Script to pass multiple files to program

jsteel · 02-08-2011, 05:13 PM

I'm playing around with a bash script to pass files to a program (such as VLC). I thought something like this would work:

vlc $( ls | sed 's/\ /\\ /g' | tr '\n' ' ' )

ls shows the files in the current directory, then sed changes spaces to "\ " (to escape them) and finally tr removes the line breaks. I end up changing:

01 - Music Track.mp3
02 - Another Track.mp3

to

01\ -\ Music\ Track.mp3 02\ -\ Another\ Track.mp3

"vlc 01\ -\ Music\ Track.mp3 02\ -\ Another\ Track.mp3" works if I type it manually but my script reports a problem with `-\'

It seems it's not escaping the spaces.

Any help is much appreciated. Thanks.

crts · 02-08-2011, 05:41 PM

Hi,

I am not sure why this is not working. If you prepend your command with an echo the output looks ok. If you copy+paste the echoed line into the terminal again and hit enter then it executes just fine. It just does not do so when you try it as you have done. Maybe I will investigate this later. Anyway, if the '-' give you trouble then this might be because it is misinterpreted as an option. To bypass the funny command substitution issue this:

Code:

vlc -- *

Since you want to play all files in that directory. The '--' is to tell vlc that there are no more options to parse.

ntubski · 02-08-2011, 07:27 PM

Quote:

I am not sure why this is not working. If you prepend your command with an echo the output looks ok.

Testing with echo isn't good enough, it doesn't show you how the arguments are parsed:

Code:

~/tmp/mp3$ ls
01 - Music Track.mp3  02 - Another Track.mp3
~/tmp/mp3$ echo vlc $( ls | sed 's/\ /\\ /g' | tr '\n' ' ' )
vlc 01\ -\ Music\ Track.mp3 02\ -\ Another\ Track.mp3
~/tmp/mp3$ args.sh vlc $( ls | sed 's/\ /\\ /g' | tr '\n' ' ' )
arg1: vlc
arg2: 01\
arg3: -\
arg4: Music\
arg5: Track.mp3
arg6: 02\
arg7: -\
arg8: Another\
arg9: Track.mp3
~/tmp/mp3$ args.sh vlc *
arg1: vlc
arg2: 01 - Music Track.mp3
arg3: 02 - Another Track.mp3

args.sh:

Code:

#!/bin/sh
i=1
while [ -n "$1" ] ; do
    echo arg$i: "$1"
    shift
    i=`expr $i + 1`
done

crts · 02-08-2011, 08:41 PM

Yes, I had noticed that the output gets split up that way. You do not really need an extra script for that. I got the corresponding 'No such file or directory' error messages in vlc.

The interesting thing at this point, however, is why are the parameters handled this way? Is vlc responsible for splitting them up or is it bash? If it were vlc then this would happen with every filename that has a space in it, thus making it impossible for vlc to play files with spaces in them. Which leaves bash as the last suspect. My guess at this point is that after a command substitution bash does not parse the result. Apparently it simply splits up the resulting string at every space and feeds the tokens to vlc one by one - with the escape character '\'.
However, this behavior does not seem very conclusive at this point. Now it would be understandable that after a command substitution the result is untouched and passed as is to vlc. But this is not the case. It does process it to some degree, i.e. splitting it up. If it does some post-processing then why not do the whole thing and recognizing the escape sequences? Are there any good reasons not to? The way it deals with it seems like going only half-way. Is there a "switch" that can trigger an alternate behavior?

That being said I am open to explanations/corrections and/or good reasons why it has to be this way and not otherwise.

ntubski · 02-08-2011, 09:04 PM

Quote:

The interesting thing at this point, however, is why are the parameters handled this way? Is vlc responsible for splitting them up or is it bash?

vlc was never called in my example.

Quote:

It does process it to some degree, i.e. splitting it up. If it does some post-processing then why not do the whole thing and recognizing the escape sequences? Are there any good reasons not to?

Quote:

The order of expansions is: brace expansion, tilde expansion, parameter, variable, and arithmetic expansion and command substitution (done in a left-to-right fashion), word splitting, and filename expansion.

Shell Expansions

After the preceding expansions, all unquoted occurrences of the characters ‘\’, ‘'’, and ‘"’ that did not result from one of the above expansions are removed.
Quote Removal

That is the current documented behaviour of bash. As for a "good" reason: I don't think bash (or sh, csh) is well designed, it seems like it just accumulates features as time goes on. It has to be this way because changing would break too many scripts.