add done: 20200608 A secure and private open source alternative to Alexa.md

sources/tech/20200608 Use this script to find a Raspberry Pi on your network.md

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+[#]: collector: (lujun9972)
+[#]: translator: ( )
+[#]: reviewer: ( )
+[#]: publisher: ( )
+[#]: url: ( )
+[#]: subject: (Use this script to find a Raspberry Pi on your network)
+[#]: via: (https://opensource.com/article/20/6/find-raspberry-pi)
+[#]: author: (Chris Collins https://opensource.com/users/clcollins)
+
+Use this script to find a Raspberry Pi on your network
+======
+Identify a specific Raspberry Pi in your cluster with a script that
+triggers an LED to flash.
+![Raspberries with pi symbol overlay][1]
+
+We've all been there. "I'm going to get this [Raspberry Pi][2] to try out. They look kinda cool." And then, like tribbles on an Enterprise, suddenly you have [Kubernetes clusters][3] and [NFS servers][4] and [Tor proxies][5]. Maybe even a [hotel booking system][6]!
+
+Pis cover the desk. They spill out onto the floor. Carrier boards for Raspberry Pi compute modules installed into lunchboxes litter the shelves.
+
+…or maybe that's just me?
+
+I'll bet if you have one Raspberry Pi, you've got _at least_ two others, though, and gosh darn it, they all look the same.
+
+This was the situation I found myself in recently while testing a network filesystem (NFS) server I set up on one of my Raspberry Pis. I needed to plug in a USB hard drive, but … to which one? Ol' Lingonberry Pi was the chosen host, and I was SSH'd into her, but which actual, _physical_ RPi was she? There was no way of knowing…
+
+Or was there?
+
+![Raspberry Pis stacked up in cluster cases][7]
+
+So, so many Raspberry Pis. Which one is Lingonberry? (Chris Collins, [CC BY-SA 4.0][8])
+
+At a previous job, I sometimes worked on servers in our data centers, and some of them had a neat feature: an ID button on the front of the server that, when pressed, started an LED flashing on the front and back of the server. If I needed to deal with the other side of the server, I could press the ID button, then walk _allllll_ the way around to the other side of the rack, and easily find the right server.
+
+I needed something like this to find Lingonberry.
+
+There aren't any buttons on the Pis, but there are LEDs, and after a quick Google search, I learned that [one of them is _controllable_][9]. _Cue maniacal laughter._
+
+There are three important bits to know. First, the LED path: on Raspberry Pis, at least those running Ubuntu 20.04, the front (and user-controllable) LED is found at `/sys/class/leds/led0`. If you navigate to it, you'll find it is a symlink to a directory that has a number of files in it. The two important files are `trigger` and `brightness`.
+
+The `trigger` file controls what lights up the LED. If you `cat` that file, you will find a list:
+
+```
+none usb-gadget usb-host rc-feedback rfkill-any
+rfkill-none kbd-scrolllock kbd-numlock kbd-capslock
+kbd-kanalock kbd-shiftlock kbd-altgrlock kbd-ctrllock
+kbd-altlock kbd-shiftllock kbd-shiftrlock kbd-ctrlllock
+kbd-ctrlrlock timer oneshot disk-activity disk-read
+disk-write ide-disk mtd nand-disk heartbeat backlight
+gpio cpu cpu0 cpu1 cpu2 cpu3 default-on input panic
+mmc1 [mmc0] bluetooth-power rfkill0
+unimac-mdio--19:01:link unimac-mdio--19:01:1Gbps
+unimac-mdio--19:01:100Mbps unimac-mdio--19:01:10Mbps
+```
+
+
+    The item in brackets indicates what triggers the LED; in the example above, it's [mmc0]—the disk activity for when the SD card plugged into the Raspberry Pi. The trigger file isn't a normal file, though. Rather than editing it directly, you change the trigger by echoing one of the triggers into the file.
+
+
+    To identify Lingonberry, I needed to temporarily disable the [mmc0] trigger, so I could make the LED work how I wanted it to work. In the script, I disabled all the triggers by echoing "none" into the trigger file:
+
+    [code]
+
+```
+# You must be root to do this
+$ echo none >trigger
+
+$ cat trigger
+[none] usb-gadget usb-host rc-feedback rfkill-any rfkill-none kbd-scrolllock kbd-numlock kbd-capslock kbd-kanalock kbd-shiftlock kbd-altgrlock kbd-ctrllock kbd-altlock kbd-shiftllock kbd-shiftrlock kbd-ctrlllock kbd-ctrlrlock timer oneshot disk-activity disk-read disk-write ide-disk mtd nand-disk heartbeat backlight gpio cpu cpu0 cpu1 cpu2 cpu3 default-on input panic mmc1 mmc0 bluetooth-power rfkill0 unimac-mdio--19:01:link unimac-mdio--19:01:1Gbps unimac-mdio--19:01:100Mbps unimac-mdio--19:01:10Mbps
+```
+
+    
+```
+In the contents of the trigger file above, you can see [none] is now the selected trigger. Now the LED is off and not flashing.
+
+
+Next up is the brightness file. You can control whether the LED is on (1) or off (0) by echoing either 0 or 1 into the file. Alternating