uptime=`uptime|grep days|awk '{print$3,$4}'`

uptime | awk -F'[, ]' '{print $3,$4}'

uptime | awk '/days/ { printf "%s%s",$3,substr($4,1,length($4)-1) ; }'

mybox $  uptime
 14:36:13 up 2 days, 23:41,  2 users,  load average: 0,17, 0,18, 0,14
mybox $  x=$(uptime | awk '/days/ { printf "%s%s",$3,substr($4,1,length($4)-1) ; }' )
mybox $  echo $x
2days
mybox $

uptime="$(( `cat /proc/uptime  | cut -f1 -d.` / 86400 )) days"

$ uptime
 1:15PM  up 2 days, 14:04, 1 user, load averages: 1.12, 1.12, 1.08

$ uptime
 1:24PM  up 8 mins, 1 user, load averages: 1.27, 1.17, 0.62

$ uptime | sed 's/^.*up ([^,]*).*$/1/'
19 mins