inode

When the file system is first established, it comes with a set number of inodes. The inode is a data structure used to store file information. The information that every inode will store consists of

• The file type
• The file’s permissions
• The file’s owner and group
• The file’s size
• The inode number
• A timestamp indicating when the inode was last modified, when the file was created,and when the file was last accessed
• A link count (the number of hard links that point at this file)
• The location of the file (i.e., the device storing the file) and pointers to the individual file blocks (if this is a regular file)

these pointers break down into

•  A set number of pointers that point directly to blocks
•  A set number of pointers that point to indirect blocks; each indirect block contains pointers that point directly to blocks
• A set number of pointers that point to doubly indirect blocks, which are blocks that have pointers that point to additional indirect blocks
• A set number of pointers that point to triply indirect blocks, which are blocks that have pointers that point to additional doubly indirect blocks

Typically, an inode will contain 15 pointers broken down as follows:

• 12 direct pointers
• 1 indirect pointer
• 1 double indirect pointer
• 1 triply indirect pointer

An inode is illustrated in Figure (which contains 1 indirect pointer and 1 doubly indirect pointer but no triply indirect pointer because of a lack of space).

 Let us take a look at how to access a Linux file through the inode. We will make a few assumptions.
First, our Linux inode will store 12 direct pointers, 1 indirect pointer, 1 doubly indirect pointer, and 1 triply indirect pointer. Blocks of pointers will store 12 pointers no matter whether they are indirect, doubly indirect, or triply indirect blocks. We will assume that our file consists of 500 blocks, numbered 0 to 499, each block storing 8 KB (the typical disk block stores between 1 KB and 8 KB depending on the file system utilized).
Our example file then stores 500 * 8 KB = 4000 KB or 4 MB. Here is the breakdown of how we access the various blocks.

• Blocks 0–11: direct pointers from the inode.
• Blocks 12–23: pointers from an indirect block pointed to by the inode’s indirect pointer.
• For the rest of the file, access is more complicated.

   • We follow the inode’s doubly indirect pointer to a doubly indirect block. This block contains 12 pointers to indirect blocks. Each indirect block contains 12 pointers to disk blocks.

−− The doubly indirect block’s first pointer points to an indirect block of 12 pointers, which point to blocks 24–35.
−− The doubly indirect block’s second pointer points to another indirect block of 12 pointers, which point to blocks 36–47.
−− …
−− The doubly indirect block’s last pointer points to an indirect block of 12 pointers, which point to blocks 156–167.

• We follow the inode’s triply indirect pointer to a triply indirect block. This block contains 12 pointers to doubly indirect blocks, each of which contains 12 pointers to indirect blocks, each of which contain 12 pointers to disk blocks. From the triply indirect block, we can reach blocks 168 through 499 (with room to increase the file to block 1895).

Earlier, we noted that the disk drive supports random access. The idea is that to track down a block, block i, we have a mechanism to locate it. This is done through the inode pointers as just described.
The above example is far from accurate. A disk block used to store an indirect, doubly indirect, or triply indirect block of pointers would be 8 KB in size. Such a sized block would store far more than 12 pointers. A pointer is usually 32 or 64 bits long (4 or 8 bytes). If we assume an 8 byte pointer and an 8 KB disk block, then an indirect, doubly indirect, or
triply indirect block would store 8 KB/8 B pointers = 1 K or 1024 pointers rather than 12.
When a file system is created, it comes with a set number of inodes. The actual number depends on the size of the file system and the size of a disk block. Typically, there is 1 inode for every 2–8 KB of file system space.
If we have a 1 TB file system (a common size for a hard disk today), we might have as many as 128 K (approximately 128 thousand) inodes. The remainder of the file system is made up of disk blocks dedicated to file storage and pointers. Unless nearly all of the files in the file system are very small, the number of inodes should be more than sufficient for any file system usage.

Linux Commands to Inspect inodes and Files

There are several tools available to inspect inodes. The following commands provide such information:

• stat—provides details on specific file usage, the option –c %i displays the file’s inode number
• ls –i displays the inodes of all entries in the directory
• df –i this command provides information on the utilization of the file system, partition by partition. The -i option includes details on the number of inodes used.

The stat command itself will respond with the name of the file, the size of the file, the blocks used to store the file, the device storing the file (specified as a device number), the inode of the file, the number of hard links to the file, the file’s permissions, UID, GID in both name and
number, and the last access, modification, and change date and time for the file. The stat command has many options. The most significant are: 

• -L : follow links to obtain inode information of files stored in other directories (without this, symbolic links in the given directory are ignored)
• -f : used to obtain statistics on an entire file system rather than a file
•  -c FORMAT : output only the requested information where FORMAT uses the characters listed in Table , additionally when used with -f (file system stats) there are other formatting characters available (see second half of Table )

Formatting Characters for -c, Bottom Half for -c -f

We use stat to provide for us the size of each file in blocks and bytes, the file name, the inode number of the file, and the time of last access. This command is given as:

$ stat   -c "%b %s %n %i %x" *
736 373337 firestarter-events.txt~ 1486 2010-04-28 14:37:10.843398916 +0300
472544 241938432 FreeBSD-10.2-RELEASE-amd64-bootonly.iso 285743 2015-09-10 20:01:12.310727882 +0300
119344 61098928 FreeBSD-10.2-RELEASE-amd64-bootonly.iso.xz 4349 2015-09-10 19:53:37.250723649 +0300

 we inspect devices from /dev  where we look at the file’s type (%F)  the device number, UID of the owner, number of hard links, inode
number, file name, and file type.
 

$ stat   -c "%d %u %h %i %n %F" /dev/*
5 0 1 4695 /dev/adsp character special file
5 0 1 4699 /dev/audio character special file
5 0 2 1901 /dev/block directory
5 0 2 2342 /dev/bsg directory
5 0 3 1801 /dev/bus directory
5 0 1 4001 /dev/cdrom symbolic link
5 0 2 2001 /dev/char directory
5 0 1 1592 /dev/console character special file
5 0 1 3249 /dev/core symbolic link
5 0 1 1834 /dev/cpu_dma_latency character special file
5 0 5 2377 /dev/disk directory
5 0 1 4698 /dev/dsp character special file
5 0 1 4004 /dev/dvd symbolic link
5 0 1 1585 /dev/ecryptfs character special file
5 0 1 2382 /dev/fb0 character special file
5 0 1 3250 /dev/fd symbolic link
5 0 1 1233 /dev/full character special file
5 0 1 1586 /dev/fuse character special file
5 0 1 2392 /dev/hidraw0 character special file
5 0 1 1662 /dev/hpet character special file
5 0 4 1811 /dev/input directory
5 0 1 1236 /dev/kmsg character special file
5 0 1 4784 /dev/log socket
5 0 1 1716 /dev/loop0 block special file
5 0 1 1719 /dev/loop1 block special file
5 0 1 1722 /dev/loop2 block special file
5 0 1 1725 /dev/loop3 block special file
5 0 1 1728 /dev/loop4 block special file
5 0 1 1731 /dev/loop5 block special file
5 0 1 1734 /dev/loop6 block special file
5 0 1 1737 /dev/loop7 block special file
5 0 2 1819 /dev/mapper directory
5 0 1 1575 /dev/mcelog character special file
5 0 1 1229 /dev/mem character special file
5 0 1 4702 /dev/mixer character special file
5 0 2 1744 /dev/net directory
5 0 1 1835 /dev/network_latency character special file
5 0 1 1836 /dev/network_throughput character special file
5 0 1 1230 /dev/null character special file
5 0 1 5256 /dev/nvidia0 character special file
5 0 1 5255 /dev/nvidiactl character special file
5 0 1 1237 /dev/oldmem character special file
5 0 2 1741 /dev/pktcdvd directory
5 0 1 1231 /dev/port character special file
5 0 1 1743 /dev/ppp character special file
5 0 1 1814 /dev/psaux character special file
5 0 1 1661 /dev/ptmx character special file
12 0 2 1 /dev/pts directory
5 0 1 1668 /dev/ram0 block special file
5 0 1 1671 /dev/ram1 block special file
5 0 1 1698 /dev/ram10 block special file
5 0 1 1701 /dev/ram11 block special file
5 0 1 1704 /dev/ram12 block special file
5 0 1 1707 /dev/ram13 block special file
5 0 1 1710 /dev/ram14 block special file
5 0 1 1713 /dev/ram15 block special file
5 0 1 1674 /dev/ram2 block special file
5 0 1 1677 /dev/ram3 block special file
5 0 1 1680 /dev/ram4 block special file
5 0 1 1683 /dev/ram5 block special file
5 0 1 1686 /dev/ram6 block special file
5 0 1 1689 /dev/ram7 block special file
5 0 1 1692 /dev/ram8 block special file
5 0 1 1695 /dev/ram9 block special file
5 0 1 1234 /dev/random character special file
5 0 1 397 /dev/rfkill character special file
5 0 1 4497 /dev/root symbolic link
5 0 1 2177 /dev/rtc symbolic link
5 0 1 1818 /dev/rtc0 character special file
5 0 1 2374 /dev/scd0 symbolic link
5 0 1 2344 /dev/sda block special file
5 0 1 2345 /dev/sda1 block special file
5 0 1 2346 /dev/sda2 block special file
5 0 1 2347 /dev/sda3 block special file
5 0 1 2348 /dev/sda4 block special file
5 0 1 2349 /dev/sda5 block special file
5 0 1 2350 /dev/sda6 block special file
5 0 1 2351 /dev/sda7 block special file
5 0 1 2352 /dev/sda8 block special file
5 0 1 2353 /dev/sda9 block special file
5 0 1 4371 /dev/sequencer character special file
5 0 1 4375 /dev/sequencer2 character special file
5 0 1 2341 /dev/sg0 character special file
5 0 1 2364 /dev/sg1 character special file
16 0 2 3261 /dev/shm directory
5 0 1 1576 /dev/snapshot character special file
5 0 3 4314 /dev/snd directory
5 0 1 3252 /dev/sndstat symbolic link
5 0 1 2361 /dev/sr0 block special file
5 0 1 3253 /dev/stderr symbolic link
5 0 1 3254 /dev/stdin symbolic link
5 0 1 3255 /dev/stdout symbolic link
5 0 1 1591 /dev/tty character special file
5 0 1 1593 /dev/tty0 character special file
5 0 1 1598 /dev/tty1 character special file
5 0 1 1607 /dev/tty10 character special file
5 0 1 1608 /dev/tty11 character special file
5 0 1 1609 /dev/tty12 character special file
5 0 1 1610 /dev/tty13 character special file
5 0 1 1611 /dev/tty14 character special file
5 0 1 1612 /dev/tty15 character special file
5 0 1 1613 /dev/tty16 character special file
5 0 1 1614 /dev/tty17 character special file
5 0 1 1615 /dev/tty18 character special file
5 0 1 1616 /dev/tty19 character special file
5 0 1 1599 /dev/tty2 character special file
5 0 1 1617 /dev/tty20 character special file
5 0 1 1618 /dev/tty21 character special file
5 0 1 1619 /dev/tty22 character special file
5 0 1 1620 /dev/tty23 character special file
5 0 1 1621 /dev/tty24 character special file
5 0 1 1622 /dev/tty25 character special file
5 0 1 1623 /dev/tty26 character special file
5 0 1 1624 /dev/tty27 character special file
5 0 1 1625 /dev/tty28 character special file
5 0 1 1626 /dev/tty29 character special file
5 0 1 1600 /dev/tty3 character special file
5 0 1 1627 /dev/tty30 character special file
5 0 1 1628 /dev/tty31 character special file
5 0 1 1629 /dev/tty32 character special file
5 0 1 1630 /dev/tty33 character special file
5 0 1 1631 /dev/tty34 character special file
5 0 1 1632 /dev/tty35 character special file
5 0 1 1633 /dev/tty36 character special file
5 0 1 1634 /dev/tty37 character special file
5 0 1 1635 /dev/tty38 character special file
5 0 1 1636 /dev/tty39 character special file
5 0 1 1601 /dev/tty4 character special file
5 0 1 1637 /dev/tty40 character special file
5 0 1 1638 /dev/tty41 character special file
5 0 1 1639 /dev/tty42 character special file
5 0 1 1640 /dev/tty43 character special file
5 0 1 1641 /dev/tty44 character special file
5 0 1 1642 /dev/tty45 character special file
5 0 1 1643 /dev/tty46 character special file
5 0 1 1644 /dev/tty47 character special file
5 0 1 1645 /dev/tty48 character special file
5 0 1 1646 /dev/tty49 character special file
5 0 1 1602 /dev/tty5 character special file
5 0 1 1647 /dev/tty50 character special file
5 0 1 1648 /dev/tty51 character special file
5 0 1 1649 /dev/tty52 character special file
5 0 1 1650 /dev/tty53 character special file
5 0 1 1651 /dev/tty54 character special file
5 0 1 1652 /dev/tty55 character special file
5 0 1 1653 /dev/tty56 character special file
5 0 1 1654 /dev/tty57 character special file
5 0 1 1655 /dev/tty58 character special file
5 0 1 1656 /dev/tty59 character special file
5 0 1 1603 /dev/tty6 character special file
5 0 1 1657 /dev/tty60 character special file
5 0 1 1658 /dev/tty61 character special file
5 0 1 1659 /dev/tty62 character special file
5 0 1 1660 /dev/tty63 character special file
5 0 1 1604 /dev/tty7 character special file
5 0 1 1605 /dev/tty8 character special file
5 0 1 1606 /dev/tty9 character special file
5 0 1 1667 /dev/ttyS0 character special file
5 0 1 1664 /dev/ttyS1 character special file
5 0 1 1665 /dev/ttyS2 character special file
5 0 1 1666 /dev/ttyS3 character special file
5 0 1 1235 /dev/urandom character special file
5 0 1 1749 /dev/usbmon0 character special file
5 0 1 1753 /dev/usbmon1 character special file
5 0 1 1808 /dev/usbmon2 character special file
5 0 1 7508 /dev/vboxdrv character special file
5 0 1 7512 /dev/vboxdrvu character special file
5 0 1 7561 /dev/vboxnetctl character special file
5 0 2 7754 /dev/vboxusb directory
5 0 1 1594 /dev/vcs character special file
5 0 1 1596 /dev/vcs1 character special file
5 0 1 2609 /dev/vcs2 character special file
5 0 1 2618 /dev/vcs3 character special file
5 0 1 2627 /dev/vcs4 character special file
5 0 1 2636 /dev/vcs5 character special file
5 0 1 2645 /dev/vcs6 character special file
5 0 1 2662 /dev/vcs7 character special file
5 0 1 1595 /dev/vcsa character special file
5 0 1 1597 /dev/vcsa1 character special file
5 0 1 2610 /dev/vcsa2 character special file
5 0 1 2619 /dev/vcsa3 character special file
5 0 1 2628 /dev/vcsa4 character special file
5 0 1 2637 /dev/vcsa5 character special file
5 0 1 2646 /dev/vcsa6 character special file
5 0 1 2663 /dev/vcsa7 character special file
5 0 1 387 /dev/vga_arbiter character special file
5 0 1 1232 /dev/zero character special file

Some considerations :
All except pts are located on device number 5.
All are owned by user 0 (root);
Most of the items have only one hard link, found in /dev. Both input and pts are directories and have more than one hard link. The inode numbers vary from 1 to 7754.
The file type demonstrates that “files” can make up a wide variety of entities from block or character files (devices) to symbolic links to directories to domain sockets. This last field varies in length from one word (directory) to three words (character special file, block special file).
Each new file is given the next inode available. As your file system is used, you will find newer files have higher inode numbers although deleted files return their inodes.

Whenever any file is used in Linux, it must first be opened. The opening of a file requires a special designator known as the file descriptor. The file descriptor is an integer assigned to the file while it is open. In Linux, three file descriptors are always made available:

• 0  stdin
• 1  stdout
• 2  stderr

 Any remaining files that are utilized during Linux command or program execution need to be opened and have a file descriptor given to that file.

When a file is to be opened, the operating system kernel gets involved.

1. First, it determines if the user has adequate access rights to the file.
2.  If so, it then generates a file descriptor. 
3. It then creates an entry in the system’s file table, a data structure that stores file pointers for every open file. The location of this pointer in the file table is equal to the file descriptor generated. For instance, if the file is given the descriptor 185, then the file’s pointer will be the 185th entry in the file table. 
4. The pointer itself will point to an inode for the given file.

As devices are treated as files, file descriptors will also exist for every device, entities such as the keyboard, terminal windows, the monitor, the network interface(s), the disk drives, as well as the open files.

You can view the file descriptors of a given process by looking at the fd subdirectory of the process’ entry in the /proc directory (e.g., /proc/16531/fd). There will always be entries labeled 0, 1, and 2 for STDIN, STDOUT, and STDERR, respectively. Other devices and files in use will require additional entries. Alternatively, the lsof command will list any open files.