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If you have more than one user set up on your system, it s possible to switch users on the fly while you re working at the shell. On our test PC, we have an additional user account called frank. While logged in as any user, we can temporarily switch to this user by typing the following command, which stands for substitute user: su frank We ll then be asked for user frank s password. Once this is typed, we will effectively have logged in as user frank. Any files we create will be saved with frank s ownership. If you created a root account (by using the command sudo passwd root), you can temporarily switch into it by typing just su, without any username afterward. To return to your own account from any other account, type exit.
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Altering Permissions
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You can easily change permissions of files and directories by using the chmod command. For example, if you want to change a file so that everyone on the system can read and write to it, type the following:
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chmod a+rw myfile
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In other words, you re adding read and write (rw) permissions for all users (a), including the owner, the group, and everybody else. Here s another example:
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chmod a-w myfile
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This tells Linux that you want to take away (-) the ability of all users (a) to write (w) to the file. However, you want to leave the other permissions as they are.
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CHAPTER 14 UN DERS TANDING LINUX FILES AN D US ERS
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Tip If you leave out the a, chmod assumes you mean all . In other words, commands like chmod a+r
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myfile and chmod +r myfile do the same thing.
If you specify u, you can change permissions just for the owner (u is for user , which is the same as owner ):
chmod u+rw
This will add (+) read/write (rw) permissions for the owner. As you might already have guessed, you can substitute a g to change group permissions:
chmod g-rw
This will configure the file so that members of the group that owns the file can t read or write to it. Using an o, which is for others , will configure the file permissions for those who aren t the owner of the file or who are not in the group that owns the file the last three digits of the permission list. A typical day-to-day use of chmod is in making a program file that you ve downloaded executable. Because of the way the Internet works, if you download a program to install on your computer, it can lose its executable status while in transit. In this case, issue the following command:
chmod u+x myprogram
This will configure the file so that the owner (u) can execute (x) it.
Changing the Ownership of a File
To change the owner of a file, use the chown command. For security reasons, this must be prefaced with the sudo command, which is to say that chown and chgrp (to change the group ownership) require superuser powers. For example, to set the owner of myfile as frank, type this command:
sudo chown frank myfile
You can also change the owner and the group of a file using chown. Simply type each separated by a period:
sudo chown frank.mygroup myfile
This will change myfile so that its owner is frank and its group is mygroup. To change just the group of a file, you can use the chgrp command in exactly the same way as chown:
sudo chgrp mygroup myfile
CHAPTER 14 UNDERS TA NDIN G LINUX FILES AN D US ERS
CREATING FILE SHORTCUTS
We touched upon the idea of file system shortcuts in 12, when we discussed creating launchers on the GNOME desktop. The problem with launchers is that they are only recognized within GNOME. In other words, they mean nothing when you re using the command prompt (or virtually every other program that loads/saves files, with the exception of some programs created specially for the GNOME desktop environment). The Ubuntu file system offers two types of genuine shortcuts, which it refers to as file links. They are symbolic links and hard links. Both are created using the ln command. Symbolic links are the most commonly used. A symbolic link is the most similar to a Windows shortcut in that a small file is created that points toward another file. Unlike a Windows shortcut, the symbolic link file exists at the file system level, so it can t be viewed in a text editor, for example. You can spot a symbolic link in a file listing, because it will be followed by an arrow and then the name and path (if necessary) of the file it links to. For example, in your /home directory, the directory Examples is symbolically linked to /usr/share/example-content. When you type ls -l, it appears as follows: Examples -> /usr/share/example-content A hard link is more complex and needs some understanding of how files work. In simple terms, all files consist of a pointer and actual data. As you might expect, the pointer tells the file system where on the disk to find the data. Creating a hard link effectively creates an additional pointer to the data that has exactly the same attributes as the original pointer, except with a different name. Performing any operation on the linked file will perform that operation on the original file. Additionally, there will be no obvious sign the hard link isn t actually a genuine file, apart from the fact that the link count the number after the file permissions will be more than 1. This indicates that more than one file links to the data. Maybe now you can see why people prefer to use the more obviously detectable symbolic links! To create a symbolic link, the -s command option is used with the ln command. First, specify the original file and then the new link s name. Here s an example, followed immediately by the output of the ls -l command, which shows the results: ln -s original_file link ls -l lrwxrwxrwx 1 keir keir 13 2006-11-22 12:05 link -> original_file -rw-r--r-- 1 keir keir 0 2006-11-21 15:30 original_file The new link has odd file permissions. It claims to have read/write/execute permissions for everybody (rwxrwxrwx) but actually, because it s a link, it mirrors the permissions of the file it links to. So if you attempt to access a shortcut that links to a file you don t have permission to access, you ll see the appropriate error message. To create a hard link, simply use ln on its own: ln original_file link
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