> ## Documentation Index > Fetch the complete documentation index at: https://notes.kodekloud.com/llms.txt > Use this file to discover all available pages before exploring further. # File descriptors > This guide explains file descriptors in Unix and Linux, including their definition, usage, and standard streams. On the command line, the numbers 1 and 2 refer to standard output and standard error—these are examples of *file descriptors*. In this guide, you will learn: 1. Additional file descriptor numbers beyond 0, 1, and 2 2. The precise definition of a file descriptor 3. Why the term “file” descriptor applies to streams like pipes and sockets A file descriptor is a nonnegative integer that indexes an open I/O resource in the operating system, such as a disk file, network socket, or pipe. Although the name originates from early Unix file-handling, modern OSes treat sockets, pipes, and devices the same way. *** ## Reserved Standard Streams By convention on Linux and most Unix-like systems, three file descriptors are preallocated for the main I/O streams: | Descriptor Number | Stream | Description | | ----------------- | ------ | --------------- | | 0 | stdin | Standard input | | 1 | stdout | Standard output | | 2 | stderr | Standard error | Any descriptor ≥ 3 can reference additional resources: open files, sockets, pipes, or terminal devices. ![The image shows a diagram labeled "File Descriptors" with numbered circles from 0 to 4, followed by ellipses, indicating a sequence.](https://kodekloud.com/kk-media/image/upload/v1752868631/notes-assets/images/Advanced-Bash-Scripting-File-descriptors/file-descriptors-diagram-sequence.jpg) File descriptors are assigned per process. When a program opens a new file, the OS returns the smallest unused descriptor number (starting at 3). *** ## Real-World Analogy: Librarian and Call Numbers Imagine a library where: * The **librarian** is your operating system * **Books** are files, sockets, or pipes * **Call numbers** on the books are file descriptors When you hand a call number (descriptor) to the librarian, they fetch the corresponding book (I/O resource). This models how your shell uses descriptors to route input/output. ![The image shows a simple illustration of a person at a desk with books, accompanied by labels "Call Number" and "Book." The title "File Descriptors" is at the top.](https://kodekloud.com/kk-media/image/upload/v1752868632/notes-assets/images/Advanced-Bash-Scripting-File-descriptors/file-descriptors-person-desk-illustration.jpg) *** ## Standard Streams Mapped to Descriptors Applying the library analogy to standard streams: * stdin (0): Asking the librarian which book you want * stdout (1): Receiving the book from the librarian * stderr (2): Getting an error message if the book is unavailable Just as a library holds many books, a running process can have dozens or even thousands of open descriptors (3, 4, 5, …). ![The image is a diagram illustrating file descriptors, showing "Stdin" as 0, "Stdout" as 1, and "Stderr" as 2, with additional descriptors labeled 3, 4, and so on.](https://kodekloud.com/kk-media/image/upload/v1752868634/notes-assets/images/Advanced-Bash-Scripting-File-descriptors/file-descriptors-diagram-stdin-stdout-stderr.jpg) *** ## Further Reading and References * [Advanced Bash-Scripting Guide: Redirection](https://tldp.org/LDP/abs/html/io-redirection.html) * [Unix File Descriptor Tutorial](https://www.geeksforgeeks.org/file-descriptors-unix-linux/) * [Bash Manual: Redirections](https://www.gnu.org/software/bash/manual/html_node/Redirections.html)