What is the difference between "chmod +x" and "chmod 755"?

When it comes to making a file executable what is the difference between chmod 755 and chmod +x and when would I use which? I so far have only used chmod +x and I just read something and it used chmod 755 and I could not tell whether or not it was better to use chmod 755 or chmod +x.


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.

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chmod +x adds the execute permission for all users to the existing permissions.

chmod 755 sets the 755 permission for a file.

755 means full permissions for the owner and read and execute permission for others.