How to decrement a number in each filename in a directory? - bash

Question

Running "ls -lrt" on my terminal I get a large list that looks something like this:
-rw-r--r-- 1 pratik staff 1849089 Jun 23 12:24 cam13-vid.webm
-rw-r--r-- 1 pratik staff 1850653 Jun 23 12:24 cam12-vid.webm
-rw-r--r-- 1 pratik staff 1839110 Jun 23 12:24 cam11-vid.webm
-rw-r--r-- 1 pratik staff 1848520 Jun 23 12:24 cam10-vid.webm
-rw-r--r-- 1 pratik staff 1839122 Jun 23 12:24 cam1-vid.webm
I have only shown part of it above as a sample.
I would like to rename all the files to have a number one less than current.
For example,
mv cam1-vid.webm cam0-vid.webm
mv cam2-vid.webm cam1-vid.webm
.....
....
mv cam 200-vid.webm cam199-vid.webm
How can this be done using a os x / linux bash script (perhaps using sed) ?

Answer 1

You can do this with plain bash:
for i in {1..200}
do
mv "cam${i}-vid.webm" "cam$((i-1))-vid.webm"
done

Answer 2

I would use find, split up the file names, to find the number, subtract one, and rename:
find . -name "cam*-vid.webm" -print0 | while read -d\$0 old_name
do
number=${old_name#cam} #Filter left to remove 'cam' prefix
number=${number%-vid.webm"} #Filter right to remove '-vid.webm' suffix
$((number -= 1))
new_name="cam${number}-vid.webm"
echo "mv \"$old_name\" \"$new_name\""
done | tee results
This will merely print out the commands (that is why I have echo). I'm piping it into a file named results. Once this command completes, look at results and make sure it does everything it should. Whenever there's an operation like this, there can be a nasty surprise. For example, if I rename cam02-vid.webm to cam01-vid.webm before I rename cam01-vid.webm, I am going to overwrite cam01-vid-webm.
Maybe a safer way is to explicitly give the file numbers I need:
for number in {1..200}
do
$((old_number = $number + 1))
echo mv "\"cam${old_number}-vid.webm\" \"cam${number}-vid.webm\""
done | tee results
Useful hint: If the result file looks good, you can actually just run it as a shell script:
$ bash results
Another possibility is to test to make sure the old file exist:
for number in {1..200}
do
$((old_number = $number + 1))
if [ -f "$cam${old_number}-vid.webm" ]
then
echo mv "\"cam${old_number}-vid.webm\" \"cam${number}-vid.webm\""
else
echo "ERROR: Can't find a file called 'cam${old_number}-vid.webm'"
fi
done | tee results

Answer 3

A perl solution.
First it traverses all input files (#ARGV) and filters those that are plain files and not links (grep), extracts the number (map) and sorts numerically in ascendant to avoid overwritting (sort). Later creates a new file decrementing the number and renames the original:
perl -e '
for (
sort { $a->[0] <=> $b->[0] }
map { m/(\d+)/; [$1, $_ ] }
grep { -f $_ && ! -l $_ }
#ARGV
) {
$n = --$_-> [0];
($newname = $_->[1]) =~ s/\A(?i)(cam)\d+(.*)\z/$1$n$2/;
print "Executing command ===> rename $_->[1], $newname\n";
rename $_->[1], $newname;
}' *
Assuming initial content of the directory as:
cam1-vid.webm
cam13-vid.webm
cam12-vid.webm
cam11-vid.webm
cam10-vid.webm
cam2-vid.webm
After running the command yields:
cam0-vid.webm
cam10-vid.webm
cam11-vid.webm
cam12-vid.webm
cam1-vid.webm
cam9-vid.webm