In the diagram below, we see how the first
portion of the listing is interpreted. It consists
of a character indicating the file type, followed
by three sets of three characters that convey the
reading, writing and execution permission for the
owner, group, and everybody else.
The
chmod command is used
to change the permissions of a file or directory.
To use it, you specify the desired permission
settings and the file or files that you wish to
modify. There are two ways to specify the
permissions, but I am only going to teach one
way.
It is easy to think of the permission settings
as a series of bits (which is how the computer
thinks about them). Here's how it works:
rwx rwx rwx = 111 111 111
rw- rw- rw- = 110 110 110
rwx --- --- = 111 000 000
and so on...
rwx = 111 in binary = 7
rw- = 110 in binary = 6
r-x = 101 in binary = 5
r-- = 100 in binary = 4
Now, if you represent each of the three sets of
permissions (owner, group, and other) as a single
digit, you have a pretty convenient way of
expressing the possible permissions settings. For
example, if we wanted to set
some_file to
have read and write permission for the owner, but
wanted to keep the file private from others, we
would:
[me@linuxbox me]$ chmod 600 some_file
Here is a table of numbers that covers all the
common settings. The ones beginning with "7" are
used with programs (since they enable execution)
and the rest are for other kinds of files.
| Value |
Meaning |
| 777 |
(rwxrwxrwx) No restrictions on
permissions. Anybody may do anything.
Generally not a desirable setting. |
| 755 |
(rwxr-xr-x) The file's owner may
read, write, and execute the file. All others
may read and execute the file. This setting
is common for programs that are used by all
users. |
| 700 |
(rwx------) The file's owner may
read, write, and execute the file. Nobody
else has any rights. This setting is useful
for programs that only the owner may use and
must be kept private from others. |
| 666 |
(rw-rw-rw-) All users may read
and write the file. |
| 644 |
(rw-r--r--) The owner may read
and write a file, while all others may only
read the file. A common setting for data
files that everybody may read, but only the
owner may change. |
| 600 |
(rw-------) The owner may read
and write a file. All others have no rights.
A common setting for data files that the
owner wants to keep private. |
The
chmod command can also
be used to control the access permissions for
directories. In most ways, the permissions scheme
for directories works the same way as they do with files.
However, the execution permission is used in a
different way. It provides control for access to
file listing and other things. Here are some useful
settings for directories:
| Value |
Meaning |
| 777 |
(rwxrwxrwx) No restrictions on
permissions. Anybody may list files, create
new files in the directory and delete files
in the directory. Generally not a good
setting. |
| 755 |
(rwxr-xr-x) The directory owner
has full access. All others may list the
directory, but cannot create files nor delete
them. This setting is common for directories
that you wish to share with other users. |
| 700 |
(rwx------) The directory owner
has full access. Nobody else has any rights.
This setting is useful for directories that
only the owner may use and must be kept
private from others. |
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