Okay, that was quite a sidetrack. Let's return to
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.
Of course, this is best shown by example. Consider
||Readable, writable and executable by owner, group and world (everyone)
||readable and writable by everyone but executable by no one
||readable, writable and executable only by the owner
||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
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