Linux Bible. Christopher Negus
typing the
ls -ld command. The named file or directory appears as those shown in this example:
$ ls -ld ch3 test -rw-rw-r-- 1 joe sales 4983 Jan 18 22:13 ch3 drwxr-xr-x 2 joe sales 1024 Jan 24 13:47 test
The first line shows that the ch3 file has read and write permission for the owner and the group. All other users have read permission, which means that they can view the file but cannot change its contents or remove it. The second line shows the test directory (indicated by the letter d before the permission bits). The owner has read, write, and execute permissions while the group and other users have only read and execute permissions. As a result, the owner can add, change, or delete files in that directory, and everyone else can only read the contents, change to that directory, and list the contents of the directory. (If you had not used the -d options to ls, you would have listed files in the test directory instead of permissions of that directory.)
Changing permissions with chmod (numbers)
If you own a file, you can use the chmod command to change the permission on it as you please. In one method of doing this, each permission (read, write, and execute) is assigned a number—r=4, w=2, and x=1—and you use each set's total number to establish the permission. For example, to make permissions wide open for yourself as owner, you would set the first number to 7 (4+2+1), and then you would give the group and others read-only permission by setting both the second and third numbers to 4 (4+0+0), so that the final number is 744. Any combination of permissions can result from 0 (no permission) through 7 (full permission).
Here are some examples of how to change permission on a file (named file) and what the resulting permission would be:
The following chmod command results in this permission: rwxrwxrwx
# chmod 777 file
The following chmod command results in this permission: rwxr-xr-x
# chmod 755 file
The following chmod command results in this permission: rw-r--r--
# chmod 644 file
The following chmod command results in this permission: ---------
# chmod 000 file
The chmod command also can be used recursively. For example, suppose that you wanted to give an entire directory structure 755 permission (rwxr-xr-x), starting at the $HOME/myapps directory. To do that, you could use the -R option, as follows:
$ chmod -R 755 $HOME/myapps
All files and directories below, and including, the myapps directory in your home directory will have 755 permissions set. Because the numbers approach to setting permission changes all permission bits at once, it's more common to use letters to change permission bits recursively over a large set of files.
Changing permissions with chmod (letters)
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.
Setting default file permission with umask
When you create a file as a regular user, it's given permission rw-rw-r-- by default. A directory is given the permission rwxrwxr-x. For the root user, file and directory permission are rw-r--r-- and rwxr-xr-x, respectively. These default values are determined by the value of umask. Enter umask to see what your umask value is. For example:
$ umask 0002
If you ignore the leading zero for the moment, the umask value masks what is considered to be fully opened permissions for a file 666 or a directory 777. The umask value of 002 results in permission for a directory of 775 (rwxrwxr-x). That same umask results in a file permission of 644 (rw-rw-r--). (Execute permissions are off by default for regular files.)
To change your umask value temporarily, run the umask command. Then try creating some files and directories to see how the umask value affects how permissions are set. For example:
$ umask 777 ; touch file01 ; mkdir dir01 ; ls -ld file01 dir01 d---------. 2 joe joe 6 Dec 19 11:03 dir01 ----------. 1 joe joe 0 Dec 19 11:02 file01 $ umask 000 ; touch file02 ; mkdir dir02 ; ls -ld file02 dir02 drwxrwxrwx. 2 joe joe 6 Dec 19 11:00 dir02/ -rw-rw-rw-. 1 joe joe 0 Dec 19 10:59 file02 $ umask 022 ; touch file03 ; mkdir dir03 ; ls -ld file03 dir03 drwxr-xr-x. 2 joe joe 6 Dec 19 11:07 dir03 -rw-r--r--. 1 joe joe 0 Dec 19 11:07 file03
If you want to change your umask value permanently, add a umask command to the .bashrc file in your home directory (near the end of that file). The next time you open a shell, your umask is set to whatever value you chose.
Changing file ownership
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