Time Machine

The backup that is already installed and already knows what to do. Point it at a disk once and stop thinking about it.

What it does

Time Machine takes an APFS snapshot of the startup volume hourly, copies it to a backup destination, and thins the history as space runs out — hourly for a day, daily for a month, weekly for as long as it fits. Restores run from Finder for a single file, from Migration Assistant for a whole account, or from macOS Recovery for the entire machine.

It is versioned, not a clone. The backup disk does not boot; it rebuilds a Mac onto a working one.

Attach a disk

Any external disk works. Erase it as APFS in Disk Utility if it is only ever going to hold backups — Time Machine has used APFS rather than HFS+ since Big Sur, and it will offer to erase a disk you hand it.

Turn it on

System Settings → General → Time Machine → Add Backup Disk, pick the disk, and choose whether to encrypt it. Encrypt it.

Set the details

Options worth changing once:

Back up frequency — hourly by default, and every 24 hours or manual are the other choices.
Exclude the directories you can rebuild — see below.

Check it works

tmutil destinationinfo
tmutil latestbackup
tmutil startbackup --block

A destination that lists, a backup that dates from this hour, and a manual run that completes. Then restore one file from the Finder’s Enter Time Machine, because a backup nobody has ever restored from is a hypothesis.

Excluding directories

Anything you can rebuild from a package manager or a git clone is worth keeping out: it costs backup space, slows every run, and restores something you did not want. macOS already skips caches, trash and temporary files through its own exclusion list, so this is about your own directories.

Add them in System Settings

System Settings → General → Time Machine → Options…, then + and pick the folders. The list shows the total size of what you have excluded, which is a useful sanity check after adding a few.

Or add them from the command line

tmutil addexclusion ~/Code/project/node_modules
tmutil addexclusion ~/Library/Developer/Xcode/DerivedData

That is a sticky exclusion: it attaches to the item, so it survives a move or a rename, and a copy of the directory inherits it.

To exclude a path rather than an item — anything that ends up there is excluded, and the exclusion stays put when the current occupant is deleted:

sudo tmutil addexclusion -p /Users/you/VMs

-p and -v both need root and Full Disk Access for the terminal you run them from. Without Full Disk Access the command exits without doing anything useful — grant it in System Settings → Privacy & Security → Full Disk Access.

Confirm what is excluded

tmutil isexcluded ~/Library/Caches ~/Code/project/node_modules

Prints [Excluded] or [Included] against each path, which is the only way to be sure a rule landed on what you meant.

Undo one

tmutil removeexclusion ~/Code/project/node_modules
sudo tmutil removeexclusion -p /Users/you/VMs

The flags have to match how the exclusion was added — removing a fixed-path rule without -p does nothing.

Worth excluding on most machines:

Path	Why
`node_modules`, `target`, `.venv`, `vendor`	Reinstallable in one command, and enormous
`~/Library/Developer/Xcode/DerivedData`	Build output, regenerated on the next build
`~/Library/Containers/com.docker.docker/Data`	A single multi-gigabyte disk image that changes constantly
VM disks — Parallels, UTM, VMware	Same problem, larger, and backed up whole every time
`~/Downloads`	Installers you already used, if you treat it as a scratch directory

A whole volume is a different verb: sudo tmutil addexclusion -v /Volumes/Scratch, which tracks it by filesystem UUID rather than by name or mount point.

Backing up to a Linux server

Time Machine will write to an SMB share that advertises itself correctly, which means any Linux box with Samba can be the destination. The three pieces are a share configured with Samba’s fruit VFS module, a user that owns it, and Avahi telling the network it exists.

Needs Samba 4.8 or newer for Time Machine support. On anything current that is what you have — smbd --version confirms it.

Install Samba and the VFS modules

sudo apt install samba samba-vfs-modules avahi-daemon

Create the user and the directory

A dedicated account that cannot log in, owning the backup directory:

sudo useradd -M -s /usr/sbin/nologin timemachine
sudo smbpasswd -a timemachine
sudo mkdir -p /srv/timemachine
sudo chown timemachine:timemachine /srv/timemachine
sudo chmod 700 /srv/timemachine

The filesystem underneath has to support extended attributes — ext4, XFS, Btrfs and ZFS all do. exFAT and FAT do not, and the backup will fail in confusing ways.

Configure the share

/etc/samba/smb.conf

[global]
fruit:aapl = yes
fruit:nfs_aces = no
fruit:model = MacSamba
fruit:metadata = stream
fruit:veto_appledouble = no
fruit:posix_rename = yes
fruit:wipe_intentionally_left_blank_rfork = yes
fruit:delete_empty_adfiles = yes
min protocol = SMB2

[timemachine]
comment = Time Machine
path = /srv/timemachine
vfs objects = catia fruit streams_xattr
fruit:time machine = yes
fruit:time machine max size = 1T
valid users = timemachine
writable = yes
browseable = yes
guest ok = no

Two things about that file:

vfs objects in a share replaces the global list rather than adding to it, so it belongs in the share stanza, and fruit must come before streams_xattr.
fruit:time machine max size is the quota advertised to the Mac. Without it Time Machine will happily grow until the server’s disk is full.

Advertise it with Avahi

/etc/avahi/services/timemachine.service

<?xml version="1.0" standalone='no'?>
<!DOCTYPE service-group SYSTEM "avahi-service.dtd">
<service-group>
  <name replace-wildcards="yes">%h</name>
  <service>
    <type>_smb._tcp</type>
    <port>445</port>
  </service>
  <service>
    <type>_device-info._tcp</type>
    <port>0</port>
    <txt-record>model=TimeCapsule8,119</txt-record>
  </service>
  <service>
    <type>_adisk._tcp</type>
    <port>9</port>
    <txt-record>dk0=adVN=timemachine,adVF=0x82</txt-record>
    <txt-record>sys=waMa=0,adVF=0x100</txt-record>
  </service>
</service-group>

adVN= is the share name and has to match the stanza. The model record is what makes Finder draw it as a Time Capsule rather than a generic server.

sudo systemctl restart smbd avahi-daemon

mDNS needs 5353/udp open, and SMB needs 445/tcp.

Add it on the Mac

The share should now appear under Add Backup Disk. If it does not, mount it first — Finder → Go → Connect to Server, smb://server.local/timemachine — and it will be listed. From the command line:

sudo tmutil setdestination -a "smb://timemachine@server.local/timemachine"
tmutil destinationinfo

Notes

A network backup lives inside a sparsebundle disk image, which is why the quota matters and why the first run is slow. Do the first backup over Ethernet; hourly increments are fine over Wi-Fi afterwards.
Apple supports Time Machine over SMB to its own and to certified devices. A Samba share is not certified, works well in practice, and deserves an occasional Verify Backups from the Time Machine menu bar item.
Local snapshots live on the startup disk and are not a backup. tmutil listlocalsnapshots / shows them; they are what lets you restore between backups, and they vanish with the disk.
A share that refuses to appear as a destination can be forced into the list with defaults write com.apple.systempreferences TMShowUnsupportedNetworkVolumes 1. Needing it means the share is not advertising Time Machine support, so fix the fruit: settings instead.
Two destinations can be configured at once and Time Machine alternates between them — a local disk and the server is a good pairing.

Alternative to

Alternative	Type	Trade-off
Carbon Copy Cloner	Paid	Control over scheduling and retention, and it can make a bootable copy
restic	Open source	Off-site destinations and any operating system, but you write the schedule
Arq	Paid	The same hands-off feeling, pointed at cloud storage you own
Kopia	Open source	Declarative retention policies rather than Apple’s fixed thinning
SuperDuper!	Freemium	A copy you can start the machine from; no version history