When people say:
Max permissions on a file: 666
they usually mean default maximum permissions when a new regular file is created, not the maximum you can manually set.
1. Default maximum for new files: 666
For a new regular file, Linux normally starts from:
666 = rw-rw-rw-
That means:
owner = read + write
group = read + write
others = read + write
No execute permission by default.
Why? Because most new files are text files, data files, documents, config files, etc. They should not automatically be executable.
Example:
touch file1.txt
ls -l file1.txt
You may see something like:
-rw-r--r-- 1 user user file1.txt
The actual permission is affected by the umask.
2. Default maximum for directories: 777
For a new directory, Linux normally starts from:
777 = rwxrwxrwx
Why? Because directories need x permission to be entered or searched.
Example:
mkdir dir1
ls -ld dir1
You may see:
drwxr-xr-x 2 user user dir1
Again, the final permission is affected by the umask.
3. What if I give a file 777?
You can manually give a file 777:
chmod 777 file1.txt
ls -l file1.txt
Output:
-rwxrwxrwx 1 user user file1.txt
This means:
owner = read + write + execute
group = read + write + execute
others = read + write + execute
So everyone can read, modify, and execute the file.
4. Is 777 allowed?
Yes, it is allowed.
But it is usually not safe.
For a regular file, 777 means any user can change the file and possibly run it as a program or script.
For example, this is risky:
chmod 777 script.sh
because any user may be able to modify the script and then execute it.
5. Better permissions
For a normal text/config/data file:
chmod 644 file.txt
Meaning:
owner can read/write
group can read
others can read
For a private file:
chmod 600 file.txt
For a script that only the owner should run:
chmod 700 script.sh
For a script others can read and execute but not modify:
chmod 755 script.sh
Simple summary
666 = normal maximum default for new files
777 = normal maximum default for new directories
777 on a file is possible, but usually unsafe
Slide-friendly version:
Linux does not give execute permission to new regular files by default. New files start from a maximum of
666, while directories start from777because directories need execute permission to be entered. A file can be changed to777manually, but this gives everyone read, write, and execute access, which is usually insecure.
REF: AI Tools/ChatGPT
