Permissions

What are file permissions?

In Linux/Unix, every file and folder has permissions that control:

• Who can read it (see its contents)

• Who can write to it (modify or delete it)

• Who can execute it (run it as a program or script)

There are three groups for permissions:

1. User (u) — the owner

2. Group (g) — users in the same group

3. Others (o) — everyone else

Each group has three permissions:

• r = read

• w = write

• x = execute

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How does the number system work (like 777)?

Each permission has a number value — these are called octal values -->

Permission	Value
read (r)	4
write (w)	2
execute (x)	1

You add up the values to get the permission number:

Example:

• rwx = 4 + 2 + 1 = 7

• rw- = 4 + 2 + 0 = 6

• r-- = 4 + 0 + 0 = 4

So when you see something like chmod 777, it means:

Group	Permissions	Meaning
User	7 (rwx)	Read, write, and execute
Group	7 (rwx)	Read, write, and execute
Others	7 (rwx)	Read, write, and execute

In short:

EVERYONE can do ANYTHING to the file!

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Example:

If you do:

chmod 777 myfile

then:

• You (user) can read/write/execute myfile

• Your group can read/write/execute myfile

• Any other random user on the system can also read/write/execute myfile

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Why is chmod 777 dangerous?

• Anyone can modify or delete the file.

• If it’s a script or executable, anyone can inject malicious code.

• It’s basically giving full control to everyone — like leaving your front door wide open!

That's why you should almost never set 777 permissions unless you're doing it in a controlled/test environment (and you understand the risks).

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Cheatsheet for Common chmod:

chmod	Meaning
755	Owner: all (rwx), Group/Others: read and execute (r-x)
700	Owner: all (rwx), Group/Others: no permissions
644	Owner: read/write (rw-), Group/Others: read (r--)
600	Owner: read/write (rw-), Group/Others: no permissions
777	Everyone: full permissions (rwx) — dangerous!

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Permissions Visualization:

For example, 755 would look like this when you do ls -l:

-rwxr-xr-x  1 user group 12345 Apr 27 10:00 myfile

Breaking that down:

• rwx → owner can read/write/execute

• r-x → group can read/execute

• r-x → others can read/execute