I am looping through a text file, using 'while read'. I read three variables from each line - two filenames and a decimal number. I know that this works in a vanilla setup (loop, read line, echo line, extract variables, nothing else), because I have tested it on my input file.
I am getting an odd result, though, once I add in the meat of my routine - which is a call to ffmpeg to merge the audio and video streams represented by the filenames. Successive 'read' operations omit leading characters from the line. I read here that calls for 'read' input within a 'while read' loop can cause problems, and wonder if the ffmpeg call is doing something that confuses bash, but this seems unlikely and bizarre! NB: I don't really understand the ffmpeg call - I got it from an answer to another question.
Any help would be appreciated!
The first 'read' works as expected (and the ffmpeg command completes):
LINE 1 vidsNeedingSound/448£generic@06_10_16-09_30_47.mp4 soundToAdd/448£generic@06_10_16-12_11_54.wav 6.988354 V: vidsNeedingSound/448£generic@06_10_16-09_30_47.mp4 A: soundToAdd/448£generic@06_10_16-12_11_54.wav D: 6.988354
The second 'read' loses 36 characters from the beginning of the line (and the ffmpeg command fails because the file represented by the variable V which takes the first item of the line doesn't point at a file):
LINE 2 6-09_30_47.mp4 soundToAdd/452£generic@06_10_16-12_11_54.wav 9.64663 V: 6-09_30_47.mp4 A: soundToAdd/452...
The third 'read' works as expected (and the ffmpeg command completes):
LINE 3 vidsNeedingSound/452£left@06_10_16-09_30_47.mp4 soundToAdd/452£left@06_10_16-12_11_54.wav 9.862118 V: vidsNeedingSound/452£left@06_10_16-09_30_47.mp4 A: soundToAdd/452...
The fourth 'read' loses 37 character (one more than the last fail) from the beginning of the line (and the ffmpeg command fails):
LINE 4 09_30_47.mp4 soundToAdd/452£right@06_10_16-12_11_54.wav 9.431392 V: 09_30_47.mp4 A: soundToAdd/452....
That's the pattern. When not preceded by a successful call to ffmpeg, the 'read' works as expected. After each successive successful ffmpeg call, characters are omitted from the beginning of the line: 36 characters to begin with, then 37, 38, 39...
The clincher is that when ffmpeg fails after a 'successful' read (due to another problem), the following 'read' works as expected.
CONCLUSION: a successful ffmpeg call results in a read fail on the next iteration through the loop.
Why does this happen, and how can I stop it?
Here's my code: ...
# Loop through lines in the merge file IT_COUNT=1 while read MERGE_LINE do echo "LINE " $IT_COUNT ((IT_COUNT+=1)) echo $MERGE_LINE # testing use read VID_FILE AUD_FILE DURATION <<< "$MERGE_LINE" # Get the data from each line echo "V: $VID_FILE A: $AUD_FILE D: $DURATION" # testing use .... # add audio to video ffmpeg -i $VID_FILE -i $AUD_FILE -filter_complex "aevalsrc=0:d=$AUD_SHIFT[s1];[s1][1:a]concat=n=2:v=0:a=1[aout]" -c:v copy -map 0:v -map [aout] $FILE_OUT -hide_banner done <$MERGE_FILE
Thanks for looking!