12

To set the sticky bit on a directory, why do the commands chmod 1777 and chmod 3777 both work?

  • 2
    You are proposing a potentially risky operation.The combination sticky bit and rwx permisions for all is bad practice. Anybody can change and execute the file and the s-bit allow switching to root user without pasword. – jippie Feb 8 '13 at 16:37
  • 1
    @jippie setuid and setgid bits go away if the file is modified, so you can't get root access that way. – Kyle Jones Feb 11 '13 at 18:47
  • @KyleJones, it's still dangerous. If the passwd binary were world-writable, you wouldn't be able to get root access by modifying it, as you say, but you could replace it with some other binary that everyone would run thereafter, thinking it was passwd. – Wildcard Jan 9 '16 at 4:22
  • @Wildcard Agreed. – Kyle Jones Jan 9 '16 at 4:24
23

Each number (also referred to as an octal because it is base8) in that grouping represents 3 bits. If you turn it into binary it makes it a lot easier.

1 = 0 0 1
3 = 0 1 1
5 = 1 0 1
7 = 1 1 1

So if you did 1777, 3777, 5777, or 7777 you would set the sticky bit because the third column would be a 1. However, with 3777, 5777, and 7777 you are additionally setting other bits (SUID for the first column, and SGID for the second column).

Conversely, any other number in that spot (up to the maximum of 7) would not set the sticky bit because the last column wouldn't be a 1 or "on."

2 = 0 1 0
4 = 1 0 0
6 = 1 1 0

  • 3
    +1 for a nice description of how octal numbers work and how it applies to the file permission bits. – a CVn Feb 8 '13 at 14:44
  • It's called "bitmask", and +1 also for explaining/showing how it can set and clear the Owner Group & Other columns. – Chris K Dec 30 '13 at 21:07
15

The permissions passed as an argument to chmod are specified as an octal value. Each numeral in the value represents three bits. If three numerals are given, you're setting the read, write and execute bits for the file's owner, group and others (everyone else). If four numerals are given, the leftmost number sets the setuid, setgid and sticky bits. Octal 1 sets the sticky bit. Octal 2 sets the setgid bit. Octal 2 + octal 1 is octal 3 which sets both the setgid bit and the sticky bit.

  • 1
    Isn't it octal 2 | octal 1 rather than octal 2 + octal 1? The operations happen to have the same result in this case, but in general it's a bitwise or that matters, isn't it? – gerrit Feb 8 '13 at 9:06
  • 1
    @gerrit Yes, in the general case you should be looking at the binary or operator. However, as you point out, in this case it works out to the same result, and plenty more people are familiar with addition. – a CVn Feb 8 '13 at 14:44

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