If you want to learn to "think like a sysadmin", the book Time Management for System Administrators by Limoncelli is short, funny, and excellent---and covers far more than just time management. That will give you some sense of what to care about and why. It's very useful even as a dev or devops person.
If you are really serious about being a professional sysadmin, Limoncelli's other book is considered outstanding---but it's huge and I haven't read it yet. Again, it's a mix of non-technical goals and technical solutions. The Nemeth book is how I started (though I'm not a sysadmin) and it's also quite good, but more purely technical.
I wouldn't say to a beginner, "Know what every command does," but try to learn a new thing every day. Keep notes. I like to write private man pages (https://github.com/pjungwir/manpj), but do what works for you.
I think learning sysadmin skills mostly just takes time, so try to make each task double as a learning opportunity. Sometimes this requires a lot of digging, or reading, or "debugging". Here I probably agree with @protomyth: the holy grail is understanding. If you strive to achieve that (and don't just go with the first thing that appears to work) you will become better and better.
While you're learning the nitty-gritty tech stuff, also try to keep in mind the different priorities of a sysadmin over a developer. Both care about reducing their own effort and annoyances. A sysadmin wants stability (no 3 a.m. downtime), automation (easy to deploy/update/scale), transparency (monitoring, logging), auditability (logging), recoverability (failover, backups), controllability (runbooks), security. Maybe a real sysadmin could chime in with what I'm leaving out. :-)
If you are really serious about being a professional sysadmin, Limoncelli's other book is considered outstanding---but it's huge and I haven't read it yet. Again, it's a mix of non-technical goals and technical solutions. The Nemeth book is how I started (though I'm not a sysadmin) and it's also quite good, but more purely technical.
I wouldn't say to a beginner, "Know what every command does," but try to learn a new thing every day. Keep notes. I like to write private man pages (https://github.com/pjungwir/manpj), but do what works for you.
I think learning sysadmin skills mostly just takes time, so try to make each task double as a learning opportunity. Sometimes this requires a lot of digging, or reading, or "debugging". Here I probably agree with @protomyth: the holy grail is understanding. If you strive to achieve that (and don't just go with the first thing that appears to work) you will become better and better.
While you're learning the nitty-gritty tech stuff, also try to keep in mind the different priorities of a sysadmin over a developer. Both care about reducing their own effort and annoyances. A sysadmin wants stability (no 3 a.m. downtime), automation (easy to deploy/update/scale), transparency (monitoring, logging), auditability (logging), recoverability (failover, backups), controllability (runbooks), security. Maybe a real sysadmin could chime in with what I'm leaving out. :-)