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Bash Stdin or Filename

If you write a shell script and want it to run with either stdin or a filename, like so:

  cat text.txt | script.sh
  script.sh text.txt

There are some options:

#!/usr/bin/env sh
awk -F, '...' "${1:-/dev/stdin}"

#!/usr/bin/env sh
cat "$@" | awk -F, '...'

#!/bin/awk -f
BEGIN{FS=","} ...