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If I have these files in a directory


how can I list them in Bash so that they are in ascending numeric order based on the number part of the string. So the resulting order is cwcch1.pdf, cwcch2.pdf, ..., cwcch9.pdf, cwcch10.pdf, etc.

What I'm ultimately trying to do is concatenate the pdfs with pdftk with something like the following

pdftk `ls *.pdf | sort -n` cat output output.pdf

but that doesn't work as my sorting is wrong.

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Thanks for all the great answers to this. As always with Unix, there are many different excellent ways to skin this cat. – ngm Dec 6 '09 at 11:21

6 Answers 6

up vote 6 down vote accepted

Something like this might do what you want, though it takes a slightly different approach:

pdftk $(for n in {1..18}; do echo cwcch$n.pdf; done) cat output output.pdf
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Aha, nice approach! It does indeed do what I what, thanks. – ngm Dec 6 '09 at 0:29

Your sort may have the ability to do this for you:

sort --version-sort
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For this particular example you could also do this:

ls *.pdf | sort -k2 -th -n

That is, sort numerically (-n) on the second field (-k2) using 'h' as the field separator (-th).

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Splitting and then sorting on one field -- that's a great tip that I'm sure will be handy in future, thanks. – ngm Dec 6 '09 at 11:30

You can use the -v option in GNU ls: natural sort of (version) numbers within text.

ls -1v cwcch*

This does not work with BSD ls (e.g. on OS X), where the -v option has a different meaning.

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Use shell expansion directly in a commandline. The expansion should order them properly. If I understand pdftk's commandline syntax properly, this will do what you want:

# shell expansion with square brackets
pdftk cwcch[1-9].pdf cwcch1[0-9].pdf cat output output.pdf

# shell expansion with curly braces
pdftk cwcch{{1..9},{10..18}}.pdf cat output output.pdf

Or you can try a different approach. When I need to do something like this, I usually try to get my numbers formatted properly ahead of time. If I'm coming into it late and the PDFs are already numbered like your example, I'll use this to renumber:

# rename is aka prename -- perl rename script
# this adds a leading zero to single-digit numbers
rename 's/(\d)/0$1/' cwcch[1-9].pdf

Now the standard ls sorting will work properly.

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Perhaps a little more succinctly: pdftk cwcch{{1..9},{10..18}}.pdf ... – Dennis Williamson Dec 6 '09 at 7:02
good tip, added in. is that a standard Bourne shell expansion syntax or a bash extension? – quack quixote Dec 6 '09 at 7:43

Here's a method just using sort:

ls | sort -k1.6n
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