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You can also turn file permissions on and off using plus (+) and minus (–) signs, respectively, along with letters to indicate what changes and for whom. Using letters, for each file you can change permission for the user (u), group (g), other (o), and all users (a). What you would change includes the read (r), write (w), and execute (x) bits. For example, start with a file that has all permissions open (rwxrwxrwx). Run the following chmod commands using minus sign options. The resulting permissions are shown to the right of each command.

The following chmod command results in this permission: r-xr-xr-x

 $ chmod a-w file

The following chmod command results in this permission: rwxrwxrw-

 $ chmod o-x file

The following chmod command results in this permission: rwx------

 $ chmod go-rwx file

Likewise, the following examples start with all permissions closed (---------). The plus sign is used with chmod to turn permissions on.

The following chmod command results in this permission: rw-------

 $ chmod u+rw files

The following chmod command results in this permission: --x--x--x

 $ chmod a+x files

The following chmod command results in this permission: r-xr-x---

 $ chmod ug+rx files

Using letters to change permission recursively with chmod generally works better than using numbers because you can change bits selectively instead of changing all permission bits at once. For example, suppose that you want to remove write permission for “other” without changing any other permission bits on a set of files and directories. You could do the following:

 $ chmod -R o-w $HOME/myapps

This example recursively removes write permissions for “other” on any files and directories below the myapps directory. If you had used numbers such as 644, execute permission would be turned off for directories; using 755, execute permission would be turned on for regular files. Using o-w, only one bit is turned off and all other bits are left alone.