Read line output in a shell script - linux

Question

I want to run a program (when executed it produces logdata) out of a shell script and write the output into a text file. I failed to do so :/
$prog is the executed prog -> socat /dev/ttyUSB0,b9600 STDOUT
$log/$FILE is just path to a .txt file
I had a Perl script to do this:
open (S,$prog) ||die "Cannot open $prog ($!)\n";
open (R,">>","$log") ||die "Cannot open logfile $log!\n";
while (<S>) {
my $date = localtime->strftime('%d.%m.%Y;%H:%M:%S;');
print "$date$_";
}
I tried to do this in a shell script like this
#!/bin/sh
FILE=/var/log/mylogfile.log
SOCAT=/usr/bin/socat
DEV=/dev/ttyUSB0
BAUD=,b9600
PROG=$SOCAT $DEV$BAUD STDOUT
exec 3<&0
exec 0<$PROG
while read -r line
do
DATE=`date +%d.%m.%Y;%H:%M:%S;`
echo $DATE$line >> $FILE
done
exec 0<&3
Doesn't work at all...
How do I read the output of that prog and pipe it into my text file using a shell script? What did I do wrong (if I didn't do everything wrong)?
Final code:
#!/bin/sh
FILE=/var/log/mylogfile.log
SOCAT=/usr/bin/socat
DEV=/dev/ttyUSB0
BAUD=,b9600
CMD="$SOCAT $DEV$BAUD STDOUT"
$CMD |
while read -r line
do
echo "$(date +'%d.%m.%Y;%H:%M:%S;')$line" >> $FILE
done

Answer 1

To read from a process, use process substitution
exec 0< <( $PROG )
/bin/sh doesn't support it, so use /bin/bash instead.
To assign several words to a variable, quote or backslash whitespace:
PROG="$SOCAT $DEV$BAUD STDOUT"
Semicolon is special in shell, quote it or backslash it:
DATE=$(date '+%d.%m.%Y;%H:%M:%S;')
Moreover, no exec's are needed:
while ...
...
done < <( $PROG )
You might even add > $FILE after done instead of adding each line separately to the file.

Answer 2

Original answer
You haven't shown the error messages — which would have been helpful.
Your problem, though, is probably this line:
DATE=`date +%d.%m.%Y;%H:%M:%S;`
where the semicolons mark the end of a command, and there likely isn't a command %H that does anything useful, etc.
You need quotes around the format argument to date, and I'd use single quotes for this job:
DATE=$(date +'%d.%m.%Y;%H:%M:%S;')
or even replace the two lines in the body of the loop with:
echo "$(date +'%d.%m.%Y;%H:%M:%S;')$line" >> $FILE
The double quotes prevent a variety of problems.
That assumes you fix a bunch of other problems, such as the setting of the variables FILE and prog. Also, I'd probably use:
exec > $FILE
to initially zap the output file and then all subsequent standard output would go to that file, so the echo line becomes:
echo "$(date +'%d.%m.%Y;%H:%M:%S;')$line"
Amended answer
The question was originally missing lots of key information. It eventually got updated to include the complete code.
The problem I identified originally remains an issue, but you weren't running into it because the input redirection was not working. If you want the input to come from a process, use a pipe, or possibly process substitution. However, note that you have #!/bin/sh as your shebang line, and /bin/sh won't recognized process substitution; either change the shebang or use the pipe notation. Note that process substitution has advantages if the loop is setting variables that need to be accessed after the loop is complete.
$SOCAT $DEV$BAUD STDOUT |
while read -r line
do
…
done
or
while read -r line
do
…
done < <($SOCAT $DEV$BAUD STDOUT)
Note that your code contains the line:
PROG=$SOCAT $DEV$BAUD STDOUT
This runs the command identified by $DEV$BAUD with the argument STDOUT and the environment variable PROG set to the value of $SOCAT. That is not what you wanted.
You could use an array:
PROG=($SOCAT $DEV$BAUD STDOUT)
and then run:
"${PROG[#]}"
either in the pipe line:
"${PROG[#]}" |
while read -r line
do
…
done
or with process substitution:
while read -r line
do
…
done < <("${PROG[#]}")
Note that unless there is code after the final exec 0<&3, there was no particular virtue in the redirections involving file descriptor 3. You should also close 3 when you're done with it:
exec 0<&3 3>&-
The 'final' code includes the lines:
CMD="$SOCAT $DEV$BAUD STDOUT"
$CMD |
while read -r line
This works OK because there are no spaces in the arguments to the command. That's a common case, but beware of spaces in arguments and file paths.