shell script strangeness

jalla · Feb 8, 2010

I made a small script to convert some mpegs to a more portable format

Code:

#!/bin/sh

usage () {
	echo "usage: $0 [-f file] [-dvh]"
}

help () {
	usage
	echo " -f <file>
 -d     dryrun
 -v     verbose
 -h     help
"
exit
}
trap "exit 1" 0 1 2 15

system () {
  test -z $verbose || echo "+ $*" 1>&2
  test -z $dryrun || return
  eval "$@"
}
while [ x"$1" != x"" ]
do
	case  $1 in
		-f)             file=$2; shift;;
		-v)             verbose=1;;
		-d)             dryrun=1;;
		-h)             help;exit;;
	esac
	shift
done

test -z "$file" && help;
test -f "$file" || help

grep -v '^#' $file | \
while read a b 
do
 	cmd="fmpeg -i $a -ac 2 -ar 48000 -b 1000k -s cif $b.m4v"
	system $cmd
done

I feed the script with a file like this

Code:

gong:/medialib/tc# cat infile.txt
#/medialib/teapot/3150_20091012215900.mpg file-1
/medialib/teapot/2150_20091026215900.mpg file-2
/medialib/teapot/3150_20091102215900.mpg file-3
/medialib/teapot/3150_20091110215900.mpg file-4

Testrun

Code:

gong:/medialib/tc# ./trans.sh -v -d -f infile.txt 
+ fmpeg -i /medialib/teapot/2150_20091026215900.mpg -ac 2 -ar 48000 -b 1000k -s cif file-2.m4v
+ fmpeg -i /medialib/teapot/3150_20091102215900.mpg -ac 2 -ar 48000 -b 1000k -s cif file-3.m4v
+ fmpeg -i /medialib/teapot/3150_20091110215900.mpg -ac 2 -ar 48000 -b 1000k -s cif file-4.m4v

However when I ommit '-d', ffmpeg runs only once and then the script exits silently

Code:

gong:/medialib/tc# ./trans.sh -f infile.txt -v 
+ fmpeg -i /medialib/teapot/2150_20091026215900.mpg -ac 2 -ar 48000 -b 1000k -s cif file-2.m4v
FFmpeg version 0.5, Copyright (c) 2000-2009 Fabrice Bellard, et al.

 .... plenty of output as ffmpeg does it's job

gong:/medialib/tc#

Does anyone see something obviously stupid I'm doing, or otherwise have an explanation for this behaviour?

SirDice · Feb 8, 2010

The bourne shell doesn't have a 'system' command. Remove the 'system' from your script.

Scratch that, I didn't notice the function. I'm betting the error is in there though.

Looks like the 'test -z $dryrun || return' doesn't do what you expect

jalla · Feb 8, 2010

Funny thing is, if I replace ffmpeg with a bogus cmd it does loop

Code:

gnome:/medialib/tc# ./trans.sh -v -f infile.txt 
+ fmpeg -i /medialib/teapot/2150_20091026215900.mpg -ac 2 -ar 48000 -b 1000k -s cif file-2.m4v
fmpeg: not found
+ fmpeg -i /medialib/teapot/3150_20091102215900.mpg -ac 2 -ar 48000 -b 1000k -s cif file-3.m4v
fmpeg: not found
+ fmpeg -i /medialib/teapot/3150_20091110215900.mpg -ac 2 -ar 48000 -b 1000k -s cif file-4.m4v
fmpeg: not found

And, btw -d work as expected, witness this

Code:

gnome:/medialib/tc# ./trans.sh -v -f infile.txt -d
+ fmpeg -i /medialib/teapot/2150_20091026215900.mpg -ac 2 -ar 48000 -b 1000k -s cif file-2.m4v
+ fmpeg -i /medialib/teapot/3150_20091102215900.mpg -ac 2 -ar 48000 -b 1000k -s cif file-3.m4v
+ fmpeg -i /medialib/teapot/3150_20091110215900.mpg -ac 2 -ar 48000 -b 1000k -s cif file-4.m4v

SirDice · Feb 8, 2010

jalla said:
And, btw -d work as expected, witness this

Now have another close look at your code.. See what happens when $dryrun isn't defined

aragon · Feb 8, 2010

Are you pasting us the same script? What you pasted seems to work here...

jalla · Feb 8, 2010

aragon said:
Are you pasting us the same script? What you pasted seems to work here...

100%
I pasted it back and compared them.

As it happens the script works with anything I throw at it except ffmpeg.

jalla · Feb 8, 2010

SirDice said:
Now have another close look at your code.. See what happens when $dryrun isn't defined

When $dryrun isn't defined the cmd is executed. Just the way I want it.

SirDice · Feb 8, 2010

You can always run your shell script with the -x option. I use it quite a lot to see what's going on.

jalla · Feb 8, 2010

SirDice said:
You can always run your shell script with the -x option. I use it quite a lot to see what's going on.

Good point. Unfortunately it makes me none the wiser. Here's the end of a run with -x. It shows a normal end of the ffmpeg cmd, then the next iteration of the while loop (read a b), and then an immediate exit 1.

Code:

...
[libx264 @ 0x806548a10]slice I:12747 Avg QP:20.52  size: 17168
[libx264 @ 0x806548a10]slice P:140164 Avg QP:22.32  size:  3990
[libx264 @ 0x806548a10]mb I  I16..4:  7.7%  0.0% 92.3%
[libx264 @ 0x806548a10]mb P  I16..4:  3.4%  0.0%  0.0%  P16..4: 83.6%  0.0%  0.0%  0.0%  0.0%    skip:13.0%
[libx264 @ 0x806548a10]final ratefactor: 16.60
[libx264 @ 0x806548a10]SSIM Mean Y:0.9747017
[libx264 @ 0x806548a10]kb/s:1017.8
+ read a b
+ exit 1

DutchDaemon · Feb 8, 2010

Well, go all out with sh -xv.

aragon · Feb 8, 2010

Remove the trap. There shouldn't be a trap on signal 0. I guess that is causing Strange Things to occur...

aragon · Feb 8, 2010

Sorry, I think it might actually be your trap on signal 2 that's causing this. Anyhow, experiment with that line as it's the only place where "exit 1" can be getting called.

jalla · Feb 9, 2010

aragon said:
Sorry, I think it might actually be your trap on signal 2 that's causing this. Anyhow, experiment with that line as it's the only place where "exit 1" can be getting called.

No it's not the trap, but I nailed it in the end anyway. It's actually ffmpeg that eats the rest of infile.
Remember when ffmpeg starts processing it says

Press [q] to stop encoding

The problem is clearly demonstrated with a slightly modified infile (add some q's in there)

Code:

#/medialib/teapot/3150_20091012215900.mpg qfile-1
#/medialib/teapot/2150_20091026215900.mpg qfile-2
/medialib/teapot/3150_20091102215900.mpg qfile-3
/medialib/teapot/3150_20091110215900.mpg qfile-4

Then another run of the script (with ffmpeg output redirected for clarity)

Code:

gong:/medialib/tc# sh -x trans.sh -f infile.txt
+ [ x-f != x ]
+ file=infile.txt
+ shift
+ shift
+ [ x != x ]
+ test -z infile.txt
+ test -f infile.txt
+ grep -v '^#' infile.txt
+ read a b
+ cmd='ffmpeg -i /medialib/teapot/3150_20091102215900.mpg -ac 2 -ar 48000 -b 1000k -s cif -t 60 -y qfile-3.m4v > qfile-3.out 2>&1'
+ system ffmpeg -i /medialib/teapot/3150_20091102215900.mpg -ac 2 -ar 48000 -b 1000k -s cif -t 60 -y qfile-3.m4v '>' qfile-3.out '2>&1'
+ test -z
+ test -z
+ eval ffmpeg -i /medialib/teapot/3150_20091102215900.mpg -ac 2 -ar 48000 -b 1000k -s cif -t 60 -y qfile-3.m4v '>' qfile-3.out '2>&1'
+ ffmpeg -i /medialib/teapot/3150_20091102215900.mpg -ac 2 -ar 48000 -b 1000k -s cif -t 60 -y qfile-3.m4v
+ read a b
+ cmd='ffmpeg -i file-4 -ac 2 -ar 48000 -b 1000k -s cif -t 60 -y .m4v > .out 2>&1'
+ system ffmpeg -i file-4 -ac 2 -ar 48000 -b 1000k -s cif -t 60 -y .m4v '>' .out '2>&1'
+ test -z
+ test -z
+ eval ffmpeg -i file-4 -ac 2 -ar 48000 -b 1000k -s cif -t 60 -y .m4v '>' .out '2>&1'
+ ffmpeg -i file-4 -ac 2 -ar 48000 -b 1000k -s cif -t 60 -y .m4v
+ read a b
gong:/medialib/tc#

The first invocation of ffmpeg continues to read characters from stdin up to the first 'q' and quits. Then the next round of the loop reads whats left, which is only 'file4'.

So the while-loop has to go. Here's a replacement to make the script work

Code:

IFS='
'
for l in `grep -v '^#' $file`
do
        a=`echo $l | cut -d " " -f 1`
        b=`echo $l | cut -d " " -f 2`
        cmd="ffmpeg -i $a -ac 2 -ar 48000 -b 1000k -s cif -t 60 -y $b.m4v > $b.out 2>&1"
        system $cmd
done

shell script strangeness

jalla

SirDice

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jalla

SirDice

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aragon

jalla

jalla

SirDice

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jalla

DutchDaemon

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aragon

aragon

jalla