Changing Who Has Access to Files

Using chmod to Set File Permissions

You can specify permissions for chmod using the letters u, g, and o, as symbolic code for the owner ("user"), group, and others (the class). This "symbolic mode" is easy to remember, since the symbols r, w, and x (the mode) are used directly as arguments in the command.

The chmod syntax uses the +, -, and = signs. The syntax is:

   chmod class[±=]mode,[ ... ] filename

For example, you can use the symbolic mode to create rw-r--r-- permissions by specifying the symbols rw, r, and r directly in the chmod command. "User" is represented by u, "group" by g, and "other" by o. To assign the permissions absolutely, use the = sign in the argument. Unspaced commas separate class permissions:

   chmod u=rw,g=r,o=r myfile

When permissions are being set the same, you can also combine the arguments as:

      chmod ugo=r myfile

With only read permission on myfile, no one can write to it. Also, if you now try to remove myfile, the rm command asks you whether you really want to remove the file:

   rm myfile

   myfile: 444 mode? (y/n) n If you do not want to remove it, enter n.



If you do want to remove it, enter y.

To create rw------- permissions and set "no permissions" for the classes g and o, use = with no symbol following:

   chmod u=rw,g=,o= filename

Permissions are added with the + sign. Again, separate each class-permission with a comma and no space:

   chmod u+rw,g+r,o+r filename

You can also subtract permissions from u, g, or o, using -, to restrict the level of permission from a previous "higher" level. For example, if you had set rwxrw-rw- and you wanted to change this to rwx------:

   chmod g-rw,o-rw filename

However, unless you began with no permissions you may find that using + or - has added to, or subtracted from, some previously existing permissions for that file. Run the ll command to check this. If in doubt, set the permissions absolutely by using =.

Later, if you want to permit yourself and members of your group to read from and write to myfile, use chmod as follows:

   chmod ug=rw,o=r myfile

The ll command now should show:

   -rw-rw-r--  1  leslie  users      154 Nov  4 10:18 myfile

Here is a summary of the various chmod commands you can use to protect myfile.