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+====== Bash Script Stdin and Stdout ======
+===== Reading from Stdin or Filename =====
+If you write a shell script and want it to run with either stdin or a filename, like so:
+    cat file.txt | script.sh
+    script.sh file.txt
+There are some options. Here we'll use ''awk'' as the command inside the script.
+=== The General, Idiomatic Way ===
+The canonical way is to cat the args to the script.
+<file bash script.sh>
+#!/usr/bin/env sh
+cat "$@" | awk -F, '...'
+</file>
+=== Shell Default Parameter Substitution ===
+Use Bash parameter substitution to try to pass in ''$1'', but if it doesn't exist, pass in ''/dev/stdin''.
+<file bash script.sh>
+#!/usr/bin/env sh
+awk -F, '...' "${1:-/dev/stdin}"
+</file>
+=== Let the Command Handle It ===
+Since we're using ''awk'' as our example, and it already supports stdin or filenames, don't be a
+bash script, be an awk script. (But in this case we can't use ''/usr/bin/env''.)
+<file bash script.awk>
+#!/bin/awk -f
+BEGIN{FS=","} ...
+</file>
+===== Writing to Stdout and/or to file =====
+To just append script output to the file
+<file bash script.sh>
+#!/usr/bin/env bash
+set -euf -o pipefail
+# Just append stdout to the file
+exec >>file.txt
+# (now the rest of your script)
+</file>
+Or write output to stdout and append the same output to the file.
+<file bash script.sh>
+#!/usr/bin/env bash
+set -euf -o pipefail
+# Write to stdout and append to the file
+exec > >(tee -a file.txt)
+# (now the rest of your script)
+</file>