chmod – Managing Permissions

chmod – Managing Permissions

Several different syntaxes can be used with chmod. The first is to enter

chmod entity = permissions filename at the shell prompt. You substitute u for Owner, g for Group, and o for Others in the entity portion of the command. You substitute r, w, and/or x for the permissions portion of the command. For example, suppose I wanted to change the mode of contacts.odt to –rw–rw–r– –

(giving the Owner and Group read and write permissions while giving Others only read access).

I would enter chmod u=rw,g=rw,o=r contacts.odt at the shell prompt (assuming the file resides in the current directory). After I do so, the mode is adjusted with the permissions assigned by chmod, as shown here:

openSUSE:/home/ksanders # chmod u=rw,g=rw,o=r contacts.odt

openSUSE:/home/ksanders # ls -l contacts.odt

-rw-rw-r-- 1 ksanders users 0 Mar 18 08:02 contacts.odt

You can also use chmod to toggle a particular permission on or off using the

+ or – sign. For example, suppose I want to turn off the write permission I just gave to Group for the contacts.odt file.

I could enter chmod g–w contacts.odt at the shell prompt. When I do, the specified permission is turned off, as shown next:

openSUSE:/home/ksanders # chmod g-w contacts.odt

openSUSE:/home/ksanders # ls -l contacts.odt

-rw-r--r-- 1 ksanders users 0 Mar 18 08:02 contacts.odt

If I wanted to turn the permission back on, I would enter chmod g+w contacts.odt. You can substitute u or o, respectively, to modify the permission to the file or directory for Owner or Others as well.

Finally, you can also use numeric permissions with chmod. This is the option I use most often.

You can modify all three entities at once with only three characters. To do this, enter chmod <numeric_permission>  <filename>

.

Going back to our earlier example, suppose I wanted to grant read and write permissions to Owner and Group, but remove all permissions from Others. That would mean Owner and Group’s permissions would be represented numerically as 6. Because Others gets no permissions, its permissions would be represented by 0.

I could implement this by entering chmod 660 contacts.odt at the shell prompt.

When I do, the appropriate changes are made, as shown here:

openSUSE:/home/ksanders # chmod 660 contacts.odt

openSUSE:/home/ksanders # ls -l contacts.odt

-rw-rw---- 1 ksanders users 0 Mar 18 08:02 contacts.odt

You can use the –R option with chmod to change permissions on many files at once, recursively.