In Linux, if we like to know for how long my machine is running. how can we find out system uptime.

Sample output
$ uptime
07:47:00 up 34 days, 8:57, 1 user, load average: 0.06, 0.01, 0.00

Description of uptime command
Curret Time — Hours:Minute:Seconds
How long machine up: — Days Hours:Minutes
How many Users login
Load Average: — 1 Minute, 5 Minutes, 15 Minutes

As we saw uptime also show some other impotant information. But uptime have two option that can use for some better output to know how long system is running. these are -s and -p

-s, --since
                      system up since, in yyyy-mm-dd MM:HH:SS format
-p, --pretty
                      show uptime in pretty format
Output
#uptime -s
2015-12-18 07:30:14
#uptime -p
up 7 hours, 18 minutes

Same information is displayed on first line of w and top command

Sample output
$ w
08:35:44 up 34 days, 9:46, 1 user, load average: 0.00, 0.00, 0.00
USER TTY FROM LOGIN@ IDLE JCPU PCPU WHAT
ssirohi pts/0 141.145.31.50 Tue04 0.00s 0.03s 0.00s w

uptime