Advanced Topics

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# 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'.

- To launch a process in background:

      $ progname [-switches] [parameters] [< input] [> output] &
      [1] 234

  the shell tells you what the `job number' (the first digit; see below) and PID (Process IDentifier) of the process are. Each process is identified by its PID.

- To see how many processes there are:

      $ ps -ax

  This will output a list of currently running processes.

- To kill a process:

      $ kill <PID>

  You may need to kill a process when you don't know how to quit it the right way... ;-). Sometimes, a process will only be killed by one of the following:

      $ kill -15 <PID>
      $ kill -9 <PID>

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'.

- To see how many jobs there are:

      $ jobs

  jobs are identified by the numbers the shell gives them, not by their PID.

- To stop a process running in foreground:

      $ CTRL-C

  (it doesn't always work)

- To suspend a process running in foreground:

      $ CTRL-Z

  (ditto)

- To send a suspended process into background (it becomes a job):

      $ bg <job>

- To bring a job to foreground:

      $ fg <job>

- To kill a job:

      $ kill <%job>

## 8.3. Files, Revisited

More information about files.

- stdin, stdout, stderr: under UNIX, every system component is treated as if it were a file. Commands and programs get their input from a `file' called `stdin` (standard input; usually, the keyboard), put their output on a `file' called `stdout` (usually, the screen), and error messages go to a `file' called `stderr` (usually, the screen). Using `<` and `>` you redirect input and output to a different file. Moreover, `>>` appends the output to a file instead of overwriting it; `2>` redirects error messages (stderr); 2>&1 redirects stderr to stdout, while 1>&2 redirects stdout to stderr. There's a `black hole' called `/dev/null`: everything redirected to it disappears;

- wildcards: '`*`' is almost the same. Usage: * matches all files except the hidden ones; .* matches all hidden files; *.* matches only those that have a '.' in the middle, followed by other characters; p*r matches both `peter' and `piper'; *c* matches both `picked' and `peck'. '`%`' becomes '`?`'. There is another wildcard: the `[]`. Usage: `[abc]*` matches files starting with a, b, c; `*[I-N,1,2,3]` matches files ending with I, J, K, L, M, N, 1, 2, 3;

- `mv` (`RENAME`) doesn't work for multiple files; that is, `mv *.xxx *.yyy` won't work;

- use `cp -i` and `mv -i` to be warned when a file is going to be overwritten.

## 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>

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