Bash: capture stdError from looped stream - bash

Question

I have a cpp executable (mycat) that continuously read a looped audio stream from the shared memory and pipes the data to stdOut and the metadata information to stdErr. mycat pipes out 12 lines for every entry of the audio stream, containing the metadata information, looking like this:
0x1 (TimeStamp) 12Bytes:2956 + 6793/(47999+1) (0.141521) delta= 0+ 1536/(47999+1) (0.032000) 2956.151418 -9.898ms 2016.04.04 16:06:37.700
0x4 (ReferenceTime) 12Bytes:2956 + 6156972/(26999999+1) (0.228036) delta= 0+ 1618519/(26999999+1) (0.059944) 2956.151426 76.610ms 2016.04.04 16:06:37.700
0x6 (ProcessDelay) 4Bytes: 64 (0x40)
0x7 (ClockAccuracy) 8Bytes: offset=0.000ppm (+-0.000ppm)
0xb (ClockId) 8Bytes: 01 00 00 00 42 22 01 00
0x20001 (SampleRate) 4Bytes: 48000 (0xbb80)
0x20002 (Channels) 4Bytes: 6 (0x6)
0x20003 (PcmLevel) 24Bytes: -21307 -20348 -31737 -42427 -28786 -26525
0x20004 (PcmPeak) 24Bytes: -14366 -13360 -25203 -39427 -19067 -21307
0x2000e (DolbyDpMetadata) 39352Bytes:
Linear Time: 2956 + 6793/(47999+1) (0.141521) delta= 0+ 1536/(47999+1) (0.032000)
2016.04.04 16:06:37.700 update: slot=0xe2840 validTo=0x3d1dd180 shmT=0x3d195200 (delta=294784) doffset=0xec2c0 msize=39552 dsize=18432 type=0x20001 (PCMS16) data bytes: df f4 f2 fc
What I want is a bash script that:
1) launch mycat eg. ./mycat shm_name > /dev/null.
2) reads stdErr from mycat till the 12th line no matter where it started.
2.1) Eventually store the 12 line into a variable (this is optional)
3) immedialy kills mycat after the 12th line, so that the bash script can continue without being annoyed by the outcoming stdError.
4) Read the value of the line "Channels" (in this case 6) and store it to a variable named "channels"
5) Read the value of the line "SampleRate" (in this case 48000) and store it the a variable named "rate"
is there a way to do it?

Answer 1

You can redirect stderr with mycat shm_name > /dev/null 2>/path/to/file. You can kill mycat with killall mycat when the time comes. For storing the variables, you want export channels =, same pattern with rate. You can find those with grep. I'm not sure how to wait for exactly twelve lines, though.

Answer 2

First, you need to redirect STDERR to STDOUT, and have STDOUT go to null.
mycat 2>&1 >/dev/null | parsing_script
Then you need a parsing script to collect your data
#!/bin/bash
declare -a data=()
count=1
while read line; do
data+=( "$line
")
((count++))
string=$(echo $line|sed 's/.*(\(.[A-Za-z]*\)).*/\1/')
case $string in
SampleRate) rate=$(echo $line|sed 's/.*: \(.[0-9]*\) .*/\1/')
;;
Channels) channels=$(echo $line|sed 's/.*: \(.[0-9]*\) .*/\1/')
;;
*) true;;
esac
if [[ $count -gt 12 ]]; then
break
fi
done <&0
echo $channels
echo $rate
I echo'd the values, but you can redirect them to a file, format them, etc. Also, ${data[#]} contains your metadata. Since you are piping, I'm reading in from STDIN, but it could be more robust if you want to tweak it.

Answer 3

I find out an elegant solution:
while read -r line; # Read mycat line by line
do
if echo "$line" | grep -qi "SampleRate"; then # Search for line containing string 'SampleRate'
rate="${line#*:}"; # Remove all charachters till ':'
rate="${rate%(*}"; # Remove all charachters from '('
rate="$(echo -e "${rate}" | tr -d '[[:space:]]')" # Remove all tralling withespace
elif echo "$line" | grep -qi "Channels"; then
channels="${line#*:}";
channels="${channels%(*}";
channels="$(echo -e "${channels}" | tr -d '[[:space:]]')"
fi
if [ -n "$channels" ] && [ -n "$rate" ]; then # If both variables 'channels' and 'rate' are not empty kill the while loop
break;
fi
done < <(./mycat $shm_name 2>&1 > /dev/null) # Mycat is processed into a while loop, standard Error is redirected to standard Output ( 2>&1 )
echo "Found datarate: $rate"
echo "Found channels: $channels"