On syncing external ssd on-mount with rsync & launchd on MacOS

Written 6 days ago

Problem Statement

I wish to backup my stuff on an external SSD; however, I don’t want to manually copy-paste files hither and thither every time I connect my ssd to my laptop. So, how do I automatically and efficiently sync all the data I wish from my laptop after connecting my disk?

Let’s break it down into two parts:

1. Script for actual syncing of data
2. Automatic running of the syncing script when disk is mounted

rsync

Create a new sync-disk.sh in PATH somewhere, like ~/.local/bin/. Give it chmod u+x sync-disk.sh permissions.

I like to use rsync for efficiency. This is the script as it exists on my machine (modify as per your usage):

#!/usr/bin/env bash

set -euo pipefail

osascript -e 'display notification "Syncing Sandisk"'

if [[ ! -e /Volumes/Sandisk ]]; then
osascript -e 'display notification "Sandisk not mounted"'
exit
fi

rsync \
--archive \
--partial \
--recursive \
--delete \
--exclude='.*' \
$HOME/Documents \
$HOME/Music \
$HOME/Notes \
$HOME/Pictures \
$HOME/Ebooks \
$HOME/.dotfiles \
/Volumes/Sandisk

osascript -e 'display notification "Sandisk synced successfully!"'

This script, ran as command ~/.local/bin/sync-disk.sh, will synchronize mentioned folders between my Macbook and Sandisk SSD. Now, let’s make it run automatically on mount.

launchd

Much like systemd on linux, MacOS comes with launchd. Create a new service called com.<username>.sync-on-mount.plist that watches for SSD to be mounted in ~/Library/LaunchAgents/:

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
  <dict>
    <key>Label</key>
    <string>com.pritesh.sync-on-mount</string>

    <key>Program</key>
    <string>/Users/pritesh/.local/bin/sync-disk.sh</string>

    <key>RunAtLoad</key>
    <true/>

    <key>StartOnMount</key>
    <true/>
  </dict>
</plist>

Save and exit. Now to start the service:

# Verify if OK
plutil ~/Library/LaunchAgents/com.pritesh.sync-on-mount.plist

# Load and forget
launchctl load ~/Library/LaunchAgents/com.pritesh.sync-on-mount.plist

At first, it may show “Sandisk not mounted” notification (if it isn’t connected already). Try attaching your disk and see if it works!

Resources

• Stackoverflow: how-to-start-an-application-when-a-specific-disk-is-mounted

• launchd.info

• rsync notes

• launchd notes