Can running `yes > /dev/null` harm a Mac?

Question

I recently ran yes > /dev/null for a 3 minutes to drain my Mac's battery. During that time, the temperature rose to 72 °C and fans spun up to 4000 RPM. I immediately quit the process.

Should I fear that has this damaged hardware or the logic board?

Answer 1

Certainly as others have said: the CPU and kernel both have their own self-preservation strategies.

I'll add some flavour concerning how yes uses computer resources.

It's worth distinguishing between the behaviour of BSD yes and GNU yes.
macOS is a BSD, so will use an (old) distribution of BSD yes.

• macOS (Darwin) yes source code
• BSD yes source code
• GNU yes source code

A good discussion of the differences exists at How is GNU yes so fast?

And a discussion of that discussion exists on the Hacker News thread of the same name.

BSD/macOS yes really does just run puts("y"); in a tight loop.
GNU yes is… somewhat more serious. It has optimizations far beyond mere buffering of I/O.

Answer 2

The instructions for yes have this little gem from the man page :

Using yes results in 100% processor usage, for this reason it is rarely used other than for testing e.g. to max out a computer's CPU.

Which means, no, you will not have damaged your hardware. Using the yes command is a way of using all (i.e. 100%) of your CPU. The symptoms you experienced (i.e. a rise in temperature and the resulting increase in fan RPM) are to be expected under these circumstances. Additionally, your CPU will "throttle back" and ultimately shut down if its thermal threshold is exceeded to prevent damage.

Answer 3

The yes command simply repeatedly writes a string to stdout, the character y by default. Redirecting (>) it to /dev/null simply causes for the stream data to be forgotten. In other words this has no lasting effect on the persistent state of your computer, it is not a harmful command through this lens.

Since the yes command writes a string to stdout without any constraint on the output speed, this will cause for the CPU to reach maximal utilization on one core. This is the cause of the processor temperature increase and associated fan speed increase.

In a modern machine, particularly a well designed one like an Apple laptop, the hardware will protect its self from overheating damage. First by increasing fan speed, then by decreasing processor clock speed, and ultimately by halting the processor. Without intentionally circumventing these features, your hardware did not overheat. The machine is fine.

You mentioned a temperature of 72 °C specifically. This is not a very high temperature for a CPU die. A modest mobile cpu, the i5-7260U, specifies a maximum permissible temperature of 100 °C. You can see the specification as T_Junction in the package specifications section of this page: http://ark.intel.com/products/97539/Intel-Core-i5-7260U-Processor-4M-Cache-up-to-3_40-GHz

Answer 4

The sad truth is: this can "Harm a Mac".

The actual example given in the body for the question: this very likely did not harm the device. Granted.

But as a general answer to the question in the title: It depends on which type of Mac we are talking about. The advice and reasoning given so far on this question or in the comments is not universally true and can be quite dangerous! There is too much trust and faith put into the mere belief that Apple hardware is just the best there is.

It is simply not true that Apple designs now or designed in the past all its systems to really not harm themselves through overheating. While it is true that this should not happen it is also true that it does happen. And did:

The prime example for this are the MacBook Pros, especially those from 2010-2012. While the Intel chip that is primarily stressed with yes on all threads for a prolonged time will throttle down, will handle high temperatures quite well and even the OS will kick in and ramp up kernel_task to do nothing useful except help cool down the machine, the discrete chip on the same heatpipe is the vulnerable counterpart there.

Stressing these systems needlessly, like with yes, hastens the RadeonGate graphics chip failure. There are numerous examples of questions to the most heavily effected 2011 8,2 on this very web-site. This GPU failure is a thermal issue. There are even guides out there on how to kill the machine by simply running heavy 3D rendering or benchmarks for a time. These systems were advertised for but not suited for e.g. rendering or gaming. Class action lawsuits (only threatened) and Repair Extension Programs speak for themselves.