Bash, serial I/O and Arduino

arduinobashcommand lineserial-port

So, I'm in a bit over my head, and I feel like I'm very close to a solution, but it's just not working quite yet. Here's my situation:

I'm working with an Arduino microcontroller, and I'm attempting to write two Bash scripts (right now running in Mac OS X 10.6) which will (a) print all serial data coming out of the Arduino unit to the standard output, and (b) allow me to send serial data to the Arduino unit. These scripts will then be called using Adobe AIR's NativeProcess API to allow a tight integration between the Arduino unit and a Flex Adobe AIR application.

My two scripts are very simple –

Here's my WriteToSerial.sh script:

echo $1 > $2

($1 is obviously my string, $2 is the location of the serial port – currently /dev/tty.usbserial-A800eIUj)

And here's my ReadSerialOutput.sh script:

tail -f $1

($1 is the location of my serial port, currently /dev/tty.usbserial-A800eIUj)

When I call either of these scripts (or even if I just type the commands directly into the Bash console), my computer just hangs – I can type characters, but nothing happens until I Ctrl + C out of the process.

However, if I open the Arduino IDE and turn on the Serial Monitor, then tail -f the port, close the serial monitor, and then echo "test" > serial port, everything works just great.

This suggests to me that opening the Serial Monitor within the Arduino IDE is somehow initializing the serial port, which in turn allows me to tail it with no problem. This in turn suggests to me that I'm simply failing to input some sort of initialization command. However, I've been searching high and low for days and can't seem to find anything that addresses this issue.

What is the solution?

Best Answer

I get the same problem too. I use Arduino Uno with Ubuntu 12.04. After a few hours of searching and trying, I find out that Arduino will reset when the serial device is opened for the first time，but will not reset when the serial device is opened again.

So, run command - echo "input string" > /dev/ttyXXX in bash will reset Arduino and send "input string" immediately. Arduino need take some time to initialize, and is not quick enough to receive this string. cat /dev/ttyXXX will reset Arduino too.

When /dev/ttyXXX is opened in somewhere firstly, these commands will work.

Here is my solution:

1) open /dev/ttyXXX by redirecting /dev/ttyXXX to file description 3

exec 3<> /dev/ttyXXX

2) wait for Arduino's initialization

sleep 1

3) communicate with Arduino

echo "input something" >&3

cat <&3

4) close /dev/ttyXXX

exec 3>&-

Related Solutions

Bash – How to check if a directory exists in a Bash shell script

To check if a directory exists in a shell script, you can use the following:

if [ -d "$DIRECTORY" ]; then
  # Control will enter here if $DIRECTORY exists.
fi

Or to check if a directory doesn't exist:

if [ ! -d "$DIRECTORY" ]; then
  # Control will enter here if $DIRECTORY doesn't exist.
fi

However, as Jon Ericson points out, subsequent commands may not work as intended if you do not take into account that a symbolic link to a directory will also pass this check. E.g. running this:

ln -s "$ACTUAL_DIR" "$SYMLINK"
if [ -d "$SYMLINK" ]; then 
  rmdir "$SYMLINK" 
fi

Will produce the error message:

rmdir: failed to remove `symlink': Not a directory

So symbolic links may have to be treated differently, if subsequent commands expect directories:

if [ -d "$LINK_OR_DIR" ]; then 
  if [ -L "$LINK_OR_DIR" ]; then
    # It is a symlink!
    # Symbolic link specific commands go here.
    rm "$LINK_OR_DIR"
  else
    # It's a directory!
    # Directory command goes here.
    rmdir "$LINK_OR_DIR"
  fi
fi

Take particular note of the double-quotes used to wrap the variables. The reason for this is explained by 8jean in another answer.

If the variables contain spaces or other unusual characters it will probably cause the script to fail.

Bash – How to get the source directory of a Bash script from within the script itself

#!/usr/bin/env bash

SCRIPT_DIR="$( cd -- "$( dirname -- "${BASH_SOURCE[0]}" )" &> /dev/null && pwd )"

is a useful one-liner which will give you the full directory name of the script no matter where it is being called from.

It will work as long as the last component of the path used to find the script is not a symlink (directory links are OK). If you also want to resolve any links to the script itself, you need a multi-line solution:

#!/usr/bin/env bash

SOURCE="${BASH_SOURCE[0]}"
while [ -h "$SOURCE" ]; do # resolve $SOURCE until the file is no longer a symlink
  DIR="$( cd -P "$( dirname "$SOURCE" )" >/dev/null 2>&1 && pwd )"
  SOURCE="$(readlink "$SOURCE")"
  [[ $SOURCE != /* ]] && SOURCE="$DIR/$SOURCE" # if $SOURCE was a relative symlink, we need to resolve it relative to the path where the symlink file was located
done
DIR="$( cd -P "$( dirname "$SOURCE" )" >/dev/null 2>&1 && pwd )"

This last one will work with any combination of aliases, source, bash -c, symlinks, etc.

Beware: if you cd to a different directory before running this snippet, the result may be incorrect!

Also, watch out for $CDPATH gotchas, and stderr output side effects if the user has smartly overridden cd to redirect output to stderr instead (including escape sequences, such as when calling update_terminal_cwd >&2 on Mac). Adding >/dev/null 2>&1 at the end of your cd command will take care of both possibilities.

To understand how it works, try running this more verbose form:

#!/usr/bin/env bash

SOURCE="${BASH_SOURCE[0]}"
while [ -h "$SOURCE" ]; do # resolve $SOURCE until the file is no longer a symlink
  TARGET="$(readlink "$SOURCE")"
  if [[ $TARGET == /* ]]; then
    echo "SOURCE '$SOURCE' is an absolute symlink to '$TARGET'"
    SOURCE="$TARGET"
  else
    DIR="$( dirname "$SOURCE" )"
    echo "SOURCE '$SOURCE' is a relative symlink to '$TARGET' (relative to '$DIR')"
    SOURCE="$DIR/$TARGET" # if $SOURCE was a relative symlink, we need to resolve it relative to the path where the symlink file was located
  fi
done
echo "SOURCE is '$SOURCE'"
RDIR="$( dirname "$SOURCE" )"
DIR="$( cd -P "$( dirname "$SOURCE" )" >/dev/null 2>&1 && pwd )"
if [ "$DIR" != "$RDIR" ]; then
  echo "DIR '$RDIR' resolves to '$DIR'"
fi
echo "DIR is '$DIR'"

And it will print something like:

SOURCE './scriptdir.sh' is a relative symlink to 'sym2/scriptdir.sh' (relative to '.')
SOURCE is './sym2/scriptdir.sh'
DIR './sym2' resolves to '/home/ubuntu/dotfiles/fo fo/real/real1/real2'
DIR is '/home/ubuntu/dotfiles/fo fo/real/real1/real2'

Best Answer

Related Solutions

Bash – How to check if a directory exists in a Bash shell script

Bash – How to get the source directory of a Bash script from within the script itself

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