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Comment #1 posted on 2018-09-30 12:51:25 by Ken Fallon

The loop issue

This fails

ls *.mp3|while read i;do ffmpeg -i "${i}" "${i}.wav" 2>&1;done


This works

for i in *.mp3;do ffmpeg -i "${i}" "${i}.wav" 2>&1;done
,/pre>

Comment #2 posted on 2018-09-30 14:00:47 by Dave Morriss

Re: The loop issue

ls *.mp3|while read i;do ffmpeg -i "${i}" "${i}.wav" 2>&1;done

You don't say how this fails, but there are several reasons not to do things this way:

1. It's unwise to feed a 'while' loop thorough a pipe because the 'while' runs
in a separate shell which can lead to problems

2. Never use 'ls' to get a list of files for consumption in a script. Unless
you can be completely sure that the 'ls' you are using isn't adding suffixes
like '@' for links, and '/' for directories and isn't adding colour codes to
the names, don't do it. Much better to use 'find'.

My test with this pipeline returned colour codes which 'ffmpeg' didn't like,
and it failed that way.

for i in *.mp3;do ffmpeg -i "${i}" "${i}.wav" 2>&1;done

This doesn't use 'ls' it simply uses file expansion therefore no additional
filename garbage!

Did I mention: don't use 'ls' as a way of feeding filenames to a loop or
whatever?

The first example would have worked if you'd written:

while read i;do ffmpeg -i "${i}" "${i}.wav" 2>&1;done < <(find . -maxdepth 1 -name "*.mp3")

The '-maxdepth 1' option prevents 'find' from going into sub-directories. The
'find' is inside a process substitution which is redirected to the 'while' so
the 'read' inside it can obtain what is produced on its STDIN channel.

Also, if it had been me I'd have written:

ffmpeg -i "${i}" "${i}.wav"

as:

ffmpeg -i "${i}" "${i%mp3}wav"

to avoid the output files being called 'xxx.mp3.wav'. I spoke about this in my
"Bash Tips" show https://hackerpublicradio.org/eps.php?id=1648

Dave

Comment #3 posted on 2018-09-30 14:13:58 by Ken Fallon

Clarify

I'm not sure you're cleare enough about using ls.

It fails as it only does one mp3, while while does them all.

Comment #4 posted on 2018-09-30 15:02:06 by Dave Morriss

Re: Clarify

I think the thing to take away is: Don't use 'ls' for this purpose. I might have said that before ;-)

One way to debug your problem (should you feel that avoiding 'ls' is not enough) might be this:

1. Create a function to display the arguments:

_ffmpeg() { printf "ffmpeg %s %s %sn" "${@}"; }

2. Run your pipeline thus:

ls *.mp3|while read i;do _ffmpeg -i "${i}" "${i}.wav" 2>&1;done

I tested things like this:

$ cd /tmp
$ touch {a..f}.mp3
$ ls *.mp3|while read i;do _ffmpeg -i "${i}" "${i}.wav" 2>&1;done
ffmpeg -i a.mp3 a.mp3.wav
ffmpeg -i b.mp3 b.mp3.wav
ffmpeg -i c.mp3 c.mp3.wav
ffmpeg -i d.mp3 d.mp3.wav
ffmpeg -i e.mp3 e.mp3.wav
ffmpeg -i f.mp3 f.mp3.wav

The names like 'a.mp3' are all coloured blue.

If I use the real 'ffmpeg' I get (output heavily truncated with only one file shown):

ffmpeg version 4.0.2-2 Copyright (c) 2000-2018 the FFmpeg developers
built with gcc 8 (Debian 8.2.0-7)
configuration: --prefix=/usr --extra-version=2 --toolchain=hardened

[snip]

libswresample 3. 1.100 / 3. 1.100
libpostproc 55. 1.100 / 55. 1.100
?[0m?[00;36ma.mp3?[0m: No such file or directory

The codes before and after 'a.mp3' are colour on/off codes.

Your environment will certainly be different of course, so your failures may not be the same.

Clear?

Comment #5 posted on 2018-09-30 15:08:27 by Dave Morriss

Does the comment system remove backslashes?

I actually wrote:

_ffmpeg() { printf "ffmpeg %s %s %s\n" "${@}"; }

but something removed the backslash.

Comment #6 posted on 2018-10-01 11:03:53 by folky

Material for a show

@Ken and @Dave
Take your comments and make a collaborative show of the material ;-)

Comment #7 posted on 2018-10-01 12:16:59 by Ken Fallon

touché Sir

touché

Comment #8 posted on 2018-10-01 14:59:05 by clacke

subshell issues

As Dave points out, the while loop you describe would work if it weren't for ls issues. Here's what doesn't work:

items=()
produce_items | while read item; items+=( "$item" ); done
do_stuff_with "${items[@]}"

Oh, how many times I have made this mistake.

"items" gets updated just fine, in a subshell, and then after the pipe has finished executing, execution continues in the parent shell where the array is still empty.

Null-terminating and giving "read" the appropriate parameters is an HPR episode of its own, no doubt already made by Dave, or in his pipeline. ;-)

Comment #9 posted on 2018-10-01 15:00:26 by clacke

Kvalificerat hemligt

Excellent pronuncation, Dave! You're spot on.

Compared to "kvalificerat hemligt", "skeptikerpodden" is trivial to say. :-)

Comment #10 posted on 2018-10-01 15:34:27 by clacke

Re: Intro volume

While it is true that we hosts could do more to manage our sound levels, the facts on the ground are that the intro music is louder than the average show.

I have thought often that the intro volume should be a bit lower, but never said anything.

Comment #11 posted on 2018-10-01 15:36:05 by Dave Morriss

She sells subshells...

I think I have to do a show on the whole issue of loops in pipelines. It's been in the "topics to cover" heap for a while but now it's being re-prioritised! I'll probably make it show 14 in the "Bash Tips" subset.

Comment #12 posted on 2018-10-01 15:39:43 by clacke

Re: TTS over intro music

We could still allow people to add arbitrary intros, and just not do TTS-over-intro in those cases. But the idea to do TTS-over-intro on a list of prepared flexible-length intros is really cool.

As for me though, as you have noticed on my shows, I really like having a standardized intro as the unified HPR brand, while allowing some variation at the end.

Comment #13 posted on 2018-10-01 16:43:35 by clacke

That brings back memories

I never listen to my own shows via the feed, so I never listen to my whistling outro either.

As I hear it now, man does that bring back memories. I remember exactly where I was walking on the way to my bus to work as I was recording it. That was three homes, one country, three offices, three kindergartens and three years ago.

I can feel the chill from the November morning fog around Järfälla Church on my cheeks as I'm typing this.

I know that I accidentally set the sample rate too low when I recorded it (11.25 kHz, I believe), but hearing it now that sound quality is worse than I remember it. :-)

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