132

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
1

7 Answers 7

142

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
2
  • 36
    Tee will also output to stdout; sometimes you don't want the contents filling the screen. To fix this, do echo test | sudo tee /tmp/foo > /dev/null Dec 14, 2010 at 15:20
  • 1
    How will you do it with heredoc?
    – user149572
    Nov 19, 2016 at 2:54
28

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).

5
  • clever! this alleviates the need to do echo test | sudo tee /tmp/foo >/dev/null to discard the output.
    – Adam Katz
    Jan 14, 2015 at 23:52
  • 2
    I may have to take that back; dd is unreliable for that unless you're using obscure GNU-only options iflag=fullblock oflag=fullblock, which remove the elegance of this answer. I'll stick with tee.
    – Adam Katz
    Jan 16, 2015 at 6:32
  • 5
    dd is reliable with the non-obscure bs=1
    – umeboshi
    Jan 25, 2015 at 3:50
  • 2
    @umeboshi But reliable only if you're experienced enough to know exactly what you're doing. Fordd can be fairly dangerous (if not to say: devastating) if only a slight mistake was made. So for new users, I'd rather recommend the tee method to be on the safe shore. Jan 29, 2016 at 17:38
  • 1
    @AdamKatz, in the case of dd of=file alone (without count/skip...), it is reliable. iflag=fullblock is not needed because here dd writes on output what it has read on input. It doesn't matter if it was not full blocks. Sep 12, 2017 at 11:30
14

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

sudo sh -c 'echo test > /tmp/foo'
0
5

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
2
  • Note that sed -i does not actually modify the file in place - it creates a temporary file and renames it on exiting. So you won't be able to do something like tail -f ... on the original file and see the output using sed -i ... while the pipeline is running Mar 14, 2018 at 10:24
  • @AndrewHenle: Yes, since the size may be increased or shrinked, and since that's probably the case for most sed invocations, and you can't even - afaik - write to the same location on SSDs it's only a pseudo 'in place' operation. As a non native english speaker, may I ask for a brief expression, which isn't so likely misinterpreted? Just -i creates a new file of same name or is there something more compact? I guess I like in place, because it explains the i. The gnu-sed manpage calls it in place too and the long flag is --in-place. Mar 14, 2018 at 10:30
2

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.

3
  • cat takes a list of files to concatinate, therefore uncat should take a list of files to un concatinate to. It would have to use magic to decide how much to put in each file. Alternative name include dog, to-file, redirect. Apr 15, 2015 at 14:51
  • I can't think of any reason why I would want uncat when I have tee.
    – Wildcard
    Sep 29, 2015 at 9:26
  • 2
    Well, tee has the trivial drawback that it writes its stdin to its stdout — which is trivially mitigated by redirecting the stdout to /dev/null.  Other alternatives include dd of=/tmp/foo (mentioned in another answer), which writes status information to stderr, and cp /dev/stdin /tmp/foo. Feb 17, 2016 at 14:44
1

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.

3
  • 1
    I'm not sure what advantage not writing to stdout presents, but you could just redirect the output to /dev/null if it were an issue Dec 18, 2016 at 4:41
  • 2
    Of course I can redirect to /dev/null, but the command is easier to read and type without the redirection. The advantage of not writing to stdout is that my terminal is not filled with rubbish. Dec 18, 2016 at 22:48
  • 1
    I would not install a package to spare a redirection, but I regularly use sponge, so it is already there. Dec 18, 2016 at 22:50
1

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

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