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voskresenie

Member

Registered: 2025-05-20

Posts: 1

Difference between prefix@[...].suffix and prefix-[...].suffix

I've been trying to figure out the difference between the @ notation and - notation with systemd, and I haven't been able to figure out what the practical difference is.

When creating units, both can be referenced with % specifiers, with %i for @ syntax and %j for - syntax. But what is the difference between creating a unit `prefix@.suffix` and referencing the instantiated units with %i, and creating a unit `prefix-.suffix` and referencing the instantiated units with %j?

Looking through default units, I see there's user@[...].service, but also user-[...].slice. What is the functional difference here that explains why the service uses @ but the slice uses -? Why isn't the slice user@[...].slice, or the service user-[...].service?

from systemd.unit(5):

       ├───────────┼──────────────────────────────┼──────────────────────────────┤
       │ "%i"      │ Instance name                │ For instantiated units this  │
       │           │                              │ is the string between the    │
       │           │                              │ first "@" character and the  │
       │           │                              │ type suffix. Empty for       │
       │           │                              │ non-instantiated units.      │
       ├───────────┼──────────────────────────────┼──────────────────────────────┤
       │ "%I"      │ Unescaped instance name      │ Same as "%i", but with       │
       │           │                              │ escaping undone.             │
       ├───────────┼──────────────────────────────┼──────────────────────────────┤
       │ "%j"      │ Final component of the       │ This is the string between   │
       │           │ prefix                       │ the last "-" and the end of  │
       │           │                              │ the prefix name. If there is │
       │           │                              │ no "-", this is the same as  │
       │           │                              │ "%p".                        │
       ├───────────┼──────────────────────────────┼──────────────────────────────┤
       │ "%J"      │ Unescaped final component of │ Same as "%j", but with       │
       │           │ the prefix                   │ escaping undone.             │
       ├───────────┼──────────────────────────────┼──────────────────────────────┤

cryptearth

Member

Registered: 2024-02-03

Posts: 1,511

Re: Difference between prefix@[...].suffix and prefix-[...].suffix

well - although I don't know but from the man I would infer that even

prefix-FINAL@INSTANCE.suffix

is valid - so there's no EITHER / OR but also a combination of both at the same time seems possible - although not / rarely used
also note the specific wording:
while the first one is an INSTANCE - so everything before the @ is the entire prefix - maybe even including an additional FINAL separated by a hyphen - the latter one is included as part of the prefix
also there seems to be a difference between service and slice files