Using sudo with cat and redirect?

NotionCommotion · 12-09-2016, 10:15 AM

How can the following be performed using sudo?

Code:

cat >> /etc/init.d/foo <<'EoT'
Hello
EoT

BW-userx · 12-09-2016, 10:21 AM

Quote:

Originally Posted by NotionCommotion

How can the following be performed using sudo?

Code:

cat >> /etc/init.d/foo <<'EoT'
Hello
EoT

did you try this?

Code:

sudo cat >> /etc/init.d/foo <<'EoT'
Hello
EoT

NotionCommotion · 12-09-2016, 10:23 AM

Quote:

Originally Posted by BW-userx

did you try this?

Code:

sudo cat >> /etc/init.d/foo <<'EoT'
Hello
EoT

Yes, I did so before asking the question. I think it doesn't work because I am applying sudo to cat and not the file in question?

Code:

michael@raspberrypi:~ $ sudo cat >> /etc/init.d/foo <<'EoT'
> Hello
> EoT
-bash: /etc/init.d/foo: Permission denied
michael@raspberrypi:~

BW-userx · 12-09-2016, 10:32 AM

Quote:

Originally Posted by NotionCommotion

Yes, I did so before asking the question. I think it doesn't work because I am applying sudo to cat and not the file in question?

Code:

michael@raspberrypi:~ $ sudo cat >> /etc/init.d/foo <<'EoT'
> Hello
> EoT
-bash: /etc/init.d/foo: Permission denied
michael@raspberrypi:~

You're doing it right, the sudo comes before the command.

1. did you make sure the user had sudo privileges in the /etc/sudoers file, and has been added to the group, wheel or sudo, (one or the other) ?

Don't forget to logout then log back in so the group changes will take effect.

NotionCommotion · 12-09-2016, 10:34 AM

Quote:

Originally Posted by BW-userx

You're doing it right, the sudo comes before the command.

1. did you make sure the user had sudo privileges in the /etc/sudoers file, and has been added to the group, wheel or sudo, (one or the other) ?

This works:

Code:

michael@raspberrypi:~ $ sudo touch /etc/init.d/foo
[sudo] password for michael:
michael@raspberrypi:~ $

BW-userx · 12-09-2016, 10:37 AM

Quote:

Originally Posted by NotionCommotion

This works:

Code:

michael@raspberrypi:~ $ sudo touch /etc/init.d/foo
[sudo] password for michael:
michael@raspberrypi:~ $

try going su
sometime even on my system it will not allow me to do certain things using sudo, so I just go su then do it.

rknichols · 12-09-2016, 10:54 AM

The problem with your redirection is that the file is opened by your current shell prior to invoking sudo. If you want the opening to occur in the privileged environment, you have to write the command line that way:

Code:

sudo sh -c "cat >> /etc/init.d/foo" <<'EoT'
> Hello
> EoT

Now it's a privileged shell that is opening the output file.

NotionCommotion · 12-09-2016, 11:02 AM

Thanks rknichols. I think you had a small typo, and this works perfect.

Code:

sudo sh -c "cat >> /etc/init.d/foo <<'EoT'
> Hello
> EoT"

Do you mind explaining why it works? man says the following about sh's -c flag. Also, what are the two > before Hello and EoT all about?

Quote:

Read commands from the command_string operand instead of from the standard input. Special parameter 0 will be set from the command_name operand and the positional parameters ($1, $2, etc.) set from the remaining argument operands.

BW-userx · 12-09-2016, 11:15 AM

Code:

> # that one adds to a file, over writing everything in that file, replacing
it with whatever you do , whereas,
>> # adds the next line underneath whatever is already there. Appending.

To redirect standard output, use the > symbol. 
Placing > after the cat command (or after any utility or application
that writes to standard output) directs its output to the file name 
following the symbol. Press [Enter] to go to an empty line
 and use the [Ctrl] - [D] key combination to quit cat.

more on that here
https://www.cyberciti.biz/faq/unix-l...ext-to-a-file/

rknichols · 12-09-2016, 11:22 AM

Quote:

Originally Posted by NotionCommotion

Thanks rknichols. I think you had a small typo, and this works perfect.

Code:

sudo sh -c "cat >> /etc/init.d/foo <<'EoT'
> Hello
> EoT"

No, I did not make a typo. The command is correct as I wrote it.

Code:

sh -c "cat >> /etc/init.d/foo"

"sh -c" spawns a shell that runs the command line that is in the quoted string that follows, in this case "cat >> /etc/init.d/foo". Running that shell under sudo gives it the elevated privilege it needs to open /etc/init.d/foo.

There is no need for elevated privilege for the stdin redirection from the HERE document. That can be done by the current shell, and the new shell will inherit that stdin.

The "> " ahead of "Hello" and "EoT" are not something you type. They are the prompt that the shell displays when reading the HERE document from the terminal. I suppose to be consistent I should have shown the "$ " prompt for the full command.

NotionCommotion · 12-09-2016, 11:25 AM

Quote:

Originally Posted by rknichols

The "> " ahead of "Hello" and "EoT" are not something you type. They are the prompt that the shell displays when reading the HERE document from the terminal. I suppose to be consistent I should have shown the "$ " prompt for the full command.

I should have known that. What is funny is that it actually worked as I showed it.

rknichols · 12-09-2016, 11:33 AM

Quote:

Originally Posted by NotionCommotion

What is funny is that it actually worked as I showed it.

It will work that way, too, with the privileged shell doing the HERE document redirection. It just gets messy trying to include all that in the quoted string, and if that "EoT" were not quoted there would security implications with the privileged shell doing the parameter expansion, command substitution, and arithmetic expansion of the HERE document lines. You generally should do only those things that need extra privilege in the elevated shell.

BW-userx · 12-10-2016, 10:49 AM

Quote:

Use output redirection again for another file and call it home.txt.
For this example, type the command cat > home.txt, then [Enter] ,
followed by: bring the coffee home take off shoes put on sneakers make
some coffee relax! Now, on an empty line,
use the [Ctrl] - [D] key combination again to quit cat.

Code:

userx@voided1.what~>> cat >> test2>>catRedirect <<'EoT'
> g
> g
> g
> g
> bash: warning: here-document at line 21 delimited by end-of-file (wanted `EoT')
userx@voided1.what~>> cat >> test2>>catRedirect <<'EoT'
> EoT


userx@voided1.what~>> cat catRedirect
g
g
g
g

with sudo

Code:

userx@voided1.what~>> sudo sh -c "cat >> testCAtWithSuDo"
hey your whata yo


userx@voided1.what~>> sudo sh -c "cat >> testCAtWithSuDo <<'HO'"
userx@voided1.what~>> 

userx@voided1.what~>> cat testCAtWithSuDo
hey your whata yo

by you doing this experiment by hand yourself, I hope you will better see what is the end point is with what you are trying to do. To gain you your actual needed results.