Linux 杂货摊

Quick Quiz 3: I typed the following command:

sudo echo 800000 > /sys/devices/system/cpu/cpu1/cpufreq/scaling_max_freq

Despite the sudo, I got “Permission denied”. Why doesn't sudo give me sufficient permissions?

Answer: Although that command does give echo sufficient permissions, the actual redirection is carried out by the parent shell process, which evidently does not have sufficient permissions to open the file for writing. One way to work around this is sudo bash followed by the echo, or to do something like:

sudo sh -c 'echo 800000 > /sys/devices/system/cpu/cpu1/cpufreq/scaling_max_freq'

Another approach is to use tee, for example:

echo 800000 | sudo tee /sys/devices/system/cpu/cpu1/cpufreq/scaling_max_freq

Yet another approach uses dd as follows:

echo 800000 | sudo dd of=/sys/devices/system/cpu/cpu1/cpufreq/scaling_max_freq