Linux - chmod for dummy

chmod.md

What is the difference between “chmod +x” and “chmod 755”?

Motivate from this question and answer https://askubuntu.com/a/932719/927010

Short version:

To be able to compare them, we should look at them from the same perspective, so:

chmod +x is equal to chmod ugo+x (Based on umask value) chmod 755 is equal to chmod u=rwx,go=rx

Explanation:

Firstly you should know that:

1. + means add this permission to the other permissions that the file already has.
2. = means ignore all permissions, set them exactly as I provide.

• So all of the "read, write, execute, sticky bit, suid and guid" will be ignored and only the ones provided will be set.

3. read = 4, write = 2, execute = 1

• Here is the binary logic behind it (if you're interested):

Symbolic:  r-- -w- --x  |  421
Binary:    100 010 001  |  -------
Decimal:    4   2   1   |  000 = 0
                        |  001 = 1
Symbolic:  rwx r-x r-x  |  010 = 2
Binary:    111 101 101  |  011 = 3
Decimal:    7   5   5   |  100 = 4
           /   /   /    |  101 = 5
Owner  ---/   /   /     |  110 = 6
Group  ------/   /      |  111 = 7
Others ---------/       |  Binary to Octal chart

Using +x you are telling to add (+) the executable bit (x) to the owner, group and others.

• it's equal to ugo+x or u+x,g+x,o+x
• When you don't specify which one of the owner, group or others is your target, in case of x it will considers all of them. And as @Rinzwind pointed out, it's based on umask value, it adds the bit to the ones umask allows. remember if you specify the target like o+r then umask doesn't have any effect anymore.
• It doesn't touch the other mods (permissions).
• You could also use u+x to only add executable bit to the owner.

Using 755 you are specifying:

• 7 --> u=rwx (4+2+1 for owner)
• 5 --> g=rx (4+1 for group)
• 5 --> o=rx (4+1 for others)

So chmod 755 is like: chmod u=rwx,g=rx,o=rx or chmod u=rwx,go=rx.