Operation not permitted when writing in sshfs

Question

I use image-backup to backup my Rpi. I backup directly on an FTP server and I do so via sshfs.

This is how I connect in my /etc/fstab:

MYUSERNAME@MYSERVER:/home/myuser/rpi_backup /mnt/MYSERVER fuse.sshfs noauto,x-systemd.automount,_netdev,reconnect,identityfile=/home/pi/.ssh/id_rsa,allow_other,default_permissions 0 0

Here my /etc/fuse.conf

user_allow_other

I get the error that I dont have permissions:

rsync: [generator] symlink "/tmp/img-backup-mnt/boot/cmdline.txt" -> "firmware/cmdline.txt" failed: Operation not permitted (1)

I have tried also the following commands but this didn't fix it:

sudo groupadd fuse
sudo usermod -a -G fuse pi

Someone has an idea of what else I could try to fix this?

Many thanks!

EDIT:

So in the end it isn't SSHFS at all.

When using sudo image-backup and then enter the destination /mnt/test.img I get the same error.

I understand that a custom program is difficult to troubleshoot and to look whats wrong but what rsync commands can I test to reproduce or narrow down the errors? Thanks.

Answer 1

I removed the part with sudo echo in my question. This works now for some reason.

@waltinator already explained it in his comment, here is the long version:

A command line is interpreted by the shell in several distinct steps. One of these steps - a rather early one - is to process redirections. This is why a command like:

sudo -s /some/command > /some/where

is NOT interpreted as "pass /some/command > /some/where to sudo -s" but the redirection is processed first and the output of sudo -s /some/command is redirected to /some/where. Only then sudo is called and passed the option -s and the command line argument /some/command.

This means, that a process running under your user ID and with your privileges writes to the redirection target and presumably your privileges don't encompass writing there.

The solution is to create a process which is indeed able to write there:

sudo -s /some/command | sudo -s tee [-a] /some/where > /dev/null

You see, both the "/some/command" and the "tee" which does the writing run with elevated privileges.

Notice that you need to use the "-a" flag if you want to append the file, leave it out for overwriting the file. Also notice that you need to redirect the output of tee to /dev/null if you don't want to have what is written to the file clutter your screen.