Linux has one directory tree, starting at /. Disks, USB sticks and network shares are mounted into that tree rather than getting drive letters. On Ubuntu 18.04.2 LTS and CentOS 7.6 the layout follows the Filesystem Hierarchy Standard, so the same map works on both.
flowchart TB
R[Root slash] --> E[etc<br/>configuration]
R --> V[var<br/>logs and spool]
R --> U[usr<br/>programs and libraries]
R --> H[home<br/>user files]
R --> P[proc and sys<br/>kernel views]
R --> T[tmp<br/>temporary files]
| Directory | Holds |
|---|---|
/etc |
System-wide configuration files, all plain text |
/var/log |
Logs such as syslog, auth.log and messages |
/usr/bin, /usr/sbin |
Installed commands (/bin and /sbin hold the essential commands on 18.04) |
/home, /root |
User data and the root user’s home |
/proc, /sys |
Live kernel and hardware information, not real files |
/opt |
Self-contained third-party software |
Try ls / and cat /proc/cpuinfo. Knowing where configuration (/etc) and logs (/var/log) live answers most first questions on an unfamiliar server.