function in loop corrupts every other iteration

Question

I made a short bash program to download podcasts and retrieve only last 20 seconds. Strange thing is it fails downloading every other iteration. There seems to be a problem with the function trim_nsec, because when I get rid of it in the loop, all the rest correctly works.

Edit : addition of double quotes, which doesn't solve the problem

<!-- language: lang-bash -->
#!/bin/bash

# Get podcast list
wget -O feed http://www.rtl.fr/podcast/on-n-est-pas-forcement-d-accord.xml

function trim_nsec () {
    # arguments : 1 : mp3file - 2 : duration - 3 : outputfile
    duration=$(ffprobe -i "${1}" -show_entries format=duration -v quiet -of csv="p=0")
    nth_second=$(echo "${duration} - ${2}"|bc)
    ffmpeg -i "${1}" -ss "${nth_second}" "${3}"
}

cpt=1

# let's work only on the 4th first files
grep -Po 'http[^<]*.mp3' feed|grep admedia| head -n 4 > list

cat list | while read i
do
    year=$(echo "$i" | cut -d"/" -f6)
    day=$(echo "$i" | cut -d"/" -f7)
    fullname=$(echo "$i" | awk -F"/" '{print $NF}')
    fullnameend=$(echo "$fullname" |sed -e 's/\.mp3$/_end\.mp3/')
    new_name=$(echo "$year"_"$day"_"$fullnameend")

    # let's download
    wget -O "$fullname" "$i"
    # let's trim last 20 sec
    trim_nsec "$fullname" 20 "$new_name"
    echo "$cpt file processed"
    #delete orig. file :
    rm "$fullname"
    ((cpt++)) 
done

Any idea ?

Use double quotes, there is most likely a problem with unexpected globbing. See: https://www.shellcheck.net/ — M. Becerra, Feb 09 '18 at 10:22
mmmh. even with alla double quotes required by shellcheck, it still fails every other iteration. — Louis D, Feb 09 '18 at 10:31
Please [edit] your question to reflect the addition of the double quotes — M. Becerra, Feb 09 '18 at 10:32

ewcz · Accepted Answer · 2018-02-09T11:29:45.403

The problem is most likely due to the fact that on errors, ffmpeg will try to get an input from user which will consume the input provided by cat list. See a similar question here or here. To prevent trim_nsec from consuming the input from cat list, you could do:

cat list | while read i
do
    year=$(echo "$i" | cut -d"/" -f6)
    day=$(echo "$i" | cut -d"/" -f7)
    fullname=$(echo "$i" | awk -F"/" '{print $NF}')
    fullnameend=$(echo "$fullname" |sed -e 's/\.mp3$/_end\.mp3/')
    new_name=$(echo "$year"_"$day"_"$fullnameend")

    # let's download
    wget -c -O "$fullname" "$i"

    # let's trim last 20 sec
    trim_nsec "$fullname" 20 "$new_name" <&3
    echo "$cpt file processed"
    #delete orig. file :
    #rm "$fullname"
    ((cpt++))

done 3<&1

Not writing to a temporary file which you then discard would also lose the silly `cat`. The `grep` can be reduced to a single command `grep -Pom 4 'http(?=[^<]*admedia)[^<]*\.mp3' feed | while read -r i; do ...` to actually make some use of the `-P` option which wasn't doing anything useful before. — tripleee, Feb 09 '18 at 11:37

function in loop corrupts every other iteration

1 Answers1