uptime
also: uptime command
A command that displays how long a system has been running since the last reboot, along with current load averages and connected users.
The uptime command shows three key pieces of information: the current time, how long the system has been running (in days, hours, and minutes), and the system's load averages over the last 1, 5, and 15 minutes.
The load average represents the average number of processes in the system run queue during those time periods. For example, on a single-core system, a load of 1.0 means the CPU is fully utilized; above 1.0 indicates queued processes waiting for CPU time.
Example output: 14:30:45 up 45 days, 3:22, 2 users, load average: 0.52, 0.48, 0.45 means the system has been running for 45 days, 3 hours, and 22 minutes with relatively light CPU load.
System administrators use uptime to quickly assess system stability, identify prolonged uptimes (indicating reliability), and spot performance issues through high load averages.