I want to exclude some directory in my script (directory name >1000) for deletion and here is my directory look like:
/home/tester/100
/home/tester/1000
/home/tester/1020 # delete all files inside
/home/tester/2000 # delete all files inside
My bash script:
cd /home/tester
for dir in */ ; do
echo -n $dir": ";
find "$dir" -type f | wc -l;
if [ $dir -gt 1000 ]; then
cd $dir;
rm *;
cd ..;
fi
done
I got error on the if line and have no idea how to fix it ... Is it possible to do with bash script ?
Thank you for your help
for dir in */ ; do will set dir to things like "1000/" -- and the "/" makes it not a valid number. You can trim off the trailing "/" with ${dir%/}. I'd also recommend double-quoting it to prevent possible weird parsing:
if [ "${dir%/}" -gt 1000 ]; then
Note that if the directory name isn't a number (even after the "/" is removed), you'll get an error from the comparison, and the then clause won't run (which is probably what you want). If you want to handle other (non-numeric) directory names more gracefully, you should add some appropriate is-this-a-number test first.
Also, using cd in scripts tends to be problematic, because if a cd fails for any reason, the rest of the script will continue running, but in the wrong place. This can cause all sorts of chaos. Consider what'd happen if one of the cd $dir commands fails: it'd run rm * in the /home/tester directory, deleting all the non-subdirectory files there, then it'd cd .., leaving it in /home. The next iteration would try to cd down to something like 2000, which doesn't exist under /home, so that cd would fail too, and then it'd delete all files in /home. This repeats indefinitely, potentially all the way up to running rm * in /, the root directory. Not good at all.
I recommend either putting error checks on cd commands, or just avoiding them entirely in favor of using explicit paths to files.
#!/bin/bash
cd /home/tester || {
echo "Couldn't cd to /home/tester, quitting here..." >&2
exit 1
}
for dir in */ ; do
echo -n "$dir: "
find "$dir" -type f | wc -l
if [ "${dir%/}" -gt 1000 ]; then
rm "$dir"/* # Explicit path -- the / is redundant, but won't hurt
fi
done
I've also added an explicit shebang line, double-quoted all the variable references (good general scripting hygiene), and removed the semicolons from the ends of lines (not needed in shell syntax).
Another recommendation: run your scripts through shellcheck.net -- it'll point out a lot of common mistakes like unquoted variable references and unchecked cds.
The value of $dir is not numeric. Add set -x at the to of your script to debug.
Use "$(basename "$dir")" to get the numeric value.
When I did not have my first cup of coffee, I would do
for dir in */ ; do
echo -n $dir": ";
find "$dir" -type f | wc -l;
done
mv /home/tester/1000 /home/tester/some_unique_name
rm /home/tester/[1-9][0-9][0-9][0-9]/*
mv /home/tester/some_unique_name /home/tester/1000
This will not work when you have directories > 9999.
Perhaps rm /home/tester/[1-9][0-9][0-9][0-9]*/* will work, when you don't have directories like 1000backup or 2000my_unique_name.
A better solution is
find . -regextype sed -regex '/home/tester/[0-9]\{4,\}' ! -name 1000 |
xargs -L1 -I{} echo rm {}/*
I have large number of files in the format x00000.jpg, X00000.jpg and xx00000.jpg.
How can I rename these files so they are all uppercase, ignoring the numeric part of the name?
for f in * ; do mv -- "$f" "$(tr [:lower:] [:upper:] <<< "$f")" ; done
You can't rename files from Bash only, because Bash doesn't have any built-in command for renaming files. You have to use at least one external command for that.
If Perl is allowed:
perl -e 'for(#ARGV){rename$_,uc}' *.jpg
If Python is allowed:
python -c 'import os, sys; [os.rename(a, a.upper()) for a in sys.argv[1:]]' *.jpg
If you have thousands or more files, the solutions above are fast, and the solutions below are noticably slower.
If AWK, ls and mv are allowed:
# Insecure if the filenames contain an apostrophe or newline!
eval "$(ls -- *.jpg | awk '{print"mv -- \x27"$0"\x27 \x27"toupper($0)"\x27"}')"
If you have a lots of file, the solutions above don't work, because *.jpg expands to a too long argument list (error: Argument list too long).
If tr and mv are allowed, then see damienfrancois' answer.
If mv is allowed:
for file in *; do mv -- "$file" "${file^^}"; done
Please note that these rename .jpg to .JPG at the end, but you can modify them to avoid that.
The bash shell has a syntax for translating a variable name to all-caps.
for file in * ; do # or *.jpg, or x*.jpg, or whatever
mv "$file" "${file^^}"
done
This feature was introduced in bash version 4.0, so first verify that your version of bash implements it. To avoid mistakes, try it once replacing mv by echo mv, just to make sure it's going to do what you want.
The documentation for this feature is here, or type info bash and search for "upper".
You should probably decide what to do if the target file already exists (say, if both x00000.jpg and X00000.JPG already exists), unless you're certain it's not an issue. To detect such name collisions, you can try:
ls *.txt | tr '[a-z]' '[A-Z]' | sort | uniq -c | sort -n
and look for any lines not starting with 1.
rename
Probably the easiest way for renaming multiple files is using Perl's rename. To translate lowercase names to upper, you'd:
rename 'y/a-z/A-Z/' *
If the files are also in subdirs you can use globstar or find:
find . -maxdepth 1 -type f -iname "*.jpg" -execdir rename "y/a-z/A-Z/" {} +
References
Howto: Linux Rename Multiple Files At a Shell Prompt – nixCraft
More info about y/, translate instead of substitute.
DistroTube - Tools For Renaming Files In Linux
Combining previous answers could yield:
for file in * ; do # or *.jpg, or x*.jpg, or whatever
basename=$(tr '[:lower:]' '[:upper:]' <<< "${file%.*}")
newname="$basename.${file#*.}"
mv "$file" "$newname"
done
Using tr:
f="x00000.jpg"
n="${f%.*}"
n=$(tr '[:lower:]' '[:upper:]' <<< "$n")
f="$n.${f#*.}"
echo "$f"
OUTPUT:
X00000.jpg
if you are using the zsh like me:
for f in * ; do mv -- "$f" "${f:u}" ; done
If only renaming files/dirs is all you want, then you can use rnm :
rnm -rs '/./\C/g' -fo -dp -1 *
Explanation:
-rs : replace string. /./\C/g replaces all match of . (regex) to it's uppercase.
-fo : file only mode
-dp : depth of directory (-1 means unlimited).
More examples can be found here.
Rename all source files to uppercase and stage to git
#!/bin/bash
SOURCE_DIRS=('./src' './Public' './Private')
FILE_EXTENSION="*.php"
for dir in $SOURCE_DIRS; do
files="$(find "$dir" -name "${FILE_EXTENSION[#]}";)"
for file in $files; do
base_name="$(basename $file)";
new_name=$(sed "s/$base_name/${base_name^}/g" <<< $file)
git mv $file ${new_name} # or
# mv $file ${new_name}
done
done
How to change extension of all *.dat files in a directory to *.txt.
Shell script should take the directory name as an argument. Can
take multiple directories as arguments. Print the log of command
result in appending mode with date and timestamp.
Bash can do all of the heavy lifting such as extracting the extension and tagging on a new one. For example:
for file in $1/*.dat ; do mv "$file" "${file%.*}.txt" ; done
Batch File Rename By File Extension in Unix
# change .htm files to .html
for file in *.htm ; do mv $file `echo $file | sed 's/\(.*\.\)htm/\1html/'` ; done
# change .html files to .htm
for file in *.html ; do mv $file `echo $file | sed 's/\(.*\.\)html/\1htm/'` ; done
#change .html files to .shtml
for file in *.html ; do mv $file `echo $file | sed 's/\(.*\.\)html/\1shtml/'` ; done
#change .html files to php
for file in *.html ; do mv $file `echo $file | sed 's/\(.*\.\)html/\1php/'` ; done
so ==>
# change .dat files to .txt
for file in *.dat ; do mv $file `echo $file | sed 's/\(.*\.\)dat /\1txt/'` ; done
#!/bin/bash
for d in $*; do
for f in $(ls $d/*.dat); do
echo $(date) $(mv -v $f ${f%.dat}.txt)
done
done
Output redirection should be done by the shell when running the script
Leaving out argument validity checks
Simple script:
#!/bin/bash
if [ $# -lt 2 ] then
echo "Usage `basename $0` <any number of directories space separated>"
exit 85 # exit status for wrong number of arguments.
fi
for directories
do
for files in $(ls $directories/*.dat); do
echo $(date) $(mv -v $files ${files%.dat}.txt)
done
done
The first for loop by default loops on the $# i.e. command-line arguments passed.
Follow Pben's solution, if your filename contains blank space, you should use double quotation marks to the variable like the following:
#remove the space in file name
#example file name:19-014-0100.mp3 .mp3
#result file name:19-014-0100.mp3
$ for file in *.mp3 ;
do target=`echo "$file" | sed 's/ //g'`;
echo "$target";
mv "$file" "$target";
done;
#remove the duplicate file extension in file name
#example file name:19-014-0100.mp3.mp3
#result file name:19-014-0100.mp3
$ for file in *.mp3 ;
do target=`echo "$file" | sed 's/\.mp3\.mp3$/.mp3/g'`;
echo "$target";
mv "$file" "$target";
done;
To rename (changing extention) all my html files on epub files I use this command line :
find . -name "*.html*" -exec rename -v 's/\.html$/\.epub/i' {} \;
Script, first finds the names of the given extensions.
It removes the extension from names. Then adds backslash()
for identification of terminal.
Then the 'mv' command executed.
Here the '.temp' folder is used to hide the process from user,
in GUI.
#!/bin/sh
if [ $# -ne 3 ]
then
echo "Usage: ./script folder current_extension modify_extension"
exit
fi
mkdir .temp
find $1 -name "*.$2" > .temp/output_1 && sed "s/$2//" .temp/output_1 > .temp/output_2 && sed -e "s/[ \t]/\\\ /g" .temp/output_2 > .temp/output_3
while read line
do
mv -v "$line""$2" "$line""$3"
done < .temp/output_3
rm -rf .temp
The output files are saved inside the '.temp' folder,later the '.temp' folder is removed.
The top voted answer didn't really work for me. I may have been doing something wrong. My scenario was trying to create a file with the original name, but with the date appended to it, along with changing the extension from .xslx to .csv. This is what worked for me:
csvname=`echo $xlsx |sed 's/\.xlsx//'`"-$now"`echo $xlsx | sed 's/\(.*\.\)xlsx/\.csv/'`
So, for all the .dat files in a directory (without the date addition), you could run something like this:
for i in *.dat
do mv $i `echo $i |sed 's/\.dat//'``echo $i | sed 's/\(.*\.\)dat/\.txt/'`
done
From the above, this section of code just removed the extension:
echo $i |sed 's/\.dat//'
And this section changes the .dat to .txt:
echo $i | sed 's/\(.*\.\)dat/\.txt/'
And by bumping them next to each other, it concatenates the two outputs into the filename. It's like doing this:
mv [filename][.dat] [filename] + [.txt]
Though, I did use STDOUT instead of the 'mv' command.
Following command to change file extention .c to .h
find . -depth -name "*.c" -exec sh -c 'dname=$(dirname {}) && fname=$(basename {} .c) && mv {} $dname/$fname.h' ";"
change js to cjs extension files recursively:
cd dist # where you place your .js
for file in $(find . -type f -name "*.js"); do mv "$file" "${file%.*}.cjs"; done
I have to rename a complete folder tree recursively so that no uppercase letter appears anywhere (it's C++ source code, but that shouldn't matter).
Bonus points for ignoring CVS and Subversion version control files/folders. The preferred way would be a shell script, since a shell should be available on any Linux box.
There were some valid arguments about details of the file renaming.
I think files with the same lowercase names should be overwritten; it's the user's problem. When checked out on a case-ignoring file system, it would overwrite the first one with the latter, too.
I would consider A-Z characters and transform them to a-z, everything else is just calling for problems (at least with source code).
The script would be needed to run a build on a Linux system, so I think changes to CVS or Subversion version control files should be omitted. After all, it's just a scratch checkout. Maybe an "export" is more appropriate.
Smaller still I quite like:
rename 'y/A-Z/a-z/' *
On case insensitive filesystems such as OS X's HFS+, you will want to add the -f flag:
rename -f 'y/A-Z/a-z/' *
A concise version using the "rename" command:
find my_root_dir -depth -exec rename 's/(.*)\/([^\/]*)/$1\/\L$2/' {} \;
This avoids problems with directories being renamed before files and trying to move files into non-existing directories (e.g. "A/A" into "a/a").
Or, a more verbose version without using "rename".
for SRC in `find my_root_dir -depth`
do
DST=`dirname "${SRC}"`/`basename "${SRC}" | tr '[A-Z]' '[a-z]'`
if [ "${SRC}" != "${DST}" ]
then
[ ! -e "${DST}" ] && mv -T "${SRC}" "${DST}" || echo "${SRC} was not renamed"
fi
done
P.S.
The latter allows more flexibility with the move command (for example, "svn mv").
for f in `find`; do mv -v "$f" "`echo $f | tr '[A-Z]' '[a-z]'`"; done
Just simply try the following if you don't need to care about efficiency.
zip -r foo.zip foo/*
unzip -LL foo.zip
One can simply use the following which is less complicated:
rename 'y/A-Z/a-z/' *
This works on CentOS/Red Hat Linux or other distributions without the rename Perl script:
for i in $( ls | grep [A-Z] ); do mv -i "$i" "`echo $i | tr 'A-Z' 'a-z'`"; done
Source: Rename all file names from uppercase to lowercase characters
(In some distributions the default rename command comes from util-linux, and that is a different, incompatible tool.)
This works if you already have or set up the rename command (e.g. through brew install in Mac):
rename --lower-case --force somedir/*
The simplest approach I found on Mac OS X was to use the rename package from http://plasmasturm.org/code/rename/:
brew install rename
rename --force --lower-case --nows *
--force Rename even when a file with the destination name already exists.
--lower-case Convert file names to all lower case.
--nows Replace all sequences of whitespace in the filename with single underscore characters.
Most of the answers above are dangerous, because they do not deal with names containing odd characters. Your safest bet for this kind of thing is to use find's -print0 option, which will terminate filenames with ASCII NUL instead of \n.
Here is a script, which only alter files and not directory names so as not to confuse find:
find . -type f -print0 | xargs -0n 1 bash -c \
's=$(dirname "$0")/$(basename "$0");
d=$(dirname "$0")/$(basename "$0"|tr "[A-Z]" "[a-z]"); mv -f "$s" "$d"'
I tested it, and it works with filenames containing spaces, all kinds of quotes, etc. This is important because if you run, as root, one of those other scripts on a tree that includes the file created by
touch \;\ echo\ hacker::0:0:hacker:\$\'\057\'root:\$\'\057\'bin\$\'\057\'bash
... well guess what ...
Here's my suboptimal solution, using a Bash shell script:
#!/bin/bash
# First, rename all folders
for f in `find . -depth ! -name CVS -type d`; do
g=`dirname "$f"`/`basename "$f" | tr '[A-Z]' '[a-z]'`
if [ "xxx$f" != "xxx$g" ]; then
echo "Renaming folder $f"
mv -f "$f" "$g"
fi
done
# Now, rename all files
for f in `find . ! -type d`; do
g=`dirname "$f"`/`basename "$f" | tr '[A-Z]' '[a-z]'`
if [ "xxx$f" != "xxx$g" ]; then
echo "Renaming file $f"
mv -f "$f" "$g"
fi
done
Folders are all renamed correctly, and mv isn't asking questions when permissions don't match, and CVS folders are not renamed (CVS control files inside that folder are still renamed, unfortunately).
Since "find -depth" and "find | sort -r" both return the folder list in a usable order for renaming, I preferred using "-depth" for searching folders.
One-liner:
for F in K*; do NEWNAME=$(echo "$F" | tr '[:upper:]' '[:lower:]'); mv "$F" "$NEWNAME"; done
Or even:
for F in K*; do mv "$F" "${F,,}"; done
Note that this will convert only files/directories starting with letter K, so adjust accordingly.
The original question asked for ignoring SVN and CVS directories, which can be done by adding -prune to the find command. E.g to ignore CVS:
find . -name CVS -prune -o -exec mv '{}' `echo {} | tr '[A-Z]' '[a-z]'` \; -print
[edit] I tried this out, and embedding the lower-case translation inside the find didn't work for reasons I don't actually understand. So, amend this to:
$> cat > tolower
#!/bin/bash
mv $1 `echo $1 | tr '[:upper:]' '[:lower:]'`
^D
$> chmod u+x tolower
$> find . -name CVS -prune -o -exec tolower '{}' \;
Ian
Not portable, Zsh only, but pretty concise.
First, make sure zmv is loaded.
autoload -U zmv
Also, make sure extendedglob is on:
setopt extendedglob
Then use:
zmv '(**/)(*)~CVS~**/CVS' '${1}${(L)2}'
To recursively lowercase files and directories where the name is not CVS.
Using Larry Wall's filename fixer:
$op = shift or die $help;
chomp(#ARGV = <STDIN>) unless #ARGV;
for (#ARGV) {
$was = $_;
eval $op;
die $# if $#;
rename($was,$_) unless $was eq $_;
}
It's as simple as
find | fix 'tr/A-Z/a-z/'
(where fix is of course the script above)
for f in `find -depth`; do mv ${f} ${f,,} ; done
find -depth prints each file and directory, with a directory's contents printed before the directory itself. ${f,,} lowercases the file name.
This works nicely on macOS too:
ruby -e "Dir['*'].each { |p| File.rename(p, p.downcase) }"
This is a small shell script that does what you requested:
root_directory="${1?-please specify parent directory}"
do_it () {
awk '{ lc= tolower($0); if (lc != $0) print "mv \"" $0 "\" \"" lc "\"" }' | sh
}
# first the folders
find "$root_directory" -depth -type d | do_it
find "$root_directory" ! -type d | do_it
Note the -depth action in the first find.
Use typeset:
typeset -l new # Always lowercase
find $topPoint | # Not using xargs to make this more readable
while read old
do new="$old" # $new is a lowercase version of $old
mv "$old" "$new" # Quotes for those annoying embedded spaces
done
On Windows, emulations, like Git Bash, may fail because Windows isn't case-sensitive under the hood. For those, add a step that mv's the file to another name first, like "$old.tmp", and then to $new.
With MacOS,
Install the rename package,
brew install rename
Use,
find . -iname "*.py" -type f | xargs -I% rename -c -f "%"
This command find all the files with a *.py extension and converts the filenames to lower case.
`f` - forces a rename
For example,
$ find . -iname "*.py" -type f
./sample/Sample_File.py
./sample_file.py
$ find . -iname "*.py" -type f | xargs -I% rename -c -f "%"
$ find . -iname "*.py" -type f
./sample/sample_file.py
./sample_file.py
Lengthy But "Works With No Surprises & No Installations"
This script handles filenames with spaces, quotes, other unusual characters and Unicode, works on case insensitive filesystems and most Unix-y environments that have bash and awk installed (i.e. almost all). It also reports collisions if any (leaving the filename in uppercase) and of course renames both files & directories and works recursively. Finally it's highly adaptable: you can tweak the find command to target the files/dirs you wish and you can tweak awk to do other name manipulations. Note that by "handles Unicode" I mean that it will indeed convert their case (not ignore them like answers that use tr).
# adapt the following command _IF_ you want to deal with specific files/dirs
find . -depth -mindepth 1 -exec bash -c '
for file do
# adapt the awk command if you wish to rename to something other than lowercase
newname=$(dirname "$file")/$(basename "$file" | awk "{print tolower(\$0)}")
if [ "$file" != "$newname" ] ; then
# the extra step with the temp filename is for case-insensitive filesystems
if [ ! -e "$newname" ] && [ ! -e "$newname.lcrnm.tmp" ] ; then
mv -T "$file" "$newname.lcrnm.tmp" && mv -T "$newname.lcrnm.tmp" "$newname"
else
echo "ERROR: Name already exists: $newname"
fi
fi
done
' sh {} +
References
My script is based on these excellent answers:
https://unix.stackexchange.com/questions/9496/looping-through-files-with-spaces-in-the-names
How to convert a string to lower case in Bash?
In OS X, mv -f shows "same file" error, so I rename twice:
for i in `find . -name "*" -type f |grep -e "[A-Z]"`; do j=`echo $i | tr '[A-Z]' '[a-z]' | sed s/\-1$//`; mv $i $i-1; mv $i-1 $j; done
I needed to do this on a Cygwin setup on Windows 7 and found that I got syntax errors with the suggestions from above that I tried (though I may have missed a working option). However, this solution straight from Ubuntu forums worked out of the can :-)
ls | while read upName; do loName=`echo "${upName}" | tr '[:upper:]' '[:lower:]'`; mv "$upName" "$loName"; done
(NB: I had previously replaced whitespace with underscores using:
for f in *\ *; do mv "$f" "${f// /_}"; done
)
Slugify Rename (regex)
It is not exactly what the OP asked for, but what I was hoping to find on this page:
A "slugify" version for renaming files so they are similar to URLs (i.e. only include alphanumeric, dots, and dashes):
rename "s/[^a-zA-Z0-9\.]+/-/g" filename
I would reach for Python in this situation, to avoid optimistically assuming paths without spaces or slashes. I've also found that python2 tends to be installed in more places than rename.
#!/usr/bin/env python2
import sys, os
def rename_dir(directory):
print('DEBUG: rename('+directory+')')
# Rename current directory if needed
os.rename(directory, directory.lower())
directory = directory.lower()
# Rename children
for fn in os.listdir(directory):
path = os.path.join(directory, fn)
os.rename(path, path.lower())
path = path.lower()
# Rename children within, if this child is a directory
if os.path.isdir(path):
rename_dir(path)
# Run program, using the first argument passed to this Python script as the name of the folder
rename_dir(sys.argv[1])
If you use Arch Linux, you can install rename) package from AUR that provides the renamexm command as /usr/bin/renamexm executable and a manual page along with it.
It is a really powerful tool to quickly rename files and directories.
Convert to lowercase
rename -l Developers.mp3 # or --lowcase
Convert to UPPER case
rename -u developers.mp3 # or --upcase, long option
Other options
-R --recursive # directory and its children
-t --test # Dry run, output but don't rename
-o --owner # Change file owner as well to user specified
-v --verbose # Output what file is renamed and its new name
-s/str/str2 # Substitute string on pattern
--yes # Confirm all actions
You can fetch the sample Developers.mp3 file from here, if needed ;)
None of the solutions here worked for me because I was on a system that didn't have access to the perl rename script, plus some of the files included spaces. However, I found a variant that works:
find . -depth -exec sh -c '
t=${0%/*}/$(printf %s "${0##*/}" | tr "[:upper:]" "[:lower:]");
[ "$t" = "$0" ] || mv -i "$0" "$t"
' {} \;
Credit goes to "Gilles 'SO- stop being evil'", see this answer on the similar question "change entire directory tree to lower-case names" on the Unix & Linux StackExchange.
I believe the one-liners can be simplified:
for f in **/*; do mv "$f" "${f:l}"; done
( find YOURDIR -type d | sort -r;
find yourdir -type f ) |
grep -v /CVS | grep -v /SVN |
while read f; do mv -v $f `echo $f | tr '[A-Z]' '[a-z]'`; done
First rename the directories bottom up sort -r (where -depth is not available), then the files.
Then grep -v /CVS instead of find ...-prune because it's simpler.
For large directories, for f in ... can overflow some shell buffers.
Use find ... | while read to avoid that.
And yes, this will clobber files which differ only in case...
find . -depth -name '*[A-Z]*'|sed -n 's/\(.*\/\)\(.*\)/mv -n -v -T \1\2 \1\L\2/p'|sh
I haven't tried the more elaborate scripts mentioned here, but none of the single commandline versions worked for me on my Synology NAS. rename is not available, and many of the variations of find fail because it seems to stick to the older name of the already renamed path (eg, if it finds ./FOO followed by ./FOO/BAR, renaming ./FOO to ./foo will still continue to list ./FOO/BAR even though that path is no longer valid). Above command worked for me without any issues.
What follows is an explanation of each part of the command:
find . -depth -name '*[A-Z]*'
This will find any file from the current directory (change . to whatever directory you want to process), using a depth-first search (eg., it will list ./foo/bar before ./foo), but only for files that contain an uppercase character. The -name filter only applies to the base file name, not the full path. So this will list ./FOO/BAR but not ./FOO/bar. This is ok, as we don't want to rename ./FOO/bar. We want to rename ./FOO though, but that one is listed later on (this is why -depth is important).
This comand in itself is particularly useful to finding the files that you want to rename in the first place. Use this after the complete rename command to search for files that still haven't been replaced because of file name collisions or errors.
sed -n 's/\(.*\/\)\(.*\)/mv -n -v -T \1\2 \1\L\2/p'
This part reads the files outputted by find and formats them in a mv command using a regular expression. The -n option stops sed from printing the input, and the p command in the search-and-replace regex outputs the replaced text.
The regex itself consists of two captures: the part up until the last / (which is the directory of the file), and the filename itself. The directory is left intact, but the filename is transformed to lowercase. So, if find outputs ./FOO/BAR, it will become mv -n -v -T ./FOO/BAR ./FOO/bar. The -n option of mv makes sure existing lowercase files are not overwritten. The -v option makes mv output every change that it makes (or doesn't make - if ./FOO/bar already exists, it outputs something like ./FOO/BAR -> ./FOO/BAR, noting that no change has been made). The -T is very important here - it treats the target file as a directory. This will make sure that ./FOO/BAR isn't moved into ./FOO/bar if that directory happens to exist.
Use this together with find to generate a list of commands that will be executed (handy to verify what will be done without actually doing it)
sh
This pretty self-explanatory. It routes all the generated mv commands to the shell interpreter. You can replace it with bash or any shell of your liking.
Using bash, without rename:
find . -exec bash -c 'mv $0 ${0,,}' {} \;