13.14. Ownership and Permissions

13.14.1. The chmod Command

Use the chmod command to change permissions. This example shows how to change the permissions on sneakers.txt with the chmod command.

The original file looks like this, with its initial permissions settings:

-rw-rw-r--    1 sam sam     150 Mar 19 08:08 sneakers.txt

If you are the owner of the file or are logged into the root account you can change any permissions for the owner, group, and others.

Right now, the owner and group can read and write to the file. Anyone outside of the group can only read the file (r--).

	Caution
	altered, or deleted. As a rule, you should only grant read and write permissions to those who truly need them.

of the file permissions.

Take a look at the file first. At the shell prompt, type:

ls -l sneakers.txt

The previous command displays this file information:

-rw-rw-r--    1 sam sam     150 Mar 19 08:08 sneakers.txt

Now, type the following:

chmod o+w sneakers.txt

The o+w command tells the system you want to give others write permission to the file sneakers.txt. To check the results, list the file's details again. Now, the file looks like this:

-rw-rw-rw-    1 sam sam     150 Mar 19 08:08 sneakers.txt

Now, everyone can read and write to the file.

To remove read and write permissions from sneakers.txt use the chmod command to take away both the read and write permissions.

chmod go-rw sneakers.txt

By typing go-rw, you are telling the system to remove read and write permissions for the group and for others from the file sneakers.txt.

The result will look like this:

-rw-------    1 sam sam    150 Mar 19 08:08 sneakers.txt

Think of these settings as a kind of shorthand when you want to change permissions with chmod, because all you really have to do is remember a few symbols and letters with the chmod command.

Here is a list of what the shorthand represents:

Want to test your permissions skills? Remove all permissions from sneakers.txt — for everyone.

chmod a-rwx sneakers.txt

Now, see if you can read the file with the command cat sneakers.txt, which should return the following:

cat: sneakers.txt: Permission denied

chmod u+rw sneakers.txt

Use the command cat sneakers.txt to verify that you, the file owner, can read the file again.

Here are some common examples of settings that can be used with chmod:

• g+w — adds write access for the group

• o-rwx — removes all permissions for others

• u+x — allows the file owner to execute the file

• a+rw — allows everyone to read and write to the file

• ug+r — allows the owner and group to read the file

• g=rx — allows only the group to read and execute (not write)

By adding the -R option, you can change permissions for entire directory trees.

permission to search through that directory.

If you do not allow others to have execute permission to tigger, it will not matter who has read or write access. No one will be able to get into the directory unless they know the exact file name.

For example, type:

chmod a-x tigger

to remove everyone's execute permissions.

Here is what happens now when you try to cd to into tigger:

bash: tigger: Permission denied

Next, restore your own and your group's access:

chmod ug+x tigger

Now, if you check your work with ls -l you will see that only others will be denied access to the tigger directory.

13.14.2. Changing Permissions With Numbers

Remember the reference to the shorthand method of chmod? Here is another way to change permissions, although it may seem a little complex at first.

Go back to the original permissions for sneakers.txt:

-rw-rw-r--    1 sam sam     150 Mar 19 08:08 sneakers.txt

Each permission setting can be represented by a numerical value:

• r = 4

• w = 2

• x = 1

• - = 0

= 6.

For sneakers.txt, here are the numerical permissions settings:

 -  (rw-)   (rw-)  (r--)
      |       |      |
    4+2+0   4+2+0  4+0+0

The total for the user is six, the total for the group is six, and the total for others is four. The permissions setting is read as 664.

If you want to change sneakers.txt so those in your group will not have write access, but can still read the file, remove the access by subtracting two (2) from that set of numbers.

The numerical values, then, would become six, four, and four (644).

To implement these new settings, type:

chmod 644 sneakers.txt

Now verify the changes by listing the file. Type:

ls -l sneakers.txt

The output should be:

-rw-r--r--    1 sam sam     150 Mar 19 08:08 sneakers.txt

Now, neither the group nor others have write permission to sneakers.txt. To return the group's write access for the file, add the value of w (2) to the second set of permissions.

chmod 664 sneakers.txt

	Warning
	could allow tampering with sensitive files, so in general, it is not a good idea to use these settings.

Here is a list of some common settings, numerical values and their meanings:

• -rw------- (600) — Only the owner has read and write permissions.

• -rw-r--r-- (644) — Only the owner has read and write permissions; the group and others have read only.

• -rwx------ (700) — Only the owner has read, write, and execute permissions.

• -rwxr-xr-x (755) — The owner has read, write, and execute permissions; the group and others have only read and execute.

• -rwx--x--x (711) — The owner has read, write, and execute permissions; the group and others have only execute.

• -rw-rw-rw- (666) — Everyone can read and write to the file. (Be careful with these permissions.)

• -rwxrwxrwx (777) — Everyone can read, write, and execute. (Again, this permissions setting can be hazardous.)

Here are some common settings for directories:

• drwx------ (700) — Only the user can read, write in this directory.

• drwxr-xr-x (755) — Everyone can read the directory; users and groups have read and execute permissions.