chown [OPTION]... [OWNER][:[GROUP]] FILE... # change file owner and group
chgrp [OPTION]... GROUP FILE... # change group ownership
```
**File Permissions**:
-`r`: Read. Can see file content
-`w`: Write. Can modify file content
-`x`: Execute. Can execute file
**Directory Permissions**:
-`r`: Read. Can see dir contents
-`w`: CRUD. Can create, rename and delete files
-`x`: Search. Can access and navigate inside the dir. Necessary to operate on files
***Common* Octal Files Permissions**:
-`777`: (`rwxrwxrwx`) No restrictions on permissions. Anybody may do anything. Generally not a desirable setting.
-`755`: (`rwxr-xr-x`) The file's owner may read, write, and execute the file. All others may read and execute the file. This setting is common for programs that are used by all users.
-`700`: (`rwx------`) The file's owner may read, write, and execute the file. Nobody else has any rights. This setting is useful for programs that only the owner may use and must be kept private from others.
-`666`: (`rw-rw-rw-`) All users may read and write the file.
-`644`: (`rw-r--r--`) The owner may read and write a file, while all others may only read the file. A common setting for data files that everybody may read, but only the owner may change.
-`600`: (`rw-------`) The owner may read and write a file. All others have no rights. A common setting for data files that the owner wants to keep private.
***Common* Octal Directory Permissions**:
-`777`: (`rwxrwxrwx`) No restrictions on permissions. Anybody may list files, create new files in the directory and delete files in the directory. Generally not a good setting.
-`755`: (`rwxr-xr-x`) The directory owner has full access. All others may list the directory, but cannot create files nor delete them. This setting is common for directories that you wish to share with other users.
-`700`: (`rwx------`) The directory owner has full access. Nobody else has any rights. This setting is useful for directories that only the owner may use and must be kept private from others.
### Finding Files And Directories
```sh
find [path] [expression] # search file in directory hierarchy
find [start-position] -type f -name FILENAME # search for a file named "filename"
find [start-position] -type d -name DIRNAME # search for a directory named "dirname"
find [path] -exec <command> {} \; # ececute command on found items (identified by {})
[ -f "path" ] # test if a file exists
[ -d "path" ] # test if a folder exists
```
### Other
```sh
tee # copy standard input and write to standard output AND files simultaneously
tee [FILE]
command | sudo tee FILE # operate on file w/o using shell as su
echo # display a line of text
echo "string" > FILE # write lin of text to file
echo "string" >> FILE # append line o ftext to end of file (EOF)
wget URL # download repositories to linux machine
curl # dovnlaod the contents of a URL
curl [-I --head] # Fetch the headers only
ps [-ax] # display processes
kill <PID> # kill process w/ Process ID <PID>
killall PROCESS # kill process by nane
grep # search through a string using a REGEX
grep [-i] # grep ignore case
source script.sh # load script as a command
diff FILES # compare files line by line
# sudo apt install shellcheck
shellcheck FILE # shell linter
xargs [COMMAND] # build and execute command lines from standard input
# xargs reads items form the standard input, delimites by blanks or newlines, and executes the COMMAND one or more times with the items as argumests
watch [OPTIONS] COMMAND # execute a program periodically, showing output fullscreen
watch -n SECONDS COMMAND # execute command every SECONDS seconds (no less than 0.1 seconds)
```
## Data Wrangling
**Data wrangling** is the process of transforming and mapping data from one "raw" data form into another format with the intent of making it more appropriate and valuable for a variety of downstream purposes such as analytics.
```bash
sed # stream editor for filtering and transforming text
sed -E "s/REGEX/replacement/" # subsitute text ONCE (-E uses modern REGEX)
sed -E "s/REGEX/replacement/g" # subsitute text multiple times (every match)
wc [FILE] # print newline, word and byte countd for each file
wc [-m --chars] FILE # print character count
wc [-c --bytes] FILE # print bytes count
wc [-l --lines] FILE # print lines count
wc [-w --words] FILE # print word count
sort [FILE] # sort lines of a text file
uniq [INPUT [OUTPUT]] # report or omit repeated lines (from INPUT to OUTPUT)
uniq [-c --count] # prefix lines w/ number of occurrences
uniq [-d --repeated] # plrint only duplicare lines, one for each group
uniq [-D] # plrint only duplicare lines
paste [FILE] # merge lines of files
paste [-d --delimiters=LIST] # use delimiters from LIST
paste [- --serial] # paste one file at a time instead of in parallel
awk '{program}' # pattern scanning and processing language
awk [-f --file PROGRAM_FILE] # read program source from PROGRAM_FILE instead of from first argument
bc [-hlwsqv long-options] [FILE] # arbitrary precision calculator language
bc [-l --mathlib] [FILE] # use standard math library
