Type "ls -l" and a listing like the following is displayed:
total 10
drwxrwxrwx 4 george team1 122 Dec 12 18:02 Projects
-rw-rw-rw- 1 george team1 1873 Aug 23 08:34 test
-rw-rw-rw- 1 george team1 1234 Sep 12 11:13 datafile
Which means the following:
Type and # of Files's File's Size in Date of last Filename
Permission field Links Owner Group Bytes modification
| | | | | | |
drwxrwxrwx 4 george team1 122 Dec 12 18:02 Projects
Wipe out all the permissions but add read permission for everybody:
$ chmod a=r testfile
After the command, the file's permissions would be -r--r--r--
Add execute permissions for group:
$ chmod g+x testfile
Now, the file's permissions would be -r--r-xr--
Add both write and execute permissions for the file's owner. Note how you can set more than one permission at the same time:
$ chmod u+wx testfile
After this, the file permissions will be -rwxr-xr--
Remove the execute permission from both the file's owner and group. Note, again, how you can set them both at once:
$ chmod ug-x testfile
Now, the permissions are -rw-r--r--
As a summary, have a look at this quick reference for setting file permissions in symbolic mode:
Which user?
u user/owner
g group
o other
a all
What to do?
+ add this permission
- remove this permission
= set exactly this permission
Which permissions?
r read
w write
x execute
The other mode in which chmod can be used is the numeric mode. In the numeric mode, the file permissions aren't represented by characters. Instead, they are represented by a three-digit octal number.
4 = read (r)
2 = write (w)
1 = execute (x)
0 = no permission (-)
To get the permission bits you want, you add up the numbers accordingly. For example, the rwx permissions would be 4+2+1=7, rx would be 4+1=5, and rw would be 4+2=6. Because you set separate permissions for the owner, group, and others, you'll need a three-digit number representing the permissions of all these groups.
Let's have an example.
$ chmod 755 testfile
This would change the testfile's permissions to -rwxr-xr-x. The owner would have full read, write, and execute permissions (7=4+2+1), the group would have read and execute permissions (5=4+1), and the world would have the read and execute permissions as well.
Let's have another example:
$ chmod 640 testfile
In this case, testfile's permissions would be -rw-r-----. The owner would have read and write permissions (6=4+2), the group would have read permissions only (4), and the others wouldn't have any access permissions (0).
The numeric mode may not be as straightforward as the symbolic mode, but with the numeric mode, you can more quickly and efficiently set the file permissions. This quick reference for setting file permissions in numeric mode might help:
Which number?
0 ---
1 --x
2 -w-
3 -wx
4 r--
5 r-x
6 rw-
7 rwx
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