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Hummingbird Computing
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Changing
File and Directory Access Permission In general, IRIX file access permissions
are set to allow ease of use among multiple users while maintaining system
security. This section discusses how you can change file access
permissions to permit or deny read, write, and execution permission for
users, groups, or everyone. Note that users can also configure a umask to
control default access to their own files (see “About Default File
Permissions (umask)” on page 123 for more information.) IRIX divides permissions into three
categories, and users into three relationships. The three relationships
are the owner of the file, the owner’s user group, and all users. If you
view a long listing of a directory, you see that the permissions field for
each file in the directory looks something like this:
Note that the line of permissions has the
string rwx repeated three times. The first instance of rwx applies to the
file owner, the next instance applies to the group members, and the third
applies to all other users on the system. The example above shows full
permissions. A more restricted permission set might look like this:
The three categories of permissions are
read, write, and execute. They are denoted as “r” for read, “w” for write,
and “x” for execute in long listings of files. To get a long listing,
enter:
at your system prompt in any directory.
Along with the permission information, the ls –l command lists the owners
and the sizes of the files and the date they were last modified.
Read permission allows you to look at the
contents of a file. Write permission allows you to make changes to or
remove a file. Execute permission allows you to run the file as a command
from your shell prompt. Each character is separately significant in
the permissions listing. Starting at the left, the first character is a
dash. A dash in any other position means that no permission is granted and
the actions associated with that permission are denied. However, in the
leftmost place, the contents of that space describes whether the file is a
file or a directory. If it is a directory, a “d” appears in that space.
Other characters in this place indicate that the file is a pipe, a block
or character special device file, or other type of file.
To see the current status of a file’s
permission settings, use the ls -l command. For example, to see the status
of permission on the file review, enter:
Now the access permissions should look like
this:
Permissions are shown as read (r), write
(w), and execute (x), for each of user, group, and other, respectively.
That is, each of the user, group, and everyone else has some combination
of read, write, and execute access to the file. After the first character
(in this example, a dash), the next three characters give the read, write,
and execute permission for the user, the next three characters give the
read, write, and execute access for the group, and the last three
characters give the read, write, and execute access for everyone else. So
in the example, user jones has read (r) and write (w), access to the file
review, while the group has only read (r) access, and other also has only
read (r) access. Nobody has execute (x) permission. The superuser or owner of the file can
change these permission settings. As superuser, you can give everyone
write access to a file with the chmod command. For example, to add write
access for the group and others to the review file, use the go+w (g for
group, o for other, and +w to permit writing) option as follows:
Now the access permissions should look like
this:
Another way of controlling permission
settings is with the octal number representation obtained by using 7 as
representing read, write and execute permission (4+2+1). In this way, to
give complete read, write, and execute permissions to a file, use the
chmod 777 command, and to give just read permission to the owner and no
other permissions at all, use chmod 400. For complete information on
setting access permissions on files and directories, refer to the chmod(1)
reference page. Note:
If you use chmod on a
device file, edit the /etc/ioperms file to reflect the change, or the
device file returns to the default access permissions after a reboot. The
format of an entry in /etc/ioperms is:
where device_name is the device filename,
owner is the file owner, group is the group, and nnn is the octal
permission setting as described above and in the chmod(1) reference page.
Refer to the ioconfig(1M) reference page for details on device permission
settings in the /etc/ioperms file. Directory
Permissions Directories use the same permissions as
files, but their meanings are slightly different. For example, read
permission on a directory means that you can use the ls command to look at
the contents of that directory. Write permission allows you to add,
change, or remove files in that directory. (However, even though you may
have write permission in that directory, you must also have write
permission on the individual files to change or remove them, unless you
own the directory.) Finally, execute permission on a directory allows you
to use the cd command to change directories into that directory. File
Permissions The first series of three places after the
leftmost place in the permissions field describe the permissions for the
owner of the file. Here is an example of a long listing for a file:
The file is not a directory, so the
leftmost space is blank. The characters rwx indicate that the owner of the
file, owner, has read, write, and execute permission on this file. The
second series of three spaces describe permissions for the owner’s group.
In this case, the group is grp. Suppose permissions for this file were
slightly different, like this:
In that case, any member of the group grp
could read or execute the file, but not change it or remove it. All
members of group grp can share a pool of files that are individually
owned. Through careful use of group read and write permissions, you can
create a set of doc source files that are owned by one person, but any
group member can work on them. The third series of spaces provides for all
other users on the system and is called the public permissions. A file
that is set to be readable by any user on the system is called publicly
readable. Here is a long listing of a sample Projects
directory:
The files in this directory have varying
permissions. Some are restricted to the owner, some can be read only by
members of the owner’s group, and some can be read, changed, or removed by
anybody. The shell script is executable by any user. Changing
Permissions You change the permissions on a file by
means of the chmod(1) command. You can use chmod only to change files that
you own. Generally, you use this command to protect files you want to keep
secret or private, to protect private directories, and to grant
permissions to files that need to be used by others. To restrict access to
a file or directory to yourself only, enter:
Other permissions may be added by using the
chmod command with the letter associated with the permission. For example,
to add general write permission to a file, enter:
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