'chmod' explained

chmod is the program that is used to change file permissions on a Linux system. As we mentioned, others cannot modify your personal user files but you may not want other people to even read these files. You can use this command to take away the possibility of others prying into your private stuff.

The syntax (parts separated by brackets) for using this command is the following:

chmod [a/o/g/u] [+ or -] (plus or minus sign) [r/w/x]

Let's analyze the syntax:

• The symbols in the first brackets refer to the four concepts of users that Linux has.
  • a=all, all users
  • o=others, (other people)
  • g=group, the members of your group
  • u=user, (this means you)
• The symbol + adds permissions and the symbol - takes them away. As we say in the Linux world chmod giveth, chmod taketh away
• Your actual rights to files - r=read rights, w=write rights (pardon the homophonic redundancy!) and x=executable rights

Practical examples of chmod

As we mentioned before, you may want to restrict rights to read a file. You would simply type:

chmod og-r my_world_domination_plan.txt

Now nobody can read your plans to take over the world except you. Best to keep those secret anyway.

We also talked earlier about those files that you may have copied from a Windows partition or a floppy formatted for Windows. Even if they're not executable files, they'll show up as such (-rwxr-xr-x). You can change them to their correct permissions, getting rid of the executable status with:

chmod a-x the_file

Remember that if you copied them as 'root', you will have to use chmod as root to do this. You could even go one step further and change the ownership of the file to the user you desire. You may want to change root ownership to yourself (your user name). We'll go into this in the next part of the lesson.

Using chmod with number values

The permissions in Linux also can also be changed with number values. An accomplished Linux/Unix user will probably use this system more. When you're beginning with Linux, the method we described before will probably be clearer to you. Without going into tedious technical detail, we'll describe the chmod number system to you and look at two of the most common examples.

As I said before, we have three sections to permissions; those of the owner of the file followed by those of the group and then the permissions of others. So if you use numbers with 'chmod', there will be three numbers following the command. One number corresponds to each group. Here's an example of a chmod command with numbers:

chmod 644 grocery_list.txt

If you had used 'pico' to write up a grocery list for your trip to the supermarket today, Linux would have given you 644 permission by default. That means, read and write permissions for the owner but no write permissions for the group and for others. So the number 6 means read and write permissions for you, the number 4 means read permissions for the group and the last number, 4 means read permissions for others. Now the question is: Why these numbers? Why not other numbers, like the number 8 or 12 or even 1345? Let's explain this.

The three types of permissions correspond to three numbers. Read permission is given a value of 4. Think of it as the most important permission. (if you can't read a file, then what else can you do with it?) Write permission is given a value of 2. Execute permission is given a value of 1. Here's what it would look like:

User (or the owner)	Group	Others
read-write-execute	read-write-execute	read-write-execute
4-2-1	4-2-1	4-2-1

It's really just a question of simple arithmetic. In our example of chmod 644 grocery_list.txt, we've added the 4 for read permission and to the 2 for write permission for the owner to get 6. We've just given the group and others read permission, so there's nothing to add in these two groups. Just give it a 4 in each case. Presto! 644.

Some practical examples with numbers

Let's take another look at my file that has my plans for dominating the world (my_world_domination_plan.txt). I had taken away the read permissions for everybody except myself using +/- letter method. (chmod og-r my_world_domination_plan.txt)

Now let's do the same with the numbers.

chmod 600 my_world_domination_plan.txt

As you can see I have read and write permissions for me (4 read + 2 write equals 6) and 0 permissions for everybody else. The Zero value means 7 (maximum permission value) minus 4 minus 2 minus 1 equals 0.

Real world examples with 'chmod'

I'd like to talk about a real world practical example for doing this. There is a command mode program called 'fetchmail' for getting your e-mail. It's a very good program if you're using your computer as different users for various jobs. It will send mail to the different users based on e-mail addresses. The different addresses are stored in a file that you create called '.fetchmailrc' This file also includes the user name and password for retrieving mail from each address. We've got a possible security hole here, particularly if you're networked. This file shouldn't be read by everybody. When you create your .fetchmailrc file, you must then use chmod 600/chmod og-r on the file to solve the security issue. In fact, 'fetchmail' won't run if you don't. It will complain about the file's permissions.

Let's look at another "real world" example. As the internet becomes more popular, websites need to be more user friendly and interactive. It's becoming more and more common to use scripts to improve the quality of a website's offerings. If you used a language like perl to write the scripts and you hosted your website with a company that runs Linux on their servers (we would hope that you would do this!), you would probably be given permission to use these scripts on your website. You would then have to give the scripts read and execute permissions so that the your visitors could use them. After you uploaded a script to your website, you would then issue this command:

chmod 755 my_awesome_script

and the script would become "executable" (and readable) for the outside world. (7 - 4 read, 2 write 1 executable for you, 4 read and 1 executable for group and others). You could also use chmod og+rx my_awesome_script The script is then "converted" into a "program" so to speak. You have also converted yourself into a "web developer" with the right to hang your shingle on the door.

chmod is one of those commands that are used most by system administrators. In our Linux system administration course, we'll go into this command in more detail. In this lesson we've shown you just a few practical examples to get your feet wet with this very important command.

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