Sync a local directory with a remote directory in Linux

Surprisingly, this can actually be done with rsync(1) alone, without shell access to the remote server.

Assuming bash(1) and GNU date(1):

#! /usr/bin/env bash

src=rsync://example.com/archives/
dst=/path/to/mirrors

cutoff=$( date -d '7 days ago' +%s )
rsync -na --no-motd --out-format='%M %f' "$src" "$dst" | \
    while IFS= read -r line; do
        d=${line%% *}
        fn=${line#* }
        fdate=$( date -d "${d/-/ }" +%s ) || continue
        if [ $fdate -ge $cutoff ]; then printf '%s\0' "$fn"; fi
    done | \
    rsync -a --files-from=- -0 "$src" "$dst"

When it comes to rsync, think of client and server as roles, that the same rsync binary can fulfill. When it's running as a daemon, you build an rsyncd.conf file, and run rsync with the --daemon argument.

One rsync binary can connect to another (remote) rsync binary either directly - if rsync is running as a daemon listening for it's own connections on a remote host, or via remote shell execution - where rsync uses ssh to connect to a remote host and execute the rsync binary at that host, with both rsync processes passing data back and forth to each other.

To hopefully answer your questions:

1. When transferring files between local and remote by accessing via rsync daemon, is the remote host required to run rsync as a daemon?

   • Yes. If you're going to access an rsync daemon, that daemon has to be running.

2. When transferring files between local and remote by accessing via a remote shell, does the remote host need to run rsync? If yes, does the remote host run rsync as a server but not as a daemon, or just as the local host runs rsync as a client?

   • Yes. When rsync has to be available on both hosts, either in the default $PATH, or with the full path to the binary specified in the command options.

3. When transferring files within the local host, does the local host need to run a rsync client and a rsync server?

   • No. The rsync client binary can function as both roles at the same time if both the SRC and DEST paths are locally available.

4. When listing the source files instead of copying, if the source is in a remote host, does the remote host need to run rsync as a server or daemon?

   • No, running an rsync daemon is not necessary, a remote client can be triggered if you use rsync -e ssh to access rsync (the client) on the remote host. (The remote-shell-spawned rsync in Q 6.)

5. Which use cases does "Note that rsync must be installed on both the source and destination machines" apply to, and not apply to, respectively?

   • The rsync binary must be installed on both machines. In the case that the SRC and DEST are the same machine, the lone rsync binary will essentially perform both roles.

6. In the GENERAL part, "a server can be either a daemon or a remote-shell spawned process". Does a remote host running rsync as a server as "a remote-shell spawned process" mean that the remote host runs rsync in the same way as the local host runs rsync?

   • Yes. Effectively so. The only item of note is that the remotely spawned rsync session is fulfilling the server role.