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Comment by alerighi | Hacker News Reader

It would also be useful in cases when the file must be written with root privileges (but the shell is run as a normal user)

Now you have to use `tee` for that, that is fine but if you don't want to echo the file back to the terminal you have to do

    command | sudo tee file > /dev/null

with this you can simply do

    command | sudo sponge file

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Can't you just use `cat`?

You cannot use `sudo cat` to open a file with root privileges, because `sudo cat > foo` means "open file foo with your current privileges, then run `sudo cat` passing the file to it", and the whole root thing only happens after you already tried and failed to open the file.

I seem to get this wrong roughly once a week.

I knew a person who would give something like this as a sysadmin / devops interview question. It was framed as "'sudo cat >/etc/foo' is giving a permission denied error! what's wrong?" Usually the interview candidate would go off on a tangent...

Will shells or posix standard ever evolve instead of being the obvious gotchas in interviews?

If Windows can evolve (PowerShell...), so can Linux/Unix.

    … | sudo tee foo

sudo sh -c "cat > file"

Cat won’t write the file. If you mean `command | sudo cat > file.txt`, that won’t work because the redirection is still happening in the non-root shell. You could do `command | sudo sh -c "cat > file.txt"` but that’s rather verbose.

I thought the same thing. What's wrong with this approach?

  cat myfile.txt | awk '{do_stuff()}' | sort -u | column --table > new-myfile.txt

cat is doing nothing useful here. Your command is equivalent to

    awk '{do_stuff()}' myfile.txt | sort -u | column --table > new-myfile.txt

The only thing you’ve changed is that you’re sending the output to a new file. That’s fine, but it’s what sponge is avoiding.

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