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Tuesday, September 30, 2014

How to redirect sudo output to a file requiring root permission

sudo is the recommended way to execute a command which requires root permission. In effect, the target command takes on the permission of root without having to provide the root password.

Consider the following scenario. In order to save the changes made to the iptables firewall rules, I need to run the following command which outputs the changes to a file with root permission.

$ sudo iptables-save > /etc/iptables/rules.v4 bash: /etc/iptables/rules.v4: Permission denied

Note that sudo responded with the Permission denied error. The problem was that the iptables-save command was run under sudo, but the output redirection to the /etc/iptables/rules.v4 file was handled by the shell and hence under the non-root user.

To overcome the problem, you can write a simple shell script and run the script using sudo like this:

$ cat > #!/bin/sh iptables-save > /etc/iptables/rules.v4 $ chmod +x $ sudo
If you don't want to write a script, the following are some alternatives.
  • $ sudo sh -c "iptables-save > /etc/iptables/rules.v4"
  • $ echo 'iptables-save > /etc/iptables/rules.v4' | sudo bash
  • $ sudo iptables-save|sudo tee /etc/iptables/rules.v4 >/dev/null

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