complex command substitution

Shell Programming and Scripting
1

hi,
I have to execute this line below from within a shell script; simply backquoting it is not doing the trick; it is mangling up all the options; but when i type it out on a command line, it executes cleanly. Please help me in getting this right;

$ vlc -I dummy --sout='#transcode{vcodec=mp4v, acodec=mp4a}:std{access=file, mux=mp4, url=temp.mp4} file.avi

As you can see, the syntax of the option specification for VLC includes many complex tokens; Can you guide me to get it working from within backquotes?

Thanks!

2

Incredible that it works at all. What shell are you using? Most shells will choke on an unmatched single quote.

3

ok, there is no unmatched single quote; thanks a lot for pointing out; this is the corrected command line:

$ vlc -I dummy --sout='#transcode{vcodec=mp4v,acodec=mp4a}:std{access=file,mux=mp4,url=temp.mp4} ' file.avi

I got this partially working by simply writing the line in the shell script (without an backquotes). The problem is my temp.mp4 and file.avi are actually inside variables $T and $F; something like this works:

#!/bin/sh

#do some string manipulation to determine values of $T and $F

vlc -I dummy --sout='#transcode{vcodec=mp4v,acodec=mp4a}:std{access=file,mux=mp4,url=temp.mp4} ' "$F"

but if I replace $T for temp.mp4, it wont work anymore; I dont care about the return value of this command, so I dont need backquotes;

So, the problem has boiled down to - how do I make the shell interpret the $ variable inside those single quotes?

Thanks!

4

What you need to do is pre-process the command line within the single quotes, performing variable substution. I don't know about your application, so here is a trivial example which should make it clear.

There is a one line script called z containing the line "print $1"
There is a variable within the current shell with the value "z" (a=z)
First run z with $a as a parameter and it prints "z" (value substituted)

$ ./z $a
$ z

Next run z with '$a' as a parameter and it prints "$a" (value not substituted because it's in single quotes)

$ ./z '$a'
$ z

Finally, run with eval and value is substuted even though in quotes (substitution is actually done as a separate step before command is executed)

$ eval ./z '$a'
$ z

hope this helps

5

Could you not just unquote at the point you need the variable interpolation:

vlc -I dummy --sout='#transcode{vcodec=mp4v,acodec=mp4a}:std{access=file,mux=mp4,url='$T'} ' "$F"

Does that work?

6

hi all, thanks a lot, I solved the problem with some generous usage of quotes..
vlc -I dummy --sout=""#transcode{vcodec=mp4v,acodec=mp4a}:std{access=file,mux=mp4,url=$T} "" $F

did the trick; the problem was that I needed some kind of quotes after --sout= as part of the commad line syntax;