You mention in your comments having smcFanControl installed; this open source project includes the command line tool
smc. You can use
smc to get fan speed information via Terminal.app:
See the smc manual page for more options.
Since Mac OS X 10.5, you need to use a third party piece of software to access the fan speed information. It appears no tool, installed by default on OS X, exposes this information through the terminal.
The open source project Fan Control includes a command line tool that provides fan speed information. This article, OS X: Current CPU temperature on command line, talks about the project and how to extract the fan speed:
smc -k TC0D -r | sed 's/.*bytes \(.*\))/\1/' |sed 's/\([0-9a-fA-F]*\)/0x\1/g' | perl -ne 'chomp; ($low,$high) = split(/ /); print (((hex($low)*256)+hex($high))/4/64); print "C\n";'
spindump requires administrator privileges and when run manually, spindump samples user and kernel stacks for every process in the system. This is a computationally expensive process, even when run for one second.
Other tools and applications exist, including Temperature Monitor. See Can I get the CPU temperature and fan speed from the command line in OS X?
Pre-Mac OS X 10.5
This article, get sensor information, shows how to use
ioreg to extract the fan speed information with:
ioreg -c IOHWSensor | grep -B3 -A11 '"type" = "fanspeed"'
The above article and the script it contains was designed for Mac OS X 10.4.3.