====== 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>