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(or stdin, or stderr, or FD 3)

grep foo < input1 < input2 > output1 > output2 > output3

# Merge inputs into a single stream, write outputs individually
cat input1 input2 | grep foo | tee output1 output2 > output3

# Pass inputs as filenames, write outputs individually
grep foo input1 input2 | tee output1 output2 > output3

A file descriptor, whether stdin, stdout, stderr, or non-standard ones, can only point to a single file/pipe.

For input, many commands support processing multiple filenames. In these cases you can just specify the filenames instead of redirecting. Alternatively, you can use cat to merge multiple filenames into a single stream.

For output, you can use tee to write to multiple output sinks in parallel.

Zsh will automatically cat inputs and tee outputs, but none of the shells supported by ShellCheck do.

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