Added find notes to the knowledge base

knowledge base/find.md

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+# Find
+
+## TL;DR
+
+```shell
+# change the permissions of all files and directories in the current directory, recursively
+find . -type d -exec chmod 755 {} +
+find . -type f -exec chmod 644 {} +
+
+# change the ownership of all files and directories owned by a specific user or group, recursively
+find . -type d -user harry -exec chown daisy {} +
+find . -type f -group users -exec chown :admin {} +
+
+# delete all empty files and directories in the 'Documents' directory
+find Documents -empty -delete               # recursively
+find Documents -maxdepth 1 -empty -delete   # non recursively
+
+# get the extensions of all files larger than 1MB
+find . -type f -size +1M -exec basename {} \; | sed 's|.*\.||' | sort -u
+
+# get all empty directories in a git repository (but not the repo's ones)
+find $PATH_TO_REPO -type d -empty -not -path "./.git/*"
+
+# find broken symlinks in the given directories, recursively
+find $DIR1 $DIR2 $DIRN -type l -exec test ! -e {} ';' -print   # posix
+find $DIR1 $DIR2 $DIRN -xtype l                                # gnu find only
+
+# get files by name, in numeric order regardless of the directory they are in
+find . -type f -o -type l \
+  | awk 'BEGIN {FS="/"; OFS="|"} {print $NF,$0}' \
+  | sort --field-separator '|' --numeric-sort \
+  | cut -d '|' -f2
+
+# print quoted file paths
+# %p is for path
+find . -type f -printf '%p\n'
+
+# sort files by size
+# %s is for size
+find . -type f -printf '%s %p\n' | sort -nr | head -50
+
+# get files which are executable but not readable
+find /sbin /usr/sbin -executable -not -readable -print
+
+# get files which are writable by either their owner or their group
+find . -perm /220
+find . -perm /u+w,g+w
+find . -perm /u=w,g=w
+
+# get files which are writable by both their owner and their group.
+find . -perm -220
+find . -perm -g+w,u+w
+
+# list set-user-ID files and directories into /root/suid.txt and list large files into /root/big.txt
+find / \( -perm -4000 -fprintf /root/suid.txt '%#m %u %p\n' \) , \( -size +100M -fprintf /root/big.txt '%-10s %p\n' \)
+```
+
+## Gotchas
+
+- in GNU's `find` the path parameter defaults to the current directory and can be avoided
+
+  ```shell
+  # delete all empty folders in the current directory only
+  find -maxdepth 1 -empty -delete
+  ```
+
+## Further readings
+
+- [How can I find broken symlinks?]
+- [find . -type f -exec chmod 644 {} ;]
+- [how to output file names surrounded with quotes in SINGLE line?]
+
+[find . -type f -exec chmod 644 {} ;]: https://stackoverflow.com/questions/19737525/find-type-f-exec-chmod-644#22083532
+[how can i find broken symlinks?]: https://unix.stackexchange.com/questions/34248/how-can-i-find-broken-symlinks
+[how to output file names surrounded with quotes in single line?]: https://stackoverflow.com/questions/6041596/how-to-output-file-names-surrounded-with-quotes-in-single-line#15137696