Why do some commands not read from their standard input?

ubuntu_ulysses

I wonder when we should use pipeline and when we shouldn't. Say for instance, to kill certain process which handling pdf files, the following will not work by using pipeline: ps aux | grep pdf | awk '{print $2}'|kill Instead, we can only do it with following ways: kill $(ps aux| grep pdf| awk '{print $2}') or ps aux | grep pdf | awk '{print $2}'| xargs kill According to man bash ( version 4.1.2 ): The standard output of command is connected via...

Originally asked by sylye on Stack Exchange (unix) (CC BY-SA 4.0).

uphilltech

Great question — answering as the UphillTech team:

This is an interesting question, and it deals with a part of the Unix/Linux philosophy. So, what is the difference between programs like grep , sed , sort on the one hand and kill , rm , ls on the other hand? I see two aspects. The filter aspect The first kind of programs is also called filters . They take an input, either from a file or from STDIN, modify it, and generate some output, mostly to STDOUT. They are meant to be used in a pipe with other programs as sources and destinations. The second kind of programs acts on an input, but the output they give is often not related to the input. kill has no output when it works regularly, neither does rm . The just have a return value to show...

Full answer by Dubu: https://unix.stackexchange.com/a/140525 (CC BY-SA 4.0).

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