The normal text output of ffprobe goes to stderr, so you usually need to add 2>&1 to parse the output. The alternative is to use the complex ffprobe options to format the output which then goes to stdout.

ffprobe "$in" 2>&1 | perl -nle '/^\s+Stream #(\d+:\d+): Audio/ && print $1'

with codec and number of channels:

ffprobe "$in" 2>&1 | perl -nle '/^\s+Stream #(\d+:\d+): Audio: (\S+).*(\d+)\s+channels/ && print join("\t", $1, $2, $3)'

or to have the tracks in a Bash array:

audio_tracks=( $(ffprobe "$in" 2>&1 | perl -nle '/^\s+Stream #(\d+:\d+): Audio/ && print $1') )

or the "official" way:

ffprobe -select_streams a -show_entries stream=index -of csv=p=0 "$in"

ffprobe -select_streams a -show_entries stream=index,codec_name,channels -of csv=p=0 "$in"

audio_tracks=( $(ffprobe -select_streams a -show_entries stream=index -of csv=p=0 "$in") )

For example to detect silent tracks/channels

ffmpeg -i "$in" -map 0:a -af "astats=measure_overall=none:measure_perchannel=Max_level" -f null -

for track in $(ffprobe -v error -select_streams a -show_entries stream=index -of csv=p=0 "$in"); do \
  ffmpeg -i "$in" -vn -map 0:$track -c:a copy -f md5 - 2>/dev/null \
  | while read md5; do \
      printf "%32s  Audio Track %2d %s\n" $md5 $track "$in"; \
    done; \
done

ffprobe -v error -show_entries stream=index,codec_name:stream_tags=timecode -of csv=p=0 "$in"

1. sections

Print sections structure and section information, and exit. The output is not meant to be parsed by a machine.

1. show_data_hash algorithm

Show a hash of payload data, for packets with -show_packets and for codec extradata with -show_streams.