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Changing Permission: chmod

You can make changes to permissions by entering a chmod command. It allows the owner of the file to add to (+) or remove from (-) existing permissions. It also allows the owner to clear existing permission and assign all permission from scratch; this is known as assigning permissions absolutely (=). The chmod command affects any of the three types of access for any of the three categories of Unix users, using one-letter symbols in the following order (left to right):

u   owner (user)
g   File's group
o   all others
x   all (default)

+   add permission
-   remove permission
=   absolute permission

r   to read
w   to write
x   to execute

Caution: It is possible for you to lock yourself out of one of your own files with chmod. Be careful when you type it.

Example:

ls -l psab

-rwxr-xr-x 1 otto rz 487 Jul 30 10:21 psab

chmod o -x psab

ls -l psab

-rwx-xr--- 1 otto rz 487 Jul 30 10:21 psab

In the above example, the first ls -l shows the default permissions for a script, which is executable and readable by everyone, but writable only by the owner. After the chmod o-x command, the execution permission for others is removed. The file permissions can also be expressed in octal numeric form; see the man pages of chmod for details.