What does " 2>&1 " mean?

Original Article ↗ Hacker News Discussion ↗

Basic meaning of 2>&1

  • Most comments agree: it redirects file descriptor 2 (stderr) to wherever file descriptor 1 (stdout) is currently going.
  • 2 = stderr, > = “send to”, &1 = “to the file descriptor 1 (not a file named 1)”.
  • This is often used to combine stdout and stderr into one stream for piping, grepping, or logging.

System call and execution model

  • Several explain it in terms of Unix syscalls: 2>&1 corresponds to dup2(1, 2) – duplicate fd 1 onto fd 2.
  • Redirections are applied left‑to‑right as the shell parses them:
    • cmd >file 2>&1 → both stdout and stderr go to file.
    • cmd 2>&1 >file → stderr still goes to the original stdout (e.g. terminal), only stdout goes to file.
  • Pipes (|) are not simple redirections but also fork and set up a pipe pair; this interacts with the ordering of redirections.

Advanced uses and extra file descriptors

  • Shells allow arbitrary fds beyond 0/1/2, e.g. 3>&1, 4>&3, etc.
  • People describe using extra fds for:
    • Multiple logging levels, keeping “chatty” output separate.
    • Wrapping build systems or tools (gpg, aws cli) to capture or filter specific streams.
    • Creating private “channels” inside shell scripts.
  • Tricks like >(...), process substitution, and /dev/fd/* are mentioned as powerful but quirky; named pipes and /dev/tcp also come up.

Syntax design, intuitiveness, and alternatives

  • Strong split:
    • Some find the & fd notation and terseness elegant and “muscle memory”.
    • Others consider 2>&1 opaque, elitist, and a sign of archaic, user‑hostile shell syntax.
  • Suggested clearer syntaxes include things like &stderr>&stdout or rc’s >[1=2].
  • Comparisons to other shells and tools (rc, fish, PowerShell, Nushell, Python+sh, Go, Node) raise the usual “shell vs real language” debate.
  • There’s discussion of abstraction vs leaky low‑level concepts and how terse syntax hides the underlying model for newcomers.

Portability, variants, and gotchas

  • Non‑portable constructs like /dev/stdout and /dev/stderr are warned against; behavior differs across Unix, Linux, macOS, and shells.
  • Bash adds extra operators like &>file and |& (pipe stdout+stderr) that are not POSIX.
  • PowerShell borrows 2>&1 syntax but has different semantics and multiple “streams”.
  • Several subtle pitfalls are highlighted:
    • Order dependence of redirections and pipelines.
    • | acting as a pipeline and also a statement boundary.
    • Confusion between & as “background” vs as part of a redirection operator.

Learning resources, tools, and meta‑discussion

  • Multiple people recommend:
    • The Bash manual’s “Redirections” and “Pipelines” sections.
    • POSIX shell specification chapters.
    • man dup, man bash, and tools like ShellCheck.
  • There is extended meta‑discussion:
    • LLMs frequently emit 2>&1 in generated commands.
    • Mixed feelings about LLMs vs Stack Overflow: some miss human explanations and context; others prefer AI’s lack of social friction.
    • Nostalgia and criticism for “old” Stack Overflow culture and the difficulty of replacing deep, curated human knowledge.