Bash variable truncated when passed into ffmpeg

Question

I have an album of 11 .flac audio files. (edit: since this issue has been resolved, it's now clear that the precise names and content of the files are irrelevant, so I've renamed them):

> find . -name "*.flac"
./two.flac
./ten.flac
./nine.flac
./eight.flac
./seven.flac
./three.flac
./four.flac
./five.flac
./one.flac
./eleven.flac
./six.flac

I was converting these to .wav files with a specific sample rate and bit depth. I used ffmpeg in a Bash shell to do this. A command like this one works perfectly for each of the 11 files if I call it manually:

ffmpeg -i "./six.flac" -sample_fmt s16 -ar 44100 "./six.wav"

However, when I wrote a while loop to run this command for each file:

find . -name "*.flac" | sed "s/\.flac$//" | while read f; do ffmpeg -i "$f.flac" -sample_fmt s16 -ar 44100 "$f.wav"; done

This worked, but only for 8/11 of the files, with ffmpeg giving the following error messages for the three failures:

/ten.flac: No such file or directory
ree.flac: No such file or directory
ix.flac: No such file or directory

For ./ten.flac, the leading . in the relative file path was truncated, resulting in an absolute path, and the other two, ./three.flac and ./six.flac, lose even more characters, including from the start of their basenames. Some of the other eight files had ./ truncated, but this resulted in the correct relative path, so ffmpeg was able to continue in these cases.

If I use the same loop structure, but with echo "$f" instead of calling ffmpeg, I see no problems with the output:

> find . -name "*.flac" | sed "s/\.flac$//" | while read f; do echo "$f"; done
./two
./ten
./nine
./seven
./three
./four
./five
./one
./eleven
./six

So I'm convinced the structure of the loop is fine, with "$f" expanding how I expect it to at each iteration. Somehow, when passing "$f.flac" into ffmpeg, part of the string is truncated, but only for some of the files.

I just want to understand why my first attempt exhibited this behaviour. I'm not looking for an alternative way to loop over or convert the files (my second attempt, with a different kind of loop, was successful for all files).

---

edit: I accidentally discovered that piping yes | into ffmpeg seems to fix this issue. I added this so I wouldn't be prompted to overwrite existing .wav files.

edit: Thanks @roaima for the explanation! Turns out that ffmpeg and read both inherit the same stdin handle, so ffmpeg could consume characters from the start of each line before read got a chance, thus mangling some of the file paths. This explains why yes | ffmpeg ... worked, since it gave ffmpeg a different stdin handle. The original loop works fine with ffmpeg -nostdin .... See http://mywiki.wooledge.org/BashFAQ/089.

Answer 1

Unless you have a directory tree of files (not mentioned, and not shown in your example dataset), you can use a simple loop to process the files

for f in *.flac
do
    w="${f%.flac}.wav"
    ffmpeg -i "$f" -sample_fmt s16 -ar 44100 "$w"
done

If you really do have a hierarchy of files you can incorporate this into a find search

find -type f -name '*.flac" -exec sh -c 'for f in "$@"; do w="${f%.flac}.wav"; ffmpeg -i "$f" -sample_fmt s16 -ar 44100 "$w"; done' _ {} +

For efficiency you might want to skip processing of any flac that already has a corresponding wav. After the w= assignment, add [ -s "$w" ] && continue. If you really don't want that then you can further optimise the command thus,

find -type f -name '*.flac" -exec sh -c 'ffmpeg -i "$1" -sample_fmt s16 -ar 44100 "${1%.flac}.wav";' _ {} \;

For pre-run testing, prefix ffmpeg with echo to see what would get executed without it actually doing so. (Quotes won't be shown, though, so bear that in mind.)

---

It turns out that the question is actually about ffmpeg chewing up the filenames it's supposed to be processing. This is because ffmpeg reads commands from stdin, and the list of files has been presented as a pipe to a while read … loop (also using stdin).

Solution: ffmpeg -nostdin … or ffmpeg … </dev/null

See

• Strange errors when using ffmpeg in a loop
• http://mywiki.wooledge.org/BashFAQ/089