leaking memory with shell script and tty

Question

I'm running a shell script on a raspberry pi to communicate with a arduino. Every hour my ram gets used up by about 50 megabytes more.

The script itself just connects to ttyACM0 (the arduino's usb terminal) and then sends a char to it. The script runs once per minute to check the Temps. Values for the first argument can be: 'a', 'b' or 'T'. In the case of 'a' or 'b' it switches on/off a relay. In the case of 'T' the arduino returns 3 temperature values, which I store.

The script runs fine (I can control the relay and receive values, without the arduinos serial getting restarted). but I'm having issues because of my limited memory.

I don't have any other processes running on the device and already tried a fresh raspbian setup.

Can somebody tell me where and if there are memory leaks in this script and how I can prevent them?

EDIT: I found the culprit: the script runs hundreds of 'cat' commands with time, how do I get rid of them? I already tried killall cat, but this resets my raspberrys serial connection (I want to prohibit that, because it 'restarts' the arduino!) @mikeserv pointed me towards using head, which automatically quits after reading a defined number of lines, unfortunately that doesn't work either. And I can't seem to get the temperatures written in my output file without a pipe

head -n3 <&3 >>/home/pi/output

doesn't work as it's not exiting too and I don't get any output

I could kill all cat's every few minutes to free the mem, but this also resets my tty to arduino (so it reboots and loses the state for the relay)

Edit2: I did not manage to get this working (I tried several possibilities including minicom, screen etc...) but the receiving part is where I have trouble. Sending chars to the arduino works fine though!

#!/bin/bash

# READ / WRITE ARDUINO

exec 3<> /dev/ttyACM0

echo "connected, sleep for 1 sec..."
sleep 1

echo "send $1..."

echo "$1" >&3

if [ "$1" = "T" ]
then
        cat <&3 | cat >> /home/pi/output &2>1
else
        echo "nothing to save"
fi

echo "closing.."

exec 3>&-

exit 0

Answer 1

When you:

exec 3<> /dev/ttyACM0

... you open a read/write file-descriptor to the usb serial tty on 3 which is automatically inherited by children - such as cloned subshells - and this is why you can read it with cat <&3 later in your backgrounded pipeline.

The thing is, though, because you background the pipeline - which puts it and all associated subshells in a separate process group - it is not closed for the pipeline processes later when you...

exec 3<&-

...and close that descriptor for the script's current shell processes. Instead...

cat </dev/ttyACM0 | cat >> file &

...hangs in the background, reading nothing for as long as there is nothing to read, and keeping an open line on that tty the whole time. This is because cat only quits its input on EOF, which, in this case, it never receives.

You can do:

cat /dev/tty

... at your prompt to approximate that behavior.

What you should be doing is explicitly quitting the input as soon as you have read as much as you need, or else explicitly killing cat after same. You might do this like:

head -n"$GUARANTEED_NUM_AVAIL_INPUT LINES" <&3 >>file

... or with sed [num]q or similar. In this way you should avoid having to |pipe at all, and likely can omit the &backgrounding entirely. Else to kill cat you might do:

cat <&3 >>file &
sleep 1 && kill "$!"

...but probably that is not necessary, as quitting the input is far more simple.

As an aside, it is worth noting that your script is already composed entirely of portable syntax, and so it is probably worth your while to alter the #!/bin/bash line and instead invoke a lighter-weight - and likely faster - shell instead. I recommend dash for something like this. Have a look here if interested in this line of thinking for a q&a on performance comparisons between various shells - to include both bash and dash.

Answer 2

I can't write you a fix, but I can explain what's happening.

1. You're opening the Port.
2. Your checking/sending values from the temperature sensor.
3. You're closing the port.

Some possibilities:

• The USB Buffer from the Arduino is Filling until you rerun the script, but the buffer is still full from the last run. You'r inadvertently causing a buffer overflow in your Comm Port, as the amounts of previous runs pile up in the buffer.
• The same issue is occurring in the Raspberry Pi, as the buffer fills in the same way from the other direction.

---

You need to find a way to clear the buffer on both sides so that the gate opens, sends 3, and closes, opens, sends 3, and closes etc. Subshelling, as Yeti suggests is one way. I'm sure there are other, such as closing and reopening the buffer but not killing the CommPort. Since I'm no expert on Comm Drivers, I can only offer a theory.

Update

If [ "$1" = "T" ]
then
        cat <&3 | cat >> /home/pi/output &2>1
        cat <&3 | cat >> /dev/null
else
        echo "nothing to save"
        cat <&3 | cat >> /dev/null
fi