To find the revolutions per minute (RPM) speed of a hard disk from a shell prompt on a Linux system, you can use the hdparm command. This command is used to get and set various hard disk parameters, including the RPM speed.
To use hdparm, you can run the command with the -I flag followed by the device name of the hard disk. For example:
hdparm -I /dev/sda
This will output detailed information about the hard disk, including the model number, serial number, firmware version, and other information. The RPM speed of the hard disk will be listed under the "Rotation Rate" field.
Keep in mind that the hdparm command may not work for all hard disks, particularly if the disk is a solid-state drive (SSD) or a virtual disk. In these cases, the RPM speed may not be relevant or may not be reported accurately.