Bash's redirection isn't just `>` and `<`. The full file descriptor model lets you route any stream anywhere. Bash Cookbook treats this as essential for production scripts. ## Standard streams Every process has three open by default: - **fd 0** (stdin) — input - **fd 1** (stdout) — normal output - **fd 2** (stderr) — errors ```bash echo "hello" # writes to fd 1 (stdout) echo "oops" >&2 # writes to fd 2 (stderr) read line # reads from fd 0 (stdin) ``` ## Basic redirection ```bash cmd > out.txt # stdout to file (truncate) cmd >> out.txt # stdout to file (append) cmd 2> err.txt # stderr to file cmd < in.txt # stdin from file cmd > out.txt 2> err.txt # split stdout and stderr ``` ## Combining streams ```bash cmd > all.txt 2>&1 # stderr to stdout, both to file # Common shortcut: cmd &> all.txt # bash-only: both to file cmd >& all.txt # alternative form cmd 2>&1 | grep foo # pipe BOTH stdout and stderr to grep cmd 2>&1 > out.txt # WRONG order — stderr to terminal, stdout to file cmd > out.txt 2>&1 # CORRECT — both to file ``` **Order matters.** `2>&1` means "redirect fd 2 to whatever fd 1 currently points to" — so it must come AFTER `> out.txt`. ## Discarding output ```bash cmd > /dev/null # discard stdout cmd 2> /dev/null # discard stderr cmd &> /dev/null # discard everything ``` Use this when you only care about the exit status: ```bash if command -v git &> /dev/null; then echo "git is installed" fi ``` ## Here-documents and here-strings ```bash # Here-doc — multi-line literal: cat < /var/log/myscript.log echo "info message" >&3 echo "error" >&2 exec 3>&- # close fd 3 ``` Advanced — useful for scripts with multiple output channels (audit log, error log, normal output). ## tee — split a stream ```bash cmd | tee output.txt # write to file AND stdout cmd | tee -a output.txt # append (don't truncate) cmd 2>&1 | tee -a log.txt # log everything ``` Useful for capturing output while still seeing it in the terminal. ## Common mistakes - **`2>&1` BEFORE `>file`** — wrong order. stderr ends up at the terminal. - **Confusing `>` with `>>`** — `>` truncates; `>>` appends. Mistakes wipe files. - **Quoting variables in here-docs** — `<

Bash Intermediate

Curriculum