The BackupTheEFS page has instructions to backup the EFS. This page instead tries to document why it is done in that way, and what are the advantages and disadvantages of various other backups methods.
This can also be useful to write more generic backup instructions to do a more complete backup.
Old versions of the EFS backup instructions used the following command:
adb shell "cat /dev/block/platform/*/by-name/EFS > /EFS.img"
At some point or under some condition, this stopped working and the backup were corrupted.
adb shell "cat /dev/block/platform/*/by-name/EFS /dev/block/platform/*/by-name/EFS" > /EFS.img" EFS.img adb pull /EFS.img ./
Doing it in two stages like that seem to be widely used in other instructions (like the ones found in XDA forums).
Normally cat should produce a valid backup, however it might be better to use dd for extra safety.
On Replicant 6.0 0004, at least the recoveries for the following devices have 'dd':The following should also work:
adb pull /dev/block/mmcblk0p3 ./EFS.img
The advantage is that it can also backup huge partitions like the user data partition or Replicant system partition.
TODO:It might be a good idea to have a list of backup applications and/or to ship one with Replicant.
It might also be a good idea to understand if the backup solution chosen is sustainable in the long term: If the development stop or upstream decide to make the new version proprietary some users might have a hard time adapting to new backup applications or systems.