What is ACID Property?

ACID Property - Atomicity Consistency Isolation Durability

A for Atomicity

The principle of atomicity states that all parts of a transaction must complete or not.

C for Consistency

The principle of consistency states that the results of a query must be consistent with the state of the database at the time the query started. It requires that the database ensure that changed values are  not seen by the query. Through the use of undo segments Oracle guarantees that if a query succeeds, the result will be consistent to all else data shown from undo segments.

I for Isolation

The principle of isolation states that an incomplete i.e. uncommitted transaction must be invisible to all others. While the transaction is in progress, only the one session that is executing the transaction is allowed to see the changes, all other sessions must see the unchanged data, not the new values.

D for Durability

The principle of durability states that once a transaction completes with a COMMIT, it must be impossible for the database to lose it. During the time that the transaction is in progress, the principle of isolation requires that no one can see the changes it has made so far.

What are the File/Directory Permissions in Unix/Linux?

Every user on a Unix/Linux system has a unique username, and is a member of at least one group. This group information is held in the password file stored in /etc/passwd. A user can also be a member of one or more other groups, stored in /etc/group.  Only the administrator can create new groups or add/delete group members.

Every directory and file on the system has an owner, and also an associated group. It also has a set of permission flags which specify separate read, write and execute permissions for the owner, group and other.

ls -l command shows the permissions and group associated with files.

ls -g command shows the group information.

The permission flags are read as follows (left to right)

1	directory flag, 'd' if a directory, '-' if a normal file, something else occasionally may appear here for special devices.
2,3,4	read, write, execute permission for User (Owner) of file
5,6,7	read, write, execute permission for Group
8,9,10	read, write, execute permission for Other
-	in any position means that flag is not set
r	file is readable by owner, group or other
w	file is writeable. On a directory, write access means you can add or delete files
x	file is executable - only for programs and shell scripts. Execute permission on a directory means you can list the files in that directory