find

# To find files following symlinks (otherwise the symlinks are not followed):
find -L . -type f

# To find files by case-insensitive extension (ex: .jpg, .JPG, .jpG):
find . -iname "*.jpg"

# To find directories:
find . -type d

# To find files:
find . -type f

# To find files by octal permission:
find . -type f -perm 777

# To find files with setuid bit set:
find . -xdev \( -perm -4000 \) -type f -print0 | xargs -0 ls -l

# To find files newer than 1 day old and copy elsewhere (remove -p flag in xargs to not be asked):
find . -type f -ctime -1 -print0 | xargs -0 -p cp -t <dir>
# or:
find . -type f -ctime -1 -print0 | xargs -0 -p -J % cp % <dir>

# To find files with extension '.txt' and remove them:
find ./path/ -name '*.txt' -delete

# To find files with tilde as postfix and remove them:
find ./path/ -name '*~' -delete

# To find files with extension '.txt' and dump their contents:
find ./path/ -name '*.txt' -exec cat '{}' \;

# To find files with extension '.txt' and look for a string into them:
find ./path/ -name '*.txt' | xargs grep 'string'

# To find files with size bigger than 5 Mebibyte and sort them by size:
find . -size +5M -type f -print0 | xargs -0 ls -Ssh | sort -z

# To find files bigger than 2 Megabyte and list them:
find . -type f -size +200000000c -exec ls -lh {} \; | awk '{ print $9 ": " $5 }'

# To find files modified more than 7 days ago and list file information:
find . -type f -mtime +7d -ls

# To find symlinks owned by a user and list file information:
find . -type l -user <username-or-userid> -ls

# To search for and delete empty directories:
find . -type d -empty -exec rmdir {} \;

# To search for directories named build at a max depth of 2 directories:
find . -maxdepth 2 -name build -type d

# To search all files who are not in .git directory:
find . ! -iwholename '*.git*' -type f

# To find all files that have the same node (hard link) as MY_FILE_HERE:
find . -type f -samefile MY_FILE_HERE 2>/dev/null

# To find all files in the current directory and modify their permissions:
find . -type f -exec chmod 644 {} \;

# To find all files changed in last 2 days:
find . -type f -ctime -48h
find . -type f -ctime -2
# Or created in last 2 days:
find . -type f -Btime -2
# Or accessed in last 2 days:
find . -type f -atime -2

# To find and rename (imperfect) all files and dirs that have a comma in the
# name (dry-run):
find . -name '*,*' | while read f; do echo mv "$f" "${f//,/}"; done

# To find all broken links. Note -L returns a file unless it is a broken link:
find -L /usr/ports/packages -type l

# To find and run multiple shell commands (without multiple execs):
# See: https://stackoverflow.com/questions/5119946/find-exec-with-multiple-commands
find . -type f -exec sh -c "echo '{}'; cat '{}';" \;

# To find files that are newer than a file:
find <path> -newer <target-file>