File and Directory Permissions

ls -l shows the permissions string to the left.
broken down into 4 groups, e.g. drwxr-xr-x or -rw-rw-r--.
First group is a single character;
d: directory
-: file
l: link

next groups are 3 characters each. In order, they are:
permissions for the user, the group and the other / world / everybody else
r: read
w: write
x: execute

Modify permissions

Suppose file testfile.txt contains the command ls -l,

make file executable for everybody:
chmod +x testfile.txt

remove execution permission for user:
chmod u-x testfile.txt

remove execution permission for group:
chmod g-x testfile.txt

remove execution permission for other:
chmod o-x testfile.txt

make file executable for user:
chmod u+x testfile.txt

remove read permission for group:
chmod g-r testfile.txt

remove read permission for other:
chmod o-r testfile.txt

remove write permission for group:
chmod g-w testfile.txt

example: chmod u-w testfile.txt
will make the following command fail:
echo "https://github.com/TheNewThinkTank/code-vault" >> testfile.txt
chmod u+w testfile.txt
echo "https://github.com/TheNewThinkTank/code-vault" >> testfile.txt
now the content was appended to the file.

Combining chmod commands

Allow group to read and write with a single command:
chmod g+rw testfile.txt

Bit scores - numerical representations of r, w and x

r: 4 w: 2 x: 1

example
chmod 770 testfile.txt
results in permission string
-rwxrwx---

generally, chmod ??? testfile.txt with ??? equal to ugo, user, group and other.

example
ls -l Downloads/

Recursively change permissions for all files in dir, without affecting the dir: chmod 600 Downloads/*

Use with caution, applies to everything beneath the object: chmod -R 700 Downloads

Or use find command for more granular controls

Change ownership

Change ownership of all files in Downloads/ dir to new-user and new-user-group
sudo chown -R new-user:new-user-group Downloads/

Shorthand (leaving out group defaults to the user's group):
sudo chown -R new-user: Downloads/