INode contains the metadata about the files in the *nix systems.
Also, the name of the file is not stored in Inodes (We will come to it later). When a file is created inside a directory then the file-name and Inode number are assigned to file i.e. directory maps file name to inode number.
The user might think that the directory contains the complete file and all the extra information related to it but this might not be the case always. So we see that a directory associates a file name with its Inode number.
When a user tries to access the file or any information related to the file then he/she uses the file name to do so but internally the file-name is first mapped with its Inode number stored in a table. Then through that Inode number the corresponding Inode is accessed. There is a table (Inode table) where this mapping of Inode numbers with the respective Inodes is provided.
For example :
Note that Hard links cannot be created on different file systems and also they cannot be created for directories.
Now, the above concept brings up another interesting fact. A file system can run out of space in two ways :
The above use-case is possible but less encountered because on a typical system the average file size is more than 2KB which makes it more prone to running out of hard disk space first. But, nevertheless there exists an algorithm which is used to create number of Inodes in a file system. This algorithm takes into consideration the size of the file system and average file size. The user can tweak the number of Inodes while creating the file system.
1) Check if the file exists:
2) Try to remove it normally:
3) Remove the file using Inode number:
As we discussed earlier in our find command examples article, you can search for a file using inode number and delete it.
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Inode Basics
An Inode number points to an Inode. An Inode is a data structure that stores the following information about a file :- Size of file
- Device ID
- User ID of the file
- Group ID of the file
- The file mode information and access privileges for owner, group and others
- File protection flags
- The timestamps for file creation, modification etc
- link counter to determine the number of hard links
- Pointers to the blocks storing file’s contents
Also, the name of the file is not stored in Inodes (We will come to it later). When a file is created inside a directory then the file-name and Inode number are assigned to file i.e. directory maps file name to inode number.
The user might think that the directory contains the complete file and all the extra information related to it but this might not be the case always. So we see that a directory associates a file name with its Inode number.
When a user tries to access the file or any information related to the file then he/she uses the file name to do so but internally the file-name is first mapped with its Inode number stored in a table. Then through that Inode number the corresponding Inode is accessed. There is a table (Inode table) where this mapping of Inode numbers with the respective Inodes is provided.
Why no file-name in Inode information?
As pointed out earlier, there is no entry for file name in the Inode, rather the file name is kept as a separate entry parallel to Inode number. The reason for separating out file name from the other information related to same file is for maintaining hard-links to files. This means that once all the other information is separated out from the file name then we can have various file names which point to same Inode.$ touch a $ ln a a1 $ ls -al drwxr-xr-x 48 himanshu himanshu 4096 2012-01-14 16:30 . drwxr-xr-x 3 root root 4096 2011-03-12 06:24 .. -rw-r--r-- 2 himanshu family 0 2012-01-14 16:29 a -rw-r--r-- 2 himanshu family 0 2012-01-14 16:29 a1In the above output, we created a file ‘a’ and then created a hard link a1. Now when the command ‘ls -al’ is run, we can see the details of both ‘a’ and ‘a1′. We see that both the files are indistinguishable. Look at the second entry in the output. This entry specifies number of hard links to the file. In this case the entry has value ’2′ for both the files.
Note that Hard links cannot be created on different file systems and also they cannot be created for directories.
When are Inodes created?
As we all now know that Inode is a data structure that contains information of a file. Since data structures occupy storage then an obvious question arises about when the Inodes are created in a system? Well, space for Inodes is allocated when the operating system or a new file system is installed and when it does its initial structuring. So this way we can see that in a file system, maximum number of Inodes and hence maximum number of files are set.Now, the above concept brings up another interesting fact. A file system can run out of space in two ways :
- No space for adding new data is left
- All the Inodes are consumed.
The above use-case is possible but less encountered because on a typical system the average file size is more than 2KB which makes it more prone to running out of hard disk space first. But, nevertheless there exists an algorithm which is used to create number of Inodes in a file system. This algorithm takes into consideration the size of the file system and average file size. The user can tweak the number of Inodes while creating the file system.
Commands to access Inode numbers
Following are some commands to access the Inode numbers for files :1) Ls -i Command
As we explained earlier in our Unix LS Command: 15 Practical Examples article, the flag -i is used to print the Inode number for each file.$ ls -i 1448240 a 1441807 Desktop 1447344 mydata 1441813 Pictures 1442737 testfile 1448145 worm 1448240 a1 1441811 Documents 1442707 my_ls 1442445 practice 1442739 test.py 1447139 alpha 1441808 Downloads 1447278 my_ls_alpha.c 1441810 Public 1447099 Unsaved Document 1 1447478 article_function_pointer.txt 1575132 google 1447274 my_ls.c 1441809 Templates 1441814 Videos 1442390 chmodOctal.txt 1441812 Music 1442363 output.log 1448800 testdisk.log 1575133 vlcSee that the Inode number for ‘a’ and ‘a1′ are same as we created ‘a1′ as hard link.
2) Df -i Command
df -i command displays the inode information of the file system.$ df -i Filesystem Inodes IUsed IFree IUse% Mounted on /dev/sda1 1875968 293264 1582704 16% / none 210613 764 209849 1% /dev none 213415 9 213406 1% /dev/shm none 213415 63 213352 1% /var/run none 213415 1 213414 1% /var/lock /dev/sda2 7643136 156663 7486473 3% /homeThe flag -i is used for displaying Inode information.
3) Stat Command
Stat command is used to display file statistics that also displays inode number of a file$ stat a File: `a' Size: 0 Blocks: 0 IO Block: 4096 regular empty file Device: 805h/2053d Inode: 1448240 Links: 2 Access: (0644/-rw-r--r--) Uid: ( 1000/himanshu) Gid: ( 1001/ family) Access: 2012-01-14 16:30:04.871719357 +0530 Modify: 2012-01-14 16:29:50.918267873 +0530 Change: 2012-01-14 16:30:03.858251514 +0530
Example Usage Scenario of an Inode number
- Suppose there exist a file name with some special character in it. For example: ”ab*
- Try to remove it normally using rm command, you will not be able to remove it.
- However using the inode number of this file you can remove it.
1) Check if the file exists:
$ ls -i 1448240 a 1447274 my_ls.c 1448240 a1 1442363 output.log 1448239 "ab* 1441813 Pictures 1447139 alphaSo we have a file with name “ab* in this directory
2) Try to remove it normally:
$ rm "ab* > ^C $ rm "ab* > ^C $See that I tried couple of times to remove the file but could not.
3) Remove the file using Inode number:
As we discussed earlier in our find command examples article, you can search for a file using inode number and delete it.
$ find . -inum 1448239 -exec rm -i {} \; rm: remove regular empty file `./"ab*'? y
$ ls -i 1448240 a 1447274 my_ls.c 1448240 a1 1442363 output.log 1447139 alpha 1441813 PicturesSo we used the find command specifying the Inode number of the file we need to delete. The file got deleted. Though we could have deleted the file otherwise also by using the command rm \”ab* instead of using the complicated find command example above but still I used it to demonstrate one of the use of Inode numbers for users.
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