🔨 Bash Shorts 🩳: How to redirect output to a protected file (a.k.a.“sudo echo”)

README.md

🔨 Bash Shorts 🩳: “sudo echo” (or “How to redirect output to a protected file”)

Why sudo echo "something" > /path/to/protected/file.txt doesn’t work and what to do instead.

If you are working on the command-line and you want to write output to a file, sooner or later you come up with this command: echo "output" > file.

This is called a “Redirection”. This works wonderfully and you go on your merry way.

Sometime later, you need to do the same thing with a protected file.

So you do:

echo "something" > /path/to/protected/file.txt

And are slightly disappointed to find that the computer says “No”. Or to be more precise:

/path/to/protected/file.txt: Permission denied

Not to worry, you are no n00b, you know how to sudo! 

sudo echo "something" > /path/to/protected/file.txt

But then horror! Madness! Despair! (or at least a minor case of WTF!?), you get that warning again!

/path/to/protected/file.txt: Permission denied

What is going on? Well, the sudo command only applies to the echo command. It does not apply to the pipe >. This means it has the same effect as being called without the sudo. That is why we are seeing that error.

So, what to do?

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In bash core-utils, there is a command called tee. This command reads from standard input and writes to standard output and any files given as a parameter. In other words: output comes in from one side and is split into two separate streams. One goes to standard output, the other to file(s).

You can think of it as a T-junction for output (hence the name).

TK: Replace the image from the wiki below with a custom SVG image (using the example from this file and custom colors).

Because this is a separate command, we can use it with sudo .

We can pipe output to tee (called with sudo):

echo "something" | sudo tee /path/to/protected/file.txt

and everything works! You will notice that the word “something” has also been written to your screen

**ben@muze.nl**:**~**$ echo "something" | sudo tee file.txt  
something

This is because that's whattee does! If you have lots of input that might not be what you want. The solution is redirecting the output of tee to the black hole called/dev/null.

echo "something" | sudo tee file.txt > /dev/null

Now no output is shown. 🎉

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What’s that you say? Your file gets overwritten? You want to append output to the file? With redirection, you would simply append the redirected output using >> :

echo "output" >> file

So how does that work with tee? The tee command comes with an append option. It is invoked by calling tee with an --append flag (or the -a shorthand). So we get:

echo "something" | sudo tee -a file.txt > /dev/null

And that is how to redirect output as sudo to a protected file on the CLI!

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Have an idea for another “Bash shorts”? Maybe a question or feedback? Feel free to comment below or send me a tweet at @potherca.

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Resources and links

Redirections (Bash Reference Manual)
_Before a command is executed, its input and output may be redirected using a special notation interpreted by the shell…_www.gnu.org

tee invocation (GNU Coreutils)
_The tee command copies standard input to standard output and also to any files given as arguments. This is useful when…_www.gnu.org