A Quick Introduction to chmod and Octal Numbers

Someone asked what the difference between doing a chmod 777 and chmod 755 is today, and hopefully this short, informal post will provide you with the answer (if you want to jump straight through to the conclusion, man chmod).

Octal Numbers

The number you provide as an argument to chmod is an octal number telling chmod what access you want to provide to a file (or a directory, device, etc – an entry on the file system). The number are in fact three discreet values, 7, 5 and 5. Each of the values correspond to a set of three bits, either one being zero or one. Three bits makes up a value from 0 – 7, hence an octal number (a decimal number has the digits 0 – 9 for each digit, an octal number has 0 – 7, a binary number has 0 – 1, a hexadecimal number has 0 – F (15)).

If you tried to count from 0 to 10 (decimal) in octal, it’d be: 0, 1, 2, 3, 4, 5, 6, 7, 10, 11, 12. 12 in octal is the same value as 10 in decimal. The big difference is that both octal and decimal maps very neatly on top of binary numbers, being exactly three or four bits.

The usual way to write an octal number in a programming language is by appending a zero in front of it, such as 0755. This tells the compiler that the number is written in octal notation, and the value is then parsed as such. chmod parses all numbers as octal, and does actually handle four digits. Since missing digits are considered to be zero, the first digit is usually not included (or simply as a zero – which will look the same as the representation used in certain programming languages). The first, usually unused digit, have a special meaning, setting the “set user id” (suid), “set group id” (guid) or the “restricted deletion” or “sticky” attributes (you can read more about these options in the manual page).

File permissions

Now that we know what an octal number is, it’s time to look at how the file permissions work. Each file has three sets of permissions, one set for the user owning the file, one set for the group owning the file and one set for anyone else. If you want to take a look at these values on a unix based system, simply type ls -l to list files in a verbose way. Your result will look something like:

-rw-r--r--  1 mats mats        35 2008-08-23 20:24 IMPORTANTFILE

The permissions are listed in the first column, containng “-rw-r–r–“. The first character “-” indicates if the file is a directory (d), if the suid or guid bits are set etc.

This leaves us with “rw-r–r–” – the three sets of permissions. “rw-” is for the user owning the file, “r–” is for the group owning the file and the last “r–” are for anyone else (or ‘other’ as it’s called). The “r” means read, the “w” means write and the currently missing letter is “x”, which means execute (for files) or search (for directories). The “execute” setting is used to let bash (or another shell) attempt to run the file as a script, attempting to parse the first line as a path to the interpreter for the file (i.e. #!/usr/bin/python).

We have three flags (read, write, execute) that can be either on or off. This should remind us of three bits, either being 0 (not set) or 1 (set). And an octal digit is exactly three bits. This means that an octal digit maps exactly to the bit sequence needed to set permissions for a file. A 7 is “111”, a 5 is “101”, a 4 is “100” and so on. Mapping this to permissions:

7 = 111 = rwx
6 = 110 = rw-
5 = 101 = r-x
4 = 100 = r--
3 = 011 = -wx
2 = 010 = -w-
1 = 001 = --x
0 = 000 = ---

When calling chmod 755 on a directory we’re telling chmod to “set the read, write and search bits for me, the read and search bits for the group and the read and search bits for other users” (‘search’ for directories, ‘execute’ for files).

Another example is 644 that maps to 110 100 100, which again maps to “rw-r–r–” which usually is the standard access mode for files (and 755 for directories).

Handling Permissions With Symbols

I’m now going to eliminate the need for remembering everything I’ve written so far in the post, but at least you’ll know what people are talking about when they’re telling you to chmod something this-or-that.

You can also use the symbols directly with chmod, either adding, removing or setting the permissions for one of the three groups.

Examples:

To remove all access for other users (but leaving group and user intact)
chmod o-rwx file

To give everyone read access
chmod a+r file

To give everyone read – and search – access
chmod a+rx directory

To set particular user modes for each group
chmod u=rw,g=w,o=w file (a file that the user can read, but anyone can write to)

And with that I chmod this post a+r.

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