在shell命令中echo是什么意思？

Question

I have the following command:

echo "- - -" | sudo tee /sys/.../scan

How do I interpret this line of command? I couldn't find anything in Google because the characters in my line.

Thanks!

Answer 1

echo "- - -"

print three dashes and two spaces to the screen

|

a pipe - take output of the left-hand command (echo) and feed it as input to the right-hand command (sudo).

sudo

run a specified command with root privileges

tee

the specified command, which will be run as the root user. Tee accepts input from its stdin (being fed the echo output via the pipe), and splits it into two outputs: the first goes to the screen, so you see - - - printed in your terminal, and also outputs to a specified file, /sys/.../scan in this case.

Since you haven't provided the full path to the /sys file, can't tell WHICH scan it is, but generally speaking, this command is probably telling the OS to rescan some hardware bus for changes (additions/removals).

Answer 2

Check the first answer here:

https://askubuntu.com/questions/192461/ubuntu-server-12-04-hotplug-sata-automount

From the link: On Linux, you can tell the system to rescan an individual SATA port.So to rescan the SATA3 port

Answer 3

echo "- - -" simply outputs - - - to the standard output. The tee command (whose name comes from the plumbing T-pipe) splits the output two ways: once to its own standard output, and also to the specified file. sudo serves to run tee as the root user, in order to be able to write to the system file in /sys .

To recap, echo "- - -" | sudo tee some_file echo "- - -" | sudo tee some_file will:

1. write - - - to some_file , as the root user.

2. repeat the same output to standard output, presumably for logging/debugging purposes.