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 canonical way is to cat the args to the script.

script.sh

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

Use Bash parameter substitution to try to pass in $1, but if it doesn't exist, pass in /dev/stdin.

script.sh

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

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.)

script.awk

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

To just append script output to the file

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)

Or write output to stdout and append the same output to the file.

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)