anyone explain why it wouldn't work?
I do have a comment, now that you mention it!Check This Out Too... |
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Another way to look at hard links - they share the same inode no matter what. Do this with ls -i and it will become obvious. The disadvantage of hard links is that you cannot hard link across 2 separate drives on your system ... say hda and hdb (when you can with soft or symlinks)
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