You can set file permissions with the chmod command. Both the root user and the file's owner can set file permissions. chmod has two modes, symbolic and numeric.
First, you decide if you set permissions for the user (u), the group (g), others (o), or all of the three (a). Then, you either add a permission (+), remove it (-), or wipe out the previous permissions and add a new one (=). Next, you decide if you set the read permission (r), write permission (w), or execute permission (x). Last, you'll tell chmod which file's permissions you want to change.
Here are a few examples.
Wipe out all the permissions but add read permission for everybody:
$ chmod a=r filename
After the command, the file's permissions would be -r--r--r--
Add execute permissions for group:
$ chmod g+x filename
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 filename
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 filename
Now, the permissions are -rw-r--r--
This is a quick reference for setting file permissions in symbolic mode:
What to do?
+ add this permission
- remove this permission
= set exactly this permission