Redirecting stdout to a file you don't have write permission on

Question

When you attempt to modify a file without having write permissions on it, you get an error:

> touch /tmp/foo && sudo chown root /tmp/foo
> echo test > /tmp/foo
zsh: permission denied: /tmp/foo

Sudoing doesn't help, because it runs the command as root, but the shell handles redirecting stdout and opens the file as you anyway:

> sudo echo test > /tmp/foo
zsh: permission denied: /tmp/foo

Is there an easy way to redirect stdout to a file you don't have permission to write to, besides opening a shell as root and manipulating the file that way?

> sudo su
# echo test > /tmp/foo

Answer 1

Yes, using tee. So echo test > /tmp/foo becomes

echo test | sudo tee /tmp/foo

You can also append (>>)

echo test | sudo tee -a /tmp/foo

Answer 2

To replace the content of the file with the output of echo (like the > shell redirection operator).

echo test | sudo dd of=/tmp/foo

To write into the file (at the beginning, though you can use seek to output at different offsets) without truncating (like the 1<> Bourne shell operator):

echo test | sudo dd of=/tmp/foo conv=notrunc

To append to the file (like >>), with GNU dd:

echo test | sudo dd of=/tmp/foo oflag=append conv=notrunc

See also GNU dd's conv=excl to avoid clobbering an existing file (like with set -o noclobber in POSIX shells) and conv=nocreat for the opposite (only update an existing file).

Answer 3

tee is probably the best choice, but depending on your situation something like this may be enough:

sudo sh -c 'echo test > /tmp/foo'

Answer 4

While I agree, that | sudo tee is the canonical way, sometimes sed (here assuming GNU sed) may work:

cat sudotest 
line 1

sudo sed -i '1iitest' sudotest && cat sudotest 
itest
line 1

sudo sed -i '$aatest' sudotest && cat sudotest 
itest
line 1
atest

-i modifies the file in place. 1i means insert before line 1. $a means append after last line.

Or copy to xclipboard:

somecommand | xclip
sudo gedit sudotest
move cursor to desired place, click middle mouse button to insert, save

Answer 5

Use sponge from the moreutils package. It has the advantage that it does not write to stdout.

echo test | sudo sponge /tmp/foo

Use the -a option to append to a file instead of overwriting it.

Answer 6

I have been kicking around in the back of my mind ideas for a similar problem, and came up with the following solutions:

• sudo uncat where uncat is a program that reads standard input and writes it to the file named on the command line, but I haven't written uncat yet.

• sudocat the variant of sudoedit that I haven't written yet that does a cleaner sudo cat or sudo uncat.

• or this little trick of using sudoedit with an EDITOR that is a shell script

  #!/bin/sh
  # uncat
  cat > "$1"
  

  which can be invoked as either |sudo ./uncat file or | EDITOR=./uncat sudoedit but that has interesting side-effects.

Answer 7

The error comes from the order in which the shell does things.

The redirection is handled before the shell even executes sudo, and is therefore done with the permissions of the user that you are currently working as. Since you don't have write permissions to create/truncate the target of the redirection, you get a permission denied error from the shell.

The solution is to guarantee that the output file is created under the identity given to you by sudo, e.g. with tee:

$ generate_output | sudo tee target_file