As with load, the data available through uptime is very difficult to parse. Again, if you have the /proc/ filesystem, take advantage of it. I wrote the following code to output just the time the system has been up:
#!/bin/bash
#
# upt - show just the system uptime, days, hours, and minutes
let upSeconds="$(cat /proc/uptime) && echo ${temp%%.*})"
let secs=$((${upSeconds}%60))
let mins=$((${upSeconds}/60%60))
let hours=$((${upSeconds}/3600%24))
let days=$((${upSeconds}/86400))
if [ "${days}" -ne "0" ]
then
echo -n "${days}d"
fi
echo -n "${hours}h${mins}m" |
Output looks like "1h31m" if the system has been up less than a day, or "14d17h3m" if it has been up more than a day. You can massage the output to look the way you want it to. This evolved after an e-mail discussion with David Osolkowski, who gave me some ideas.
Before I wrote that script, I had a couple emails with David O, who said "me and a couple guys got on irc and started hacking with sed and got this: uptime | sed -e 's/.* \(.* days,\)\? \(.*:..,\) .*/\1 \2/' -e's/,//g' -e 's/ days/d/' -e 's/ up //'. It's ugly, and doesn't use regex nearly as well as it should, but it works. It's pretty slow on a P75, though, so I removed it." Considering how much uptime output varies depending on how long a system has been up, I was impressed they managed as well as they did. You can use this on systems without /proc/ filesystem, but as he says, it may be slow.
Relative speed: the "upt" script takes about 0.68 seconds on an unloaded 486SX25 (half that as a function). Contrary to David's guess, his use of sed to parse the output of "uptime" takes only 0.22 seconds.