using if statement directly in bash prompt?

Question

I am not sure if this is even possible directly. But I am trying to get the current temperature of the CPU using:

cpu=$(</sys/class/thermal/thermal_zone0/temp)

I want to pipe the output of the same command using echo to a if statement to carry out a shutdown if the temperature is higher then 70000. (Output of the command needs to be divided by 1000 to temperature in centigrade).

something like

cpu=$(</sys/class/thermal/thermal_zone0/temp) | if $cpu -gt 70000 then shutdown fi

I am trying to do this to stop my pi from overheating. I will schedule a cronjob that checks every 15 minutes for CPU temperature of my Pi and shut it down if it is over 70.

Thank you.

Answer 1

Your last command is almost correct, but the syntax is wrong. Corrected version:

cpu=$(</sys/class/thermal/thermal_zone0/temp); if [ "$cpu" -gt 70000 ]; then shutdown; fi

Or, shorter,

cpu=$(</sys/class/thermal/thermal_zone0/temp); [ "$cpu" -gt 70000 ] && shutdown

Or, shorter,

[ "$(</sys/class/thermal/thermal_zone0/temp)" -gt 70000 ] && shutdown

The issue with your command (apart from the syntax of the if-statement) is that you use a pipeline. The right hand side of a pipeline (your if statement) should read from the left hand side of the pipeline (your variable assignment). A variable assignment does not produce any output, and your if-statement does not consume any input. Furthermore, the value of $cpu is unknown on the right hand side of the pipe as the two parts of the pipeline are executed concurrently.

For a cron job, the following may be a better formulation of the command:

[ "$(</sys/class/thermal/thermal_zone0/temp)" -le 70000 ] || shutdown

This ensures that the command exits successfully even if the temperature is low. The [ ... -gt 70000 ] && shutdown variation would return a non-zero exit from the [ ... ] test for low temperatures, which would make cron think the job failed.

Note also that this requires that the crontab is using bash as the shell to run the jobs, or the $(<...) syntax may not work properly. You may change $(<...) to $(cat ...) or you may set SHELL=/bin/bash at the top of the crontab. Also, to run shutdown, you likely need root privileges.