Mastering Rsync: From Basic Sync to Daemon and Real‑Time Inotify Integration

1. Introduction

Rsync is a remote data synchronization tool that uses the Rsync algorithm to transfer only file differences, making it fast over LAN or WAN. It runs on most Unix‑like systems and has Windows ports such as cwRsync.

2. SSH Mode

2.1 Local synchronization

# mkdir src
# touch src/{1,2,3,4}
# mkdir dest
# rsync -av src/ dest/   # sync contents of src to dest (excluding src itself)
# rsync -av src dest/    # sync the src directory itself
# rsync -avR src/ dest/  # same effect even with trailing slash

2.2 LAN synchronization

# mkdir src
# touch src/{a,b,c,d}
# mkdir dest
# rsync -av 172.16.22.12:/data/test/src/ dest/   # pull from remote host
# rsync -av src/ 172.16.22.12:/data/test/dest/   # push to remote host
# rsync -av src 172.16.22.12:/data/test/dest/   # sync whole directory
# rm -rf src/d
# rsync -av --delete src/ 172.16.22.12:/data/test/dest/   # delete extraneous files on target

2.3 Synchronization with a specific user

# useradd george
# passwd george
# mkdir /home/george/test
# touch /home/george/test/g{1,2,3,4}
# rsync -av src -e ssh -l george 172.16.22.12:/home/george
# rsync -av 172.16.22.12:/home/george/test/g* -e ssh -l george -p 22 dest/

3. Daemon Mode

3.1 Service start methods

# yum install rsync xinetd   # install packages
# /usr/bin/rsync --daemon    # run as standalone daemon
# vim /etc/xinetd.d/rsync   # edit xinetd config, set disable = no
# /etc/init.d/xinetd start   # start xinetd
# chkconfig rsync on
# netstat -ntpl | grep 873   # verify daemon is listening

3.2 rsyncd.conf configuration

Both daemon start methods require an rsyncd.conf file whose syntax resembles Samba's configuration. Global parameters control address, port, PID file, log file, timeout, etc. Module sections define the shared path and access rules.

# vim /etc/rsyncd.conf
uid = root
gid = root
use chroot = no
max connections = 5
timeout = 600
pid file = /var/run/rsyncd.pid
lockfile = /var/run/rsyncd.lock
log file = /var/log/rsyncd.log

[web1]
path = /data/test/src
ignore errors = yes
read only = no
write only = no
hosts allow = 192.168.22.12
auth users = web
secrets file = /etc/web.passwd

4. Client testing

Install rsync, create a target directory, then run typical commands:

# yum -y install rsync
# mkdir /data/test
# rsync -avzP [email protected]::web1 /data/test/
# rsync -avzP --delete [email protected]::web1 /data/test/
# rsync -avzP --exclude "*3*" [email protected]::web1 /data/test/
# echo "123" > /tmp/rsync.password && chmod 600 /tmp/rsync.password
# rsync -avzP --delete --password-file=/tmp/rsync.password [email protected]::web1 /data/test/

Schedule automatic synchronization with cron:

# crontab -e
10 0 * * * rsync -avzP --delete --password-file=/tmp/rsync.password [email protected]::web1 /data/test/

5. Real‑time synchronization with inotify‑tools

5.1 Install inotify‑tools

# yum install -y gcc
# mkdir /usr/local/inotify
# tar -xf inotify-tools-3.14.tar.gz
# cd inotify-tools-3.14
# ./configure --prefix=/usr/local/inotify/
# make && make install

5.2 Environment setup

# echo 'export PATH=/usr/local/inotify/bin:$PATH' >> /root/.bash_profile
# source /root/.bash_profile
# echo '/usr/local/inotify/lib' >> /etc/ld.so.conf
# ldconfig
# ln -s /usr/local/inotify/include /usr/include/inotify

5.3 Monitoring script (push side)

#!/bin/bash
inotifywait -mrq -e modify,create,move,delete,attrib /data/test/src |
while read events; do
    rsync -a --delete /data/test/src/ 192.168.22.12:/data/test/dest
    echo "`date +'%F %T'` event: $events" >> /tmp/rsync.log 2>&1
done

Make the script executable, run it in the background, and add the command to /etc/rc.local for boot‑time start. A symmetric script on the destination can pull changes, achieving bidirectional real‑time synchronization.