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Operating on Files

In addition to the commands like cd, mv, and rm you learned in Chapter gif, there are other commands that just operate on files but not the data in them. These include touch, chmod, du, and df. All of these files don't care what is in the file--the merely change some of the things Unix remembers about the file.

Some of the things these commands manipulate:

touch  file1 file2 ...fileN

touch will update the time stamps of the files listed on the command line to the current time. If a file doesn't exist, touch will create it. It is also possible to specify the time that touch will set files to--consult the the manpage for touch.

chmod  [-Rfv] mode file1 file2 ...fileN

  The command used to change the permissions on a file is called chmod, short for change mode. Before I go into how to use the command, let's discuss what permissions are in Unix. Each file has a group of permissions associated with it. These permissions tell Unix whether or not the file can be read from, written to, or executed as a program. (In the next few paragraphs, I'll talk about users doing these things. Naturally, any programs a user runs are allowed to do the same things a user is. This can be a security problem if you don't know what a particular program does.)

However, Unix recognizes three different people: first, the owner of the file (and the person allowed to use chmod on that file). The group of most of your files might be ``users'', meaning the normal users of the system. (To find out the group of a particular file, use ls -l file.) Then, there's everybody else who isn't the owner and isn't a member of the group.

So, a file could have read and write permissions for the owner, read permissions for the group, and no permissions for all others. Or, for some reason, a file could have read/write permissions for the group and others, but no permissions for the owner!

Let's try using chmod to change a few permissions. First, create a new file using cat, emacs, or any other program. By default, you'll be able to read and write this file. (The permissions given other people will vary depending on how the system and your account is setup.) Make sure you can read the file using cat. Now, let's take away your read privilege by using chmod u-r filename. (The parameter u-r decodes to ``user minus read''.) Now if you try to read the file, you get a Permission denied error! Add read privileges back by using chmod u+r filename.      

Directory permissions  use the same three ideas: read, write, and execute, but act slightly differently. The read privilege allows the user (or group or others) to read the directory--list the names of the files. The write permission allows the user (or group or others) to add or remove files. The execute permission allows the user to access files in the directory or any subdirectories. (If a user doesn't have execute permissions for a directory, they can't even cd to it!)

To use chmod, replace the mode with what to operate on, either user, group, other, or all, and what to do with them. (That is, use a plus sign to indicate adding a privilege or a minus sign to indicate taking one away. Or, an equals sign will specify the exact permissions.) The possible permissions to add are read, write, and execute.

chmod's R flag will change a directory's permissions, and all files in that directory, and all subdirecties, all the way down the line. (The `R' stands for recursive.) The f flag forces chmod to attempt to change permissions, even if the user isn't the owner of the file. (If chmod is given the f flag, it won't print an error message when it fails to change a file's permissions.) The v flag makes chmod verbose--it will report on what it's done.

next up previous contents index
Next: System Statistics Up: Powerful Little Programs Previous: The Power of Unix

Converted on:
Mon Apr 1 08:59:56 EST 1996