Pipe, standard input and command line arguments in Bash

Question

Consider:

command1 | command2

Is the output of command1 used as standard input of command2 or as command line arguments to command2?

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Answer 1

It's used as the stdin.

Try:

grep "hehe" - $(cat test.sh)

That might be wrong; I can't test it out on this computer. If you do it without the pipe like you tried, grep treats the last argument as a filename, ie, looks for a file called [contents of test.sh]. If you pass it a - (or don't put a last argument), you tell it to use stdin as the file.

You can also just pass grep a file to scan through:

grep "hehe" test.sh

...but you seem to be asking more of a generalized bash question, not really a grep usage question, so that's probably not too helpful.