[sf-lug] permissions problem

[Michael Paoli]

Or alternatively:
$ sudo sh -c 'echo "added line" >> junk'

And why is (typically) more important than how.  :-)
Understand they why and you'll generally well be able to figure out how.

$ sudo echo "added line" >> junk
The above is parsed/interpreted by the shell as follows:
command/arg0: sudo
first argument: echo
second argument added line (taken as a single argument due to the quoting)
the >> gives us: redirection done by shell, open junk for append,
(attempt to) create junk if it doesn't already exist and
set that up as file descriptor 1 (standard output) for our command.
The shell sets up redirection before attempting the command,
so, in the earlier failure case, since the redirection fails
(EPERM - fails to open the file for append(/write) due to insufficient
permissions), the shell gives an error diagnostic and non-zero return
at that point, before bothering to attempt the command itself.
E.g.:
$ sudo 'echo dLtjVLPyCuN' >> /etc/shadow
-bash: /etc/shadow: Permission denied
$ echo $?
1
$ type foo
-bash: type: foo: not found
$ sudo foo bar baz >> /etc/shadow
-bash: /etc/shadow: Permission denied
$ foo bar baz >> /etc/shadow
-bash: /etc/shadow: Permission denied
$
Note also with the above with the redirection, the shell
didn't even attempt to execute sudo nor foo, as the redirection failed,
so the shell never bothered to attempt to execute foo or sudo.

> From: "Alex Kleider" <akleider at sonic.net>
> Subject: Re: [sf-lug] permissions problem
> Date: Sat, 27 May 2017 20:28:53 -0700

> Thanks, Daniel. Much appreciated.
>
> On 2017-05-27 20:20, Daniel Gimpelevich wrote:
>> On Sat, 2017-05-27 at 20:16 -0700, Alex Kleider wrote:
>>> $ sudo echo "added line" >> junk
>>> -bash: junk: Permission denied  # why didn't sudo allow me to do the
>>> job?
>>>
>>> I don't understand why the last command didn't go through.
>>
>> It's because the ">>" is given to the bash instance in which you're
>> typing it, rather than to sudo.
>>
>>> Has anyone a suggestion why and (more importantly) how to add a line
>>> to
>>> a privileged file? I suppose I could temporarily change ownership but
>>> that seems a long way round.
>>> tia
>>
>> $ echo "added line"|sudo tee -a junk >/dev/null