bash 通过shell脚本控制串口
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Serial port control through a shell script
提问by Adriano Carvalho
I am developing an orientation controller. I have a development board which communicates with the sensor (a compass) through I2C. Because the board is pretty limited (no OS), I developed a simple program to receive things like: (1) 'get 0' to read the sensor's register 0; (2) 'set 0 10' to set the sensor's register 0 with the value 10. For each of these cases the board returns: (1) 'Done: 10.' (register 0 has the value 10); (2) 'Done.'; and (3) 'error: ...' in case of error. With this, I am trying to develop a shell script (bash) to send commands and retrieve data in order to understand the sensor and develop the controller.
我正在开发一个方向控制器。我有一个通过 I2C 与传感器（指南针）通信的开发板。因为板子非常有限（没有操作系统），我开发了一个简单的程序来接收以下内容：（1）'get 0' 读取传感器的寄存器 0；(2) 'set 0 10' 将传感器的寄存器 0 设置为值 10。对于这些情况中的每一种，电路板都会返回： (1) 'Done: 10'。（寄存器 0 的值为 10）；(2) '完成。'; 和 (3) 'error: ...' 以防出错。有了这个，我正在尝试开发一个 shell 脚本 (bash) 来发送命令和检索数据，以便了解传感器并开发控制器。
My problem is with the following code:
我的问题是以下代码：
# read device output in the background.
head -n 1 /dev/ttyUSB0 &
head=$!
# (#1): without the following stmt I get:
# head: cannot open `/dev/ttyUSB0' for reading: : Protocol error
sleep 0.1
# send command to the device.
echo "get 0" > /dev/ttyUSB0
# (#2) wait for head.
while kill -0 $head 2>/dev/null ; do : ; done
I guess (#1) is caused by a read/write conflict between 'head' and 'echo', but I don't know why and I have no idea on how to solve it.
我猜（#1）是由“head”和“echo”之间的读/写冲突引起的，但我不知道为什么，也不知道如何解决。
Another issue is in (#2) where I would like to use a timeout. I've tried something like:
另一个问题是在 (#2) 中，我想使用超时。我试过这样的事情：
timeout 1 bash -c "while kill -0 $head 2>/dev/null ; do : ; done"
But I get: Timeout: aborting command ``bash'' with signal 9and the program gets stuck.
但我明白了：Timeout: aborting command ``bash'' with signal 9程序卡住了。
By the way, before the code above is executed I do initialize the serial port with:
顺便说一下，在执行上面的代码之前，我会使用以下命令初始化串行端口：
stty -F /dev/ttyUSB0 9600 cs8 -cstopb
EDIT: I don't want an interactive terminal. I want to use this routine as necessary. This routine is the necessary foundation of the controller (read/write sensor's registers) which later will be implemented in the board.
编辑：我不想要交互式终端。我想在必要时使用这个例程。该例程是控制器（读/写传感器的寄存器）的必要基础，稍后将在板上实现。
回答by Adriano Carvalho
To solve (#1) I modified the routine to use a fd:
为了解决（#1），我修改了例程以使用 fd：
# : the device filename, eg. /dev/ttyS0
# : number of lines to read before exit.
exec 3<>
head -n "" 0<&3 &
wait_pid=$!
cat - 1>&3
wait $wait_pid
exec 3>&-
EDIT: To solve (#2), instead of providing the routine with timeout support I delegate that responsibility to the caller. However, in case of timeout we need to clean up. For that I've added the following after wait_pid=$!:
编辑：为了解决（#2），我没有为例程提供超时支持，而是将该责任委托给调用者。但是，如果超时，我们需要清理。为此，我在之后添加了以下内容wait_pid=$!：
trap="if kill -0 $wait_pid ; then kill -TERM $wait_pid ; fi"
trap "$trap" SIGINT SIGKILL SIGTERM
