rsyncsync

Synchronise multiple locations, 2-way, with backups, using only rsync and basic POSIX tools. Optionally encrypt remotes using encfs (encryption performed locally before sending to remote.)

Note: this assumes a mostly single-user scenario, where files will generally only be modified in one place at a time. There is no fancy merging, only backing up of files that might have changed, to save your bacon if you lose synchronisation for a time.

This attempts to ensure that data are never overwritten and always backed up. Can use multiple backup locations and sync multiple working directories. (Seems to work well in my testing but no theoretical proof for N:M consistency, YMMV, etc.)

Backups are kept using rsnapshot-style hard links. Potential conflicts are detected locally and files copied to a separate local directory, and then backed up to the remote.

behaviour

local state	backup state	action
empty/missing	empty/missing	init local & push
tracked	empty/missing	push (init remote)
empty	tracked	pull (restore)
missing	tracked	abort
unchanged	unchanged	do nothing
changed	unchanged	push
unchanged	changed	pull and overwrite
changed	changed	pull+backup then push

where:

empty: directory empty or non-existent
missing: directory not empty but missing state data
tracked: directory exists with state data

Local changed status is determined by comparing modified times to the time of the most recent pull or push (so only the local clock is used).

Remote changed status is determined by comparing an opaque stamp that changes whenever a remote is updated. Remotes are only changed by pushing from another (synchronised) local. Multiple remotes can be used, and the state of each is kept distinct.

Note that files are never overwritten when pushing so it is safe to push to a remote that is missing state data.

usage

For each source and backup you want, do:

rsyncsync SOURCE-DIR [HOST:]BACKUP-PATH

requirements

backup requirements

backup can be local or remote

• ssh-server (if remote)
• rsync
• coreutils
• sh
• filesystem with hard link support, e.g. ext2

client requirements

• bash
• rsync
• coreutils
• findutils
• ssh for remote backup, public-key auth recommended