I/O

Core Idea

I/O means input and output: the bytes a process reads and writes through terminals, files, pipes, sockets, and devices.

Standard Streams

Every normal command starts with three standard streams:

• stdin: input stream, file descriptor 0.
• stdout: normal output stream, file descriptor 1.
• stderr: error output stream, file descriptor 2.

Commands To Try

printf 'hello\n'
cat < /etc/hostname
ls /etc /no/such/path > ~/playground/stdout.txt 2> ~/playground/stderr.txt

Why It Matters

Pipes and redirection are I/O plumbing. They let small programs cooperate without knowing about each other. grep can read from a file or from stdin. wc -l can count file lines or pipeline lines.

Common Confusions

• Output on the terminal can be stdout or stderr; they look similar until redirected.
• A command can succeed and still print warnings.
• A command can fail and still print useful partial output.
• Network sockets and device paths are I/O too, not only files and terminals.

Proof Check

Redirect stdout and stderr to different files, then explain which message landed where.

Docs Pointers

• Run man bash, then search for REDIRECTION.
• Read File Descriptor, Stream Redirection, Pipes, Devices And `/dev, and Sockets.