mjhjmtu

As I was trying to fix the episode numbers of a TV series I toyed with the rename function and trying to pass the return from printf as the rename parameters, but could not do it. I tried just about every combination of

for j in $(seq 1 9);
do 
 rename 's/"$( - "`printf "%d" $j`" - )"/"$( - "`printf "0%d" $j`" - )"/g' *
done

that I could think of but nothing seemed to work. I ended up just doing it manually since I was only concerned with 1-9 in this case, so

rename 's/- 1 -/- 01 -/g' *
rename 's/- 2 -/- 02 -/g' *
...

Can someone point out why the above didn't work for future reference?

You are single quoting the entire expression, so no shell interpolation will happen, and Perl will see the most curious regular expression of "$( - "printf "%d" $j" - )" which is interpolated by Perl into something like

% perl -E 'say qq"$( - "`printf "%d" $j`" - )"'
"42 640 - "`printf "%d" `" - )"
% 

because you asked Perl to interpolate the $( variable which you can read about via perldoc -v '$('. Suffice to say, this regular expression will probably not match your files. (And demonstrates one of the several pitfalls of attempting to embed multiple languages into one gloriously complicated string; there is really no need as Perl can do it all.)

A better approach would be to match and adjust what you are interested in, which here appears to be a number enclosed by hypens; assuming there is only one number in the filename simply match on that saving a backreference ([0-9]+) and use the Perl sprintf function to pad those numbers as appropriate (plus the important /e flag to treat the right hand side as an expression so sprintf actually gets called).

% touch "blah - "1..9" - blah"
% ls
blah - 1 - blah blah - 3 - blah blah - 5 - blah blah - 7 - blah blah - 9 - blah
blah - 2 - blah blah - 4 - blah blah - 6 - blah blah - 8 - blah
% rename 's/([0-9]+)/sprintf "%02d", $1/e' *
% ls
blah - 01 - blah blah - 04 - blah blah - 07 - blah
blah - 02 - blah blah - 05 - blah blah - 08 - blah
blah - 03 - blah blah - 06 - blah blah - 09 - blah
% 

If there are numbers elsewhere in the filename, then make the regular expression account for that, perhaps something like

% rename 's/- ([0-9]+) -/sprintf "- %d -", $1/e' *
% ls
blah - 1 - blah blah - 3 - blah blah - 5 - blah blah - 7 - blah blah - 9 - blah
blah - 2 - blah blah - 4 - blah blah - 6 - blah blah - 8 - blah
% 

To use regular expressions in rename most effectively, see

• http://perl-begin.org/topics/regular-expressions/


• http://perldoc.perl.org/perlrequick.html


• http://perldoc.perl.org/perlretut.html