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I have to run this command sudo sh -c "echo 'nameserver' >> /etc/resolv.conf" to append a line nameserver into /etc/resolv.conf file. I know, it could be possible only through subshell.

My questions:

  • Is that it could be possible without running the command in subshell?

  • On which cases, a command should be runned in subshell?

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marked as duplicate by Eliah Kagan, Aditya, Sylvain Pineau, Warren Hill, Andrea Corbellini Sep 22 '14 at 16:24

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

Can you clarify? You want to know why you should warp the redirection while using sudo or in which cases you have to run subshells? – Braiam May 29 '14 at 23:25
Can you clarify? In your examplesudo sh -c ... doesn't create a subshell, but a new shell process (and you don't need this new shell as long as you can use sed or tee or something else instead of >>). And >> is not a real command to use sudo on it, but is an redirection operator. So, your question doesn't have sense, or logic... – Radu Rădeanu May 30 '14 at 10:40
sudo echo 'nameserver' >> /etc/resolv.conf

fails because it gives elevated permissions to the echo command (which doesn't need it), but not to the >> redirection (which does, since the destination file is owned by root). Wrapping the whole command sequence in sudo sh overcomes that.

You could also do

echo 'nameserver' | sudo tee -a /etc/resolv.conf
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Using the pipeline, you are using in fact not one subshell, but two. See man bash in this sense (line 279): "Each command in a pipeline is executed as a separate process (i.e., in a subshell)." – Radu Rădeanu May 30 '14 at 19:08

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