8. Advanced Topics

Here the game gets tough. Learn these features, then you'll be ready to say that you `know something about Linux' ;-)

8.1. Permissions and Ownership

Files and directories have permissions (`protections') and ownership, just like under VMS. If you can't run a program, or can't modify a file, or can't access a directory, it's because you don't have the permission to do so, and/or because the file doesn't belong to you. Let's have a look at the following example:

$ ls -l /bin/ls
-rwxr-xr-x   1 root     bin         27281 Aug 15  1995 /bin/ls*

The first field shows the permissions of the file ls (owner root, group bin). There are three types of ownership: owner, group, and others (similar to VMS owner, group, world), and three types of permissions: read, write (and delete), and execute.

From left to right, - is the file type (- = ordinary file, d = directory, l = link, etc); rwx are the permissions for the file owner (read, write, execute); r-x are the permissions for the group of the file owner (read, execute); r-x are the permissions for all other users (read, execute).

To change a file's permissions:

$ chmod <whoXperm> <file>

where who is u (user, that is owner), g (group), o (other), X is either + or -, perm is r (read), w (write), or x (execute). Examples:

$ chmod u+x file

this sets the execute permission for the file owner. Shortcut: chmod +x file.

$ chmod go-wx file

this removes write and execute permission for everyone except the owner.

$ chmod ugo+rwx file

this gives everyone read, write, and execute permission.

A shorter way to refer to permissions is with numbers: rwxr-xr-x can be expressed as 755 (every letter corresponds to a bit: --- is 0, --x is 1, -w- is 2...).

For a directory, rx means that you can cd to that directory, and w means that you can delete a file in the directory (according to the file's permissions, of course), or the directory itself. All this is only part of the matter---RMP.

To change a file's owner:

$ chown username file

To sum up, a table:

VMS				Linux			Notes

SET PROT=(O:RW) file.txt	$ chmod u+rw file.txt
				$ chmod 600 file.txt
SET PROT=(O:RWED,W) file	$ chmod u+rwx file
				$ chmod 700 file
SET PROT=(O:RWED,W:RE) file	$ chmod 755 file
SET PROT=(O:RW,G:RW,W) file	$ chmod 660 file
SET FILE/OWNER_UIC=JOE file	$ chown joe file
SET DIR/OWNER_UIC=JOE [.dir]	$ chown joe dir/

8.2. Multitasking: Processes and Jobs

More about running programs. There are no `batch queues' under Linux as you're used to; multitasking is handled very differently. Again, this is what the typical command line looks like:

$ command -s1 -s2 ... -sn par1 par2 ... parn < input > output &

where -s1, ..., -sn are the program switches, par1, ..., parn are the program parameters.

Now let's see how multitasking works. Programs, running in foreground or background, are called `processes'.

In addition to this, the shell allows you to stop or temporarily suspend a process, send a process to background, and bring a process from background to foreground. In this context, processes are called `jobs'.

8.3. Files, Revisited

More information about files.

8.4. Print Queues

Your prints are queued, like under VMS. When you issue a print command, you may specify a printer name. Example:

$ lpr file.txt          # this goes to the standard printer
$ lpr -Plaser file.ps   # this goes to the printer named 'laser'

To handle the print queues, you use the following commands:

VMS					Linux

$ PRINT file.ps				$ lpr file.ps
$ PRINT/QUEUE=laser file.ps		$ lpr -Plaser file.ps
$ SHOW QUEUE				$ lpq
$ SHOW QUEUE/QUEUE=laser       		$ lpq -Plaser
$ STOP/QUEUE				$ lprm <item>