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I was playing with the raspberry pi leds (How do I control the system LEDs using my software?)
Interestingly, on my pi the following command fails:
sudo echo heartbeat >/sys/class/leds/led0/trigger
The error is bash: /sys/class/leds/led0/trigger: Permission denied
However if I run the following pair of commands things work:
sudo -s
echo heartbeat >/sys/class/leds/led0/trigger
I would have thought the two ways of triggering the LEDs are the same. Are there some things sudo cannot do?
The shell interprets and handles redirection before the command is executed. So the redirection (>/sys/class/leds/led0/trigger) is attempted with the user's permissions, thus fails.
The generally recognized solution is to use the tee command: (man page and wikipedia)
echo heartbeat | sudo tee /sys/class/leds/led0/trigger >/dev/null
The tee command splits (tees!) its input into two streams, one going to a file specified on the command line, the other to stdout. In this example, I've used the ability to specify a filename to write to that file with sudo privileges, and discard the duplicate output going to stdout.
As explained by lornix it's because the shell handles redirection before the command is executed.
I prefer this type of invocation
sudo sh -c "echo 4 >/sys/class/gpio/export"
In your case
sudo echo heartbeat >/sys/class/leds/led0/trigger
would become
sudo sh -c "echo heartbeat >/sys/class/leds/led0/trigger"
