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I want to replace a string with sed and save changes to same file which needs sudo as its a root owned file.

But while doing that I am getting below error

raja@raja-UbuntuVM:~$ sudo sed 's|gateway 192.168.56.1| |g' /etc/network/interfaces
# interfaces(5) file used by ifup(8) and ifdown(8)
auto lo
iface lo inet loopback
iface eth1 inet static
        address 192.168.56.102
        netmask 255.255.255.0

-bash: syntax error near unexpected token `newline'
raja@raja-UbuntuVM:~$ sudo sed 's|gateway 192.168.56.1| |g' /etc/network/interfaces > /etc/network/interfaces
-bash: /etc/network/interfaces: Permission denied
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sudo sed 's|gateway 192.168.56.1| |g' /etc/network/interfaces > /etc/network/interfaces

That fails because your current (non-sudo) shell attempts to open the interfaces file before sudo is executing. To work around this, use the -i (inplace edit) option of sed:

sudo sed -i 's|gateway 192.168.56.1| |g' /etc/network/interfaces

edits and replaces the file at the same time.

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    Worse, even if the perms allowed you to > /etc/network/interfaces, it would have erased the contents of the file because the shell redirection happens before the sed command is run...sed would have an empty input file and produce an empty output file. That's why you have to use -i for in place edits, or a tmpfile if your version of sed doesn't support -i. Commented Nov 14, 2015 at 5:46
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    Another thing worth noting is that the sudo applies to the command (sed) but not to the redirection. The redirection is happening in your shell, which is running as you (and you don't have permission to overwrite that file). Which is a good thing in this case, otherwise you would have wiped /etc/network/interfaces Commented Nov 14, 2015 at 5:51
  • Both cas and Robert Thank you for clear explanation . Commented Nov 14, 2015 at 5:57
  • The comment by cas above is crucial—never use a redirect to try to edit a file in place—but for a case where you want to run a bunch of arbitrary commands in a pipeline and pipe the result to a privileged file, the command tee is worth knowing. pipeline_of_commands | sudo tee privileged_file Commented Nov 14, 2015 at 6:37
  • And for editing a file in place with arbitrary commands (which unlike sed may not have a flag for it), the best answer I've seen is serverfault.com/a/547331/313521 Commented Nov 14, 2015 at 6:38

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