Linux/Ubuntu File and Directory Permissions

Linux (and therefore Ubuntu) has file permissions on each file and directory for the owner, group and everyone else. Those permissions determine if the file can be viewed, executed or edited.

Only the owner of a file or directory (or a privileged user, root for example) may change its mode.

Ownership of a file

To change the ownership of the file or directory: chown new_owner_username directory

chown john public_html

to change the ownership of directory (and all the files and folders in the directory) and also the group: chown -R new_owner_username:new_groupname directory

chown -R john:developers public_html

to change the ownership of all the files in the current directory and also the group: chown -R new_owner_username:new_groupname *

chown john:developers *

File permissions

The easiest way to set Linux file permissions is using a 3 digit sequence. The first digit designates owner permission; the second, the group permission; and the third, everyone else’s permission.

Read = 4
Write = 2
Execute = 1

The digit is the sum of those. So if you want to grant only read permission you use 4; read and execute 5; read, write and execute = 7.

chmod 775 index.html

That will set the permissions on index.html so the owner, and a user in the group specified can read, write and execute the file and everyone else can read and execute.

chmod -R 755 public_html

That will set the permissions on files and directories (recursively through all subdirectories) so the owner can read, write and execute; members of the group and everyone else can read and execute (but not write).

ls - l

That will give you a list of files and directories, in a directory, with the owner and group settings and the permissions for all 3 (those 2 and everyone else), which will look something like:

-rw-r--r-- 1 root developers   397 2008-05-25 20:33 index.html
-rw-r--r-- 1 mary developers  9177 2010-05-02 22:18 unix_file_permissions.html
...

The lines start with the permissions for the owner, group and then everyone else. There are 9 total characters, 3 for each. Taking the top line above:

rw-r--r--
rw-  (means the owner has read and write permission but not execute)
r--  (means the group has only read permission)
r--  (means everyone else has only read permission)

The next column tells you the number of hard links to the file or directory. Then column tells you the owner, then the group. Then the byte size of the file, the date it was last change and then the file name.

root
means the username of this file is named root

developers
group (means those users in the group named developers have the group permissions indicated)

Related: Ubuntu command line interface syntax examples

Using the Host File in Ubuntu

You can use the host file to have your computer route to whatever addresses you desire (instead of using your nameserver). For example, by putting

sudo nano /etc/hosts

Then add a line to the file with the ip address and the name you will use.

204.11.50.136 wastetime

One useful way to use this is to test out a website on a new host prior to changing the nameserver to point to the new host. In this case, if you want to make sure your host file is being read you can ping wastetime and if it is working it will show the results for a ping to 204.11.50.136

Using scp (secure copy) to Copy Files Between Computers

Copy a file from your local computer to a remote host using secure copy, scp (which uses ssh for data transfer and provides the same security as using ssh).

scp [filename] [username]@remotehost:[location]

scp file_to_copy.txt username@example.net:/some/remote/directory

copy a directory to your home computer from the remote computer.

scp -r directory_to_copy username@example.net:

copy a directory from a remote server to the current directory on your computer.

scp -r folder [username]@remotehost:[location] .

scp -r username@example.net:/some/remote/directory .

If you don’t have automated keys setup you will be asked for the password for that user.

An example for copying a MySQL database. Including the : without a location puts the file in the home folder.

mysqldump database_name -uroot > database_dump.sql
scp database_dump.sql user@example.net:

Then ssh into the remote server and open the mysql prompt

mysql -uroot -p
 mysql> create database database_name;
 mysql> exit

Then run the mysqldump file

mysql database_name -uuser -p < database_dump.sql

Remember to create the database user on the new machine (this has to match what is in the wp-config.php file).

Ubuntu/Linux cli syntax

Command line interface syntax for various actions

Make new directory, make [directory_name]

sudo mkdir new_directory

Remove (file or directory)

rm [name]

rm -i file_to_remove.txt

Using -i prompts you to confirm the deletion.
Remove directory and all of its contents without having to confirm. Obviously be careful.
[/bash]sudo rm -r directory_to_remove[/bash]

Move a file

mv [name] [new_location]

mv file.txt new_sub_directory/file.txt

Rename a file (similar to moving)
mv [name] [new_name]

mv file.txt new_file_name.txt

Copy a fold

cp -r [folder]/* [new_location]

cp -r folder/* /some_place/else/

Keep ssh sessions live

If you want to stop your SSH sessions from being shut down you can add the following line to /etc/ssh/ssh_config on your local machine.

sudo gedit /etc/ssh/ssh_config

Then add:

ServerAliveInterval 30

This does remove the security feature of closing your session in case you leave your computer, but you may decide to take that risk. This sends a SSH package every 30 seconds so your server doesn’t close the connection due to inactivity.

Edit DNS Name Server

Set the DNS name server to use for your machine.

sudo nano /etc/resolv.conf

Then you can use for example (8.8.8.8 is Google public DNS, 208.67.222.222 is OpenDNS):

nameserver 208.67.222.222
nameserver 8.8.8.8
sudo lshw

will provide infor on your system, CPU types, 32 v 64 bit…