Computer Science › Operating Systems

stdin, stdout, stderr

The three standard streams every process starts with.

Also known as: standard input, standard output, standard error, stdio streams

Every process starts with three standard streams already open.

import sys
name = input("Name: ")                  # reads from stdin
print("Hello")                          # writes to stdout
print("Something failed", file=sys.stderr)   # writes to stderr

console.log("result");     // stdout
console.error("problem");  // stderr

Why two output streams

It lets you separate results from messages about the results. When you redirect output to a file, errors still reach your screen:

./report > report.txt           # data goes to the file, errors show on screen
./report 2> errors.log          # errors go to a file
./report > all.log 2>&1         # both together
./report | grep total           # a pipe carries stdout only

(pipes and redirection)

Good habits for programs

• Write data to stdout and diagnostics to stderr, so output can be piped without noise.
• Read from stdin when no filename is given, so your tool works in pipelines.
• Set a non-zero exit code on errors.
• In containers and cloud platforms, log to stdout and stderr, and let the platform collect them (logging, twelve-factor app).

Under the hood

They’re file descriptors 0, 1 and 2 (file descriptors), and redirection just reconnects them to files or pipes.

Output may be buffered: when stdout is a file or pipe, text can appear late, or be lost if the program crashes before flushing. In Python, flush=True or python -u helps when debugging.