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I am trying to do a more complicated task inside a running lxc ubuntu container, but my problem can be explained using this simple example. When I run
sudo lxc-attach -n container1 -- echo "test" > test.txt
inside of a shell script, I expect to find test.txt inside of my container, but instead I find it on my host machine! What has gone wrong?
After some playing around I figured out the issue. I'll leave my question and answer here for the poor soul who inevitably runs into the same problem in the future and finds this question.
The key is to only attach to the container when accessing the file, not before running the whole command. Counterintuitively, the shell does not first connect into the container and execute the command, but rather interprets the > and creates a file locally first. To get around this, we use tee to connect to the container only when we need to. The solution is below.
echo "test" | sudo lxc-attach container1 -- tee test.txt
sudo lxc-attach -n container1 -- 'echo "test" > test.txt'?>. It createstest.txt(or empties it, if it exists), and only then does it launch thesudocommand, whose output it connects totest.txt.