How to display time elapsed since last system boot using "uptime"?

To get the time elapsed since last system boot in hh:mm:ss format, you can use:

awk '{print int($1/3600)":"int(($1%3600)/60)":"int($1%60)}' /proc/uptime

/proc/uptime pseudo-file contains two numbers:

The first number is how long the system has been up in seconds.
The second number is how much of that time the machine has spent idle in seconds.

So, using awk you can take firs number and convert it in hh:mm:ss format.

To get uptime in seconds:

awk '{print $1}' /proc/uptime

To get uptime in minutes:

 echo $(awk '{print $1}' /proc/uptime) / 60 | bc

To get uptime in hours:

 echo $(awk '{print $1}' /proc/uptime) / 3600 | bc

To get x digits of precision you can add scale=x, e.g. for x=2

echo "scale=2; $(awk '{print $1}' /proc/uptime) / 3600" | bc

A trivial modification to show days:

awk '{print int($1/86400)"days "int($1%86400/3600)":"int(($1%3600)/60)":"int($1%60)}' /proc/uptime

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