Pipes & Redirection

Bash · Reference cheat sheet

📋 Overview

Redirection connects command stdin/stdout/stderr to files or other FDs. Pipes (|) connect stdout of one process to stdin of the next. Master these to build reliable pipelines and capture errors separately.

🔧 Core concepts

FD 0 stdin, 1 stdout, 2 stderr. Order matters: cmd >out 2>&1 merges both into out; cmd 2>&1 >out leaves stderr on the terminal.

💡 Examples

Capture and merge:

make >build.log 2>&1
make 2>&1 | tee build.log
curl -fsS "$url" -o file || echo "failed" >&2

Here-doc and here-string:

ssh host bash <<'EOF'
  hostname
  uptime
EOF

grep -F <<<"$needle" file.txt

Swap / silence:

cmd 2>/dev/null
cmd >/dev/null 2>&1
cmd 3>&1 1>&2 2>&3 3>&-    # swap stdout/stderr (classic trick)

Process with pipefail:

set -o pipefail
curl -fsS "$url" | jq '.id'

⚠️ Pitfalls

• Without pipefail, pipeline status is the last command only—failures earlier are hidden.
• > overwrites silently; prefer set -o noclobber (>| to force) for safety.
• cmd > file 2>&1 vs cmd 2>&1 > file—order changes which FD lands where.
• Pipes introduce subshells; side effects (vars, cd) don’t affect the parent.
• Here-docs without quotes expand variables; use <<'EOF' for literals.
• Large pipes buffer; for huge data consider temp files or streaming tools.

🔗 Related

• exit_codes.md
• process_substitution.md
• subshells.md
• debugging.md
• scripts_best_practices.md