Permission Bits
July 19, 1998
Okay, that was quite a sidetrack. Let's return to
permissions.
If you recall, the "ls -l" utility displayed several
permission-related fields: Namely, the permission bits, the
owner and the group name. We can use these three bits of
information to determine who can read a file, delete or modify
a file, or execute it if it is executable.
Let's look at the permission bits.
The permission bits specify three sets of three
security specifications. Specifically, they define:
- Is the file readable by the owner?
- Is the file writable (modify or delete) by the owner?
- Is the file executable by the owner?
- Is the file readable by users in the specified group?
- Is the file writable by users in the specified group?
- Is the file executable by users in the specified group?
- Is the file readable by any old user?
- Is the file writable by any old user?
- Is the file executable by any old user?
These 9 fields are represented in that order in the permission
bits field of the ls-l command. If there is a "-" in the field
that means that the file is "not" readable, writable, or
executable depending on the field.
![[Permission Bits]](Images/permission_bits.gif)
Of course, this is best shown by example. Consider
the following:
| Listing |
Explanation |
| rwxrwxrwx |
Readable, writable and executable by owner, group and world
(everyone) |
| rw-rw-rw |
readable and writable by everyone but executable by no one
|
| rwx------ |
readable, writable and executable only by the owner |
| rwxrw-r-- |
readable, writable and executable by the owner. Readable
and writable by group members and readable by world. |
Note that directories work just the same as files except that
execution privilege represents the ability to move into the
directory. If you do not give executable privileges to world
and group, nobody can "cd" into the directory but you.
So the next obvious question is, how do you modify these
permission fields? How do you change the permission bits, the
owner and the user?
Additional Resources:
Who Are you and Who else is here?
Introduction to UNIX for Web Developers | Table of Contents
The "chmod" Utility
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