standard input
also: stdin, file descriptor 0, fd 0
Standard input (stdin) is the default data stream from which a program reads input, typically from the keyboard or a piped source.
Standard input is one of three standard I/O streams in Unix-like systems. By default, it receives data from your keyboard when you run a command interactively. However, stdin can be redirected from files, pipes, or other programs.
In Linux, stdin is represented by file descriptor 0. You can redirect it using the < operator or pipe data into a command with the | operator. For example, cat < file.txt reads from file.txt instead of the keyboard, and echo "hello" | cat pipes the output of echo as input to cat.
Most command-line tools read from stdin when no file arguments are provided, making it fundamental to Unix command chaining and data pipelines.