Renaming files with sed, escaping issues - regex

Question

I'm trying to write a bash script to remove spaces, underscores and dots and replace them with dashes. I also set to lowercase and remove brackets. That's the (long) second sed command, which seems to work.
The first sed call escapes the original names with spaces with '\ ' like when I tab complete, and this is the issue I think.
If I replace 'mv -i' with 'echo' I get what I think I want: the original filename escaped with backslashes and then the new name. If I paste this into the terminal it works, but with mv in the script the spaces cause problems. The escaping doesn't work.
#!/bin/bash
for a in "$#"; do
mv -i $(echo "$a" | sed -e 's/ /\\\ /g') $(echo "$a" | sed -e 's/\(.*\)/\L\1/' -e 's/_/-/g' -e 's/ /-/g' -e 's/---/--/g' -e 's/(//g' -e 's/)//g' -e 's/\[//g' -e 's/\]//g' -e 's/\./-/g' -e 's/-\([^-]*\)$/\.\1/')
done
The other solution is to put quotes around the names, but I can't work out how I would do this. I feel like I've got close, but I'm stumped.
I've also considered the 'rename' command, but you cannot do multiple operations like you can with sed.
Please point out any other issues, this is one of my first scripts. I'm not sure I got the "$#" or "$a" bits completely correct.
Cheers.
edit:
sample input filename
I am a Badly [named] (file) - PLEASE.rename_me.JPG
should become
i-am-a-badly-named-file--please-rename-me.jpg
edit2: my solution, tweaked from gniourf_gniourf's really helpful pure bash answer:
#!/bin/bash
for a in "$#"; do
b=${a,,} #lowercase
b=${b//[_[:space:]\.]/-} #subst dot,space,underscore with dash
b=${b//---/--} #remove triple dash
b=${b//[()\[\]]/} #remove brackets
if [ "${b%-*}" != "$b" ]; then #if there is a dash (prevents filename.filename)
b=${b%-*}.${b##*-} #replace final dash with a dot for extension
fi
if [ "$a" != "$b" ]; then #if there has been a change
echo '--->' "$b" #
#mv -i -- "$a" "$b" #rename
fi
done
This only fails if the file had spaces etc and no extension (e.g this BAD_filename becomes this-bad.filename. But these are media files and should have an extension, so I would have to sort them anyway.
Again, corrections and improvements welcome. I'm new at this stuff

Answer 1

Try doing this with rename :
rename 's/[_\s\.]/-/g' *files
from the shell prompt. It's very useful, you can put some perl code inside if needed.
You can remove the -n (dry-run mode switch) when your tests become valids.
There are other tools with the same name which may or may not be able to do this, so be careful.
If you run the following command (linux)
$ file $(readlink -f $(type -p rename))
and you have a result like
.../rename: Perl script, ASCII text executable
then this seems to be the right tool =)
If not, to make it the default (usually already the case) on Debian and derivative like Ubuntu :
$ sudo update-alternatives --set rename /path/to/rename
(replace /path/to/rename to the path of your perl's rename command.
Last but not least, this tool was originally written by Larry Wall, the Perl's dad.

Answer 2

Just for the records, look:
$ a='I am a Badly [named] (file) - PLEASE.rename_me.JPG'
$ # lowercase that
$ echo "${a,,}"
i am a badly [named] (file) - please.rename_me.jpg
$ # Cool! let's save that somewhere
$ b=${a,,}
$ # substitution 's/[_ ]/-/g:
$ echo "${b//[_ ]/-}"
i-am-a-badly-[named]-(file)---please.rename-me.jpg
$ # or better, yet:
$ echo "${b//[_[:space:]]/-}"
i-am-a-badly-[named]-(file)---please.rename-me.jpg
$ # Cool! let's save that somewhere
$ c=${b//[_[:space:]]/-}
$ # substitution 's/---/--/g' (??)
$ echo "${c//---/--}"
i-am-a-badly-[named]-(file)--please.rename-me.jpg
$ d=${c//---/--}
$ # substitution 's/()[]//g':
$ echo "${d//[()\[\]]/}"
i-am-a-badly-named-file--please.rename-me.jpg
$ e="${d//[()\[\]]/}"
$ # substitution 's/\./-/g':
$ echo "${e//\./-}"
i-am-a-badly-named-file--please-rename-me-jpg
$ f=${e//\./-}
$ # substitution 's/-\([^-]*\)$/\.\1/':
$ echo "${f%-*}.${f##*-}"
i-am-a-badly-named-file--please-rename-me.jpg
$ # Done!
Now, here's a 100% bash implementation of what you're trying to achieve:
#!/bin/bash
for a in "$#"; do
b=${a,,}
b=${b//[_[:space:]]/-}
b=${b//---/--}
b=${b//[()\[\]]/}
b=${b//\./-}
b=${b%-*}.${b##*-}
mv -i -- "$a" "$b"
done
yeah, done!
All this standard and known as shell parameter expansion.
Remark. For a more robust script, you could check whether a has an extension (read: a period in its name), otherwise the last substitution of the algorithm fails a little bit. For this, put the following line just below the for statement:
[[ a != *.* ]] && { echo "Oh no, file \`$a' has no extension..."; continue; }
(and isn't the *.* part of this line so cute?)