Re: [tip] Create and Insert a public Nextcloud/Owncloud link

[Juan Manuel Macías]

Max Nikulin writes:

>> Many times I need to create and share a public link to a file
>> in my local folder. In the Nextcloud forum I learned how it can be done
>> from the command line using curl,
> ..
>> │     (result-raw (shell-command-to-string
>> │                  (concat "curl -u "
>> │                          "\""
>> │                          my-username
>> │                          ":"
>> │                          my-passwd
>> │                          "\""
>
> Juan Manuel, your function is a nice proof of concept, but posting
> such code you are responsible for users who may try to use it verbatim
> having less experience with elisp.
>
> Use at least `shell-quote-argument' (though it docstring has a link to
> info "(elisp)Security Considerations"). Just adding quote characters
> is unsafe. You may avoid non-alphanumeric characters in passwords and
> file names for good reasons, but for other users a quote character may
> dramatically change the executed command.
>
> When TRAMP support is not necessary, arguments should be passed to
> external binary as a list without intermediate shell command. I know,
> Emacs does not have a convenience function with such calling
> convention similar to `shell-command-to-string'.
>
> I am almost sure that Emacs has a package to send HTTP POST requests
> directly from elisp. Unsure it has convenient enough API (reasonable
> default timeouts, etc.), but it should be safer for working with
> peculiar file names and passwords stuffed with characters having
> special meaning in shell. I admit that the code would be more verbose.
> It may save you time for recovering you system from damage caused by
> unexpected interpretation of a shell command.

Maxim, you are right that the use of shell-quote-argument is preferable
in cases like these to avoid unexpected problems with filenames,
passwords, and so on. I try to use it almost always. If I don't use it
more often, it's either because I'm lazy (because of my way of naming
the files, I don't expect this type of problems) or because I think it's
unnecessary, although not 100% free of danger[1], as in this case. I'm not
saying my behavior is exemplary, I'm just saying what I tend to do. I
should probably always use shell-quote-argument. In this case, the
affected part of my function would perhaps look better like this:

(shell-command-to-string
 (mapconcat #'shell-quote-argument
            `("curl"
              "-u"
              ,(format
                "%s:%s"
                my-user
                my-password)
              "-H"
              "OCS-APIRequest:true"
              "-X"
              "POST"
              ,(format 
                "%s/ocs/v1.php/apps/files_sharing/api/v1/shares"
                nextcloud-url)
              "-d"
              ,(format
                "path=%s/%s"
                nextcloud-public-folder-name
                file)
              "-d"
              "shareType=3"
              "-d"
              "permissions=1")
            " "))

[1] I think that a problem in this context would not go beyond the fact
that the function simply did not work as expected.

Perhaps it would have been better to use call-process-shell-command,
instead of shell-command-to-string, and extract the resulting string
from the output buffer.

On the other hand, I agree with you that whenever possible it is better
to use an Elisp solution than a shell command.

Best regards,

Juan Manuel 

-- 
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------------------------------------------------------
Juan Manuel Macías 

https://juanmanuelmacias.com

https://lunotipia.juanmanuelmacias.com

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