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