Generate a UUID on a Mac
A UUID from a website is generated on someone else's machine, which is a strange place to source an identifier you are about to treat as unique and unguessable.
Version 4, and why it is the default
A version 4 UUID is 122 random bits with six bits reserved to mark the version and variant. No timestamp, no MAC address, no coordination between machines — which is the entire point: two systems that have never communicated can generate identifiers without colliding.
The collision probability is small enough to ignore. You would need to generate billions of them before a duplicate became likely.
When random is the wrong shape
Random UUIDs make poor clustered primary keys. Because each new value lands in an arbitrary position, inserts scatter across the index instead of appending to the end, which fragments pages and hurts write performance on large tables.
Where that matters, a time-ordered identifier — UUIDv7, ULID, or a database-native sequence — keeps the uniqueness while restoring locality. UUIDv4 remains the right default for anything that is not a hot clustered index.
Formatting details that cause bugs
- The canonical form is 36 characters: 8-4-4-4-12 hexadecimal digits with hyphens.
- Case is not significant, but comparisons often are — normalise before comparing, or you will get false mismatches.
- Some systems store them as a 16-byte binary value and others as a 36-character string; conversions between the two are a routine source of confusion.
- Braces around the value are a Microsoft convention and are not part of the canonical form.
Without any tool at all
macOS has a generator built in, which is enough when you need exactly one:
uuidgenPrints an uppercase v4 UUID. Bellows generates them in bulk and in the case you want.
How to check the claim rather than trust it
Bellows is signed without a network entitlement. That is not a policy someone wrote down — it is a permission the binary does not have, and macOS enforces it whether the app wants to connect or not. You can read the entitlements it was signed with:
codesign -d --entitlements - /Applications/Bellows.appNeither com.apple.security.network.client nor com.apple.security.network.server appears in the output. Without them the sandbox refuses to open a socket.
UUID Generator is one of 41 tools in Bellows
A native Mac app that ships with no network entitlement, so macOS blocks it from connecting at all. One-time purchase, every future update included.
What Bellows is →Questions
Are UUIDs guaranteed unique?
Not guaranteed, but the probability of a collision among version 4 UUIDs is so small it is not a practical concern. You would need to generate on the order of a billion per second for decades before it became likely.
Is a UUID secure enough to use as a token?
A version 4 UUID has 122 bits of randomness, which is plenty of entropy — provided it came from a cryptographic random source. The greater risk is treating an identifier that appears in URLs and logs as if it were a secret.
How do I generate a UUID in macOS Terminal?
Run uuidgen. It produces an uppercase version 4 UUID.