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It's a question on copying in Linux in general and not just Singularity. I'm trying to recursively copy from remote directory to local directory. In my definition file I have:

scp -r $USER@$HOST_MACHINE:/a/b/c ${SINGULARITY_ROOTFS}/a/b 

The problem is that I need group abc to be able to access /a/b/c on $USER@$HOST_MACHINE. I have this group but not in my active groups on the remote machine. I tried some variance of sg but they all failed. For example:

sg abc scp -r $USER@$HOST_MACHINE:/a/b/c ${SINGULARITY_ROOTFS}/a/b 

Fails because I have abc only in $HOST_MACHINE and not locally. I also tried to use ssh but I need to copy to local directory and not on remote directory.

I was looking in the docs of rsync and scp but could not find a way to add a group into my active groups on remote machines, before copying. Basically I'm looking for a way to do:

scp -active_group abc -r $USER@$HOST_MACHINE:/a/b/c ${SINGULARITY_ROOTFS}/a/b 

How can I copy files from remote to local with a special group?

Basically I could do the following steps:

  • Ssh the remote machine.
  • Wash or use the sg command.
  • Copy from remote to local.
  • exit remote machine.

But the script should be automatic so it's breaks it. I also guess there is a way to do this and I'm just missing it.

2 Answers 2

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Your local UID/GID names are not guaranteed to match the remote UID/GID names.

But its the UID/GID values that are important in inter-machine transfers. The only UID value that is guaranteed is root - with a UID=0.

If this is a one-off copy, then you will need root access to create a group on the remote machine. So you could - with great care - run your script as root - just to create a new group.

You need to create a group with a GID that matches on both machines [group name is your choice]; presumably this should have the same set of UID's on both machines - [user names are your choice].

Ideally, group names and user names would be the same on the two machines, "for simplicity".

/etc/group is a delimited file listing 'groupname':'x':GID:user1,user2,...

/etc/passwd gives a well-known cross-reference of username,UID

However, this seems to be a very high risk procedure - even if you ran it in a dry-run mode and carefully desk-checked the results.

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  • Thank you for you reply! But the group already exists in the remote machine and my user has it there. It's just not in the active groups. This means that I can access it if I can see it in the groups output. If it was in my active groups, then it would be solved. The question is how do I make in active?
    – vesii
    Commented Jun 24, 2021 at 7:14
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If possible, you may just set the set GID bit on the remote machine:

$chmod g+s abc -R /a/b/c 

(or start at /a - depending on your needs). This is a one-time configuration change.

So whatever is done on the remote machine will have abc set as primary group and your scp would work fine as is.

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  • So basically you are saying, I need to run ssh <hostname> chmod g+s abc -R /a/b/c and then scp?
    – vesii
    Commented Jun 28, 2021 at 7:13
  • Yes. The set GID-bit will ensure that independently of the user's (active) group the actual group of the file will be used for any commands. I hereby assumed that a/b/c already is owned by group abc, though. If not you would need to change the group accordingly (chgrp -R abc /a/b/c). Again: the chmod command would only be run once and then persist. I.e. no need to run it with every scp.
    – FelixJN
    Commented Jun 28, 2021 at 7:23

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