Copyright © 2000 Paul Sheer - Click here for copying permissions

Permission and Modification Times

Permissions

Every file and directory on a UNIX system, besides being owned by a user and a group, has access flags^13.1 dictating what kind of access that user and group has to the file.

Doing an ls -ald /bin/cp /etc/passwd /tmp will give you a listing:

-rwxr-xr-x   1 root     root        28628 Mar 24  1999 /bin/cp -rw-r--r--   1 root     root         1151 Jul 23 22:42 /etc/passwd drwxrwxrwt   5 root     root         4096 Sep 25 15:23 /tmp

In the left most column are these flags, which give a complete description of the access rights to the file.

The furthest flag to the left is, so far, either - or d indicating an ordinary file or directory. The remaining nine have a - to indicate an unset value or one of several possible characters. Table 13.1 gives a complete description of file system permissions.

Table 13.1: File and directory permissions

The chmod command is used to change the permissions of a file. It usually used like:

chmod [-R] [u|g|o|a][+|-][r|w|x|s|t] <file> [<file>] ...

For example

chmod u+x myfile

adds execute permissions for the user of myfile. And,

chmod a-rx myfile

removes read and execute permissions for all -- i.e. user, group and other.

The -R options once again means recursive, diving into subdirectories as usual.

Permission bits are often represented in their binary form, especially when programming. It is convenient to show the rwxrwxrwx set in octal, where each digit fits conveniently into three bits. Files on the system are usually created with mode 0644, meaning rw-r-r-. You can set permissions explicitly with an octal number:

chmod 0755 myfile

Gives myfile the permissions rwxr-xr-x.

In the table you can see s, the setuid or setgid bit. If it is used without execute permissions then it has no meaning and is written capitalised as an S. This bit effectively colourises a x into an s, hence you should read and s as execute with the setuid or setgid bit set. t is known as the sticky bit. It also has no meaning if there are no execute permissions and is written as a capital T.

The leading 0 can in be ignored, but is preferred in order to be explicit. It can take on a value representing the three bits, setuid (4), setgid (2) and sticky (1). Hence a value of 5764 is $101~111~110~100$ in binary and gives -rwsrw-r-T.

Modification times and the stat command

In addition to permissions, each file has three integers associated with it that represent in seconds, the last time the file was accessed (read), when it was last modified, and when it was created. These are known as the atime, mtime and ctime of a file respectively.

To get a complete listing of the file's permissions, use the stat command. Here is the result of stat /etc:

5

File: "/etc"   Size: 4096         Filetype: Directory   Mode: (0755/drwxr-xr-x)         Uid: (    0/    root)  Gid: (    0/    root) Device:  3,1   Inode: 14057     Links: 41    Access: Sat Sep 25 04:09:08 1999(00000.15:02:23) Modify: Fri Sep 24 20:55:14 1999(00000.22:16:17) Change: Fri Sep 24 20:55:14 1999(00000.22:16:17)

The Size: quoted here is the actual amount of disk space used in order to store the directory listing, and is the same as reported by ls. In this case it is probably four disk blocks of 1024 bytes each. The size of a directory as quoted here does not mean the sum of all files contained under it.