108

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
110

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
  • 27
    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 – Shawn J. Goff Dec 14 '10 at 15:20
  • How will you do it with heredoc? – JohnDoea Nov 19 '16 at 2:54
26

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).

  • clever! this alleviates the need to do echo test | sudo tee /tmp/foo >/dev/null to discard the output. – Adam Katz Jan 14 '15 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 '15 at 6:32
  • 5
    dd is reliable with the non-obscure bs=1 – umeboshi Jan 25 '15 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. – syntaxerror Jan 29 '16 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. – Stéphane Chazelas Sep 12 '17 at 11:30
12

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

sudo sh -c 'echo test > /tmp/foo'
  • 1
    3 problems: eval instead of a simple sh -c; -i, which will change your working dir; running the whole command as root, which might change it's behaviour or may introduce an unnecessary risk. why did this ever get an upvote? – user601 Aug 31 '10 at 10:38
  • 4
    you didn't, but it is misleading, generally bad and maybe even dangerous. – user601 Aug 31 '10 at 20:24
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
  • 1
    This is worded like an Ancient Chinese riddle. Can't see to understand how this got so many upvotes. (hrmph) – syntaxerror Jan 29 '16 at 17:36
  • @syntaxerror: Sir, I'm sorry, I'm not a native English speaker. If you specify what is unclear to you, I could try to improve my answer. Compared with 65 upvotes for Gert, 4 doesn't seem to be that many upvotes, either, don't you think? – user unknown Jan 30 '16 at 1:35
  • Well I would be pretty satisfied with 4 :-D Your English is not bad at all. It's just worded in a kind of terse techie nerdspeak. I must guess you're a coder. Coders amongst each other will understand themselves perfectly that way, but a non-coder must think it's worded like...as said. – syntaxerror Jan 31 '16 at 20:22
  • 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 – Andrew Henle Mar 14 '18 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. – user unknown Mar 14 '18 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.

  • 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. – ctrl-alt-delor Apr 15 '15 at 14:51
  • I can't think of any reason why I would want uncat when I have tee. – Wildcard Sep 29 '15 at 9:26
  • 1
    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. – Scott Feb 17 '16 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.

  • 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 – Michael Mrozek Dec 18 '16 at 4:41
  • 1
    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. – Hontvári Levente Dec 18 '16 at 22:48
  • 1
    I would not install a package to spare a redirection, but I regularly use sponge, so it is already there. – Hontvári Levente Dec 18 '16 at 22:50
0

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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