What is a File Descriptor?
A file descriptor is a unique identifier assigned by the operating system to each open file, socket, pipe, or other I/O resource within a process. It is represented as a non-negative integer and serves as a reference to the underlying file or resource. File descriptors are managed by the kernel and are used by processes to perform read, write, and other operations on files and I/O devices.
File Descriptor Table: Each process in Linux has its own file descriptor table, which is a data structure maintained by the kernel to keep track of open files and their associated file descriptors. The file descriptor table is typically implemented as an array, where each entry corresponds to a file descriptor. When a process opens a file or I/O resource, the kernel assigns a file descriptor and adds an entry to the process’s file descriptor table, pointing to the opened resource.
Global File Table: In addition to the file descriptor table maintained by each process, Linux also maintains a global file table, also known as the system file table. The global file table contains information about all open files in the system, including their current position, access mode, and other metadata. It serves as a centralized repository for managing open files across all processes.
Connection between File Descriptor Table and Global File Table: When a process opens a file, the kernel consults the global file table to find the corresponding file resource. It then assigns a file descriptor to the opened file and adds an entry to the process’s file descriptor table, pointing to the file resource in the global file table. This linkage allows processes to refer to files using file descriptors and perform I/O operations on them.
Every Linux process comes pre-equipped with three standard file descriptors:
- stdin (Standard Input): File descriptor 0, representing input from the keyboard.
- stdout (Standard Output): File descriptor 1, representing output to the terminal.
- stderr (Standard Error): File descriptor 2, representing error output to the terminal.
Example: Let’s illustrate the concept of file descriptors with a simple example in a Linux shell:
# Open a file using the 'cat' command
$ cat example.txtIn this example, the cat command opens the file example.txt for reading. When the cat command is executed, the operating system assigns a file descriptor (let’s say 3) to the opened file and adds an entry to the current shell’s file descriptor table, mapping file descriptor 3 to the file example.txt. Simultaneously, the operating system updates the global file table with information about the opened file, such as its current position and access mode.
What are Inodes?
Inode numbers that uniquely exist for all the files on Linux or Unix systems. An Inode number points to an Inode. An Inode, short for index node, is a data structure used by the Linux file system to store metadata about files and directories. Each file or directory in a Linux file system is represented by an Inode, which contains information such as file type, permissions, ownership, timestamps, and pointers to data blocks. Inodes serve as a mapping between file names and data blocks on disk and are essential for accessing and managing files efficiently.
Structure of an Inode: An Inode consists of several fields that store metadata about the associated file or directory. The typical fields found in an Inode include:
- File type: Specifies whether the Inode represents a regular file, directory, symbolic link, or special file.
- Permissions: Define the access permissions for the file or directory, including read, write, and execute permissions for the owner, group, and others.
- Owner and group: Identify the user and group that owns the file or directory.
- Timestamps: Record the time of last access, modification, and status change (creation) of the file or directory.
- File size: Indicates the size of the file’s data content in bytes.
- Pointers to data blocks: Store references to the data blocks on disk that contain the actual content of the file or directory.
When a user tries to access the file or any information related to the file the user 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.
Role of Inodes in File System Operations: Inodes play a critical role in various file system operations, including file creation, modification, deletion, and access. When a new file is created, the file system allocates a new Inode to represent the file and initializes its metadata fields. Subsequently, when the file is accessed or modified, the file system uses the corresponding Inode to locate and update the file’s metadata and data blocks on the disk.
Furthermore, Inodes enable efficient file system navigation and traversal by providing a mapping between file names and their associated metadata. When a user requests to access or manipulate a file by its name, the file system uses the Inode corresponding to the file’s name to locate and retrieve its metadata and data content.
Example: Let’s illustrate the concept of Inodes with a simple example in a Linux shell:
# Create a new file named 'example.txt'
$ touch example.txt
# Display detailed information about the file using the 'ls' command with the '-i' option
$ ls -i example.txt
123456 example.txt
# The first number (123456) represents the Inode number assigned to the file 'example.txt'In this example, the ‘ls -i’ command displays the Inode number (123456) assigned to the file ‘example.txt’, which uniquely identifies the file within the file system.
That’s all for now.
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