Linux-User message #24 from Janis Kracht to All.
   Entered on 30th October, 1999 at 19:44, 243 lines.

Subject: Linux Tips 003
=======================

Linux Tips 003

==================================================================
Identifying what you see in a long directory list
==================================================================

Under Linux, every item you see listed in a directory list is a file.
That includes directories, binary (executible) files, data files,
etc.  You can tell what a particular listing is by looking at the
first column of a long directory display. The following is a clip of
a directory listing using ls -ltr -d (long format, sorted by time/date,
reverse, -d lists directory entries instead of the contents of the
directory).

for example:

[bbs@jkracht bbbs]$ ls -ltr -d test2

drwxrwxr-x   2 bbs      bbs          1024 Oct 30 13:41 test2/

The first character describes the type of file it is: directory (d).
If it is not a directory, a '-' is listed.

for example:
-rw-rw-r--   1 bbs      bbs            12 Jul 21 13:11 tst.txt


Under DOS, you are given information in directory listings regarding
type, creation date/time:

=====
DOS
=====
<DIR> TEST2 (date/time created, size)


Under linux, you not only see the creation date/time and type, but
you also see who has read, write and execute privileges, who "owns"
the file, etc.:

=====
Linux
=====

(permissions) (owner)  (group)       (size)(date/time) (name)
drwxrwxr-x   2 bbs      bbs          1024 Oct 30 13:41 test2/


In each listing, you see the d (or -) in the first field, followed by
rwx rwx rwx.

These characters display who has read/write/execute permissions for
this file. These characters list the permissions for this file in the
order of  user/group/everyone-else:

 (user/group/everyone-else)
drwxrwxr-x   2 bbs      bbs          1024 Oct 30 13:41 test2/

So here you can see that the user has read/write/execute permission,
anyone in the group listed has the same permissions, and "everyone-else"
who may try to access this file may only read it/execute it.
Everyone-else then, cannot modify the file.


|usr
 ---
 |  |group
 |   ---
 |   |  |everyone-else
 |   |   ---
 |   |   |
 |   |   |
 --- --- ---
drwx rwx r-x   2 bbs      bbs          1024 Oct 30 13:41 test2/
----------     --------------               ------------ ------
d direcotry
-r who can see this
--w who can write to it
---x it is executible or not (- means not) for this person

              |ownername groupname

                                           |Creation Date/time

                                                        |filename

So, the above file, test2, is a directory, owned by user bbs.  User
bbs has read/write/execute permissions for this file, while anyone
else in the _group_ bbs may also read/write/execute.  All other
persons may only read/execute.


==================================================================
chmod - change file access permissions
==================================================================

There may be times when you would like to keep a directory "private"
so that it is only visible (readable) by yourself.  Or there may be
times when you want a file to be only readable and not executible,
etc.  You can make any file on a linux system private by changing the
attributes with the chmod command. i.e., Since directories are only
files, you can therefore change the directory's attributes.

In order to change a files attributes, you need to understand the bit
pattern of the chmod command.

Using this bit pattern, you assign a number to the
user/group/everone-else fields of one or all files.  It is the
cumulative value of octal digits 0-7 for bits 4, 2 and 1 that defines
the permissions as to who may r/w/x.

A value of 4 gives the field read access.
A value of 2 gives write access.
A value of 1 gives execute access.

If you add the values of the bits you want to change, you have the value of
the field.

Sounds scary, but it's easier to see when you look at it with this table:


adding  up  the bits with values of bits 4, 2, and 1.

(USER GROUP EVERYONE-ELSE/USER GROUP EVERYONE-ELSE/USER GROUP EVERYONE-ELSE)
xxx xxx xxx
--- --- ---
421 421 421 These are values that are added.
rwx rwx rwx

775
4+2+1 4+2+1 4+1
rwx   rwx   r-x

660
4+2+0 4+2+0 0+0+0
rw-   rw-   ---



So to make our new directory rwx by user bbs ONLY,

Original at creation:
drwxrwxr-x   2 bbs      bbs          1024 Oct 30 13:41 test2/

you would type:

chmod 700 test2

This changes the permissons to:

drwx------   2 bbs      bbs          1024 Oct 30 13:41 test2/
(4+2+1/0+0+0/0+0+0)
 7     0     0

To make it readable by all, but not executible:

chmod 666 test2

drw-rw-rw-   2 bbs      bbs          1024 Oct 30 13:41 test2/
4+2/4+2/4+2
6   6   6

To make it rw for everyone, but rwx only for user bbs:

chmod 766 test2

So now the permissions look like this:

drwxrw-rw-   2 bbs      bbs          1024 Oct 30 13:41 test2/
4+2+1/4+2+0/4+2+0
7    6      6

There is something you learn very quickly when you run a *nix
operating system.. #1, don't log in as root (superuser) to perform
general tasks.  If a file is removed, changed, whatever, user root
can do it anywhere on the system, in any directory.  Therefore user
root could delete an entire directory tree "by mistake" and nothing
on the system would hinder this. I.e., there is no prompt "Are you
sure?", like you'd see on a DOS command like DEL *.*.

Likewise, when you use chmod to change the access of a file, it is
generally a good idea to log in to the system as "non-root", for
example, user janis, or user bbs to change the permisions of a file.
That way, if you mistakenly change the attributes to something you
didn't mean, or if you attempt to change a file that you didn't MEAN
to change, your limited permissions can generally prevent total
disaster <smile>.


========================================================================
More options to chmod
========================================================================
As usual, see man chmod for all options.

You can change the permissions of entire directories/subdirectories
with the -R switch (recursive):

chmod -R 775  /home/ftp/pub

This makes every file and directory rwx by the user that owns it,
everyone in the group, and r-x (read/execute) for everyone-else.

Also, another often used switch to chmod is a+x.  a+x adds the
executible flag to a file while not affecting the other previously
set flags (such as who can execute the file).

To use the a+x switch, you would type:
chmod a+x filename

========================================================================
Some bonuses to having everything be a file under Linux
========================================================================

One of the benefits once you realize that everything is considered a
file under linux, is that you can make a simple text file executible
with chmod, and then execute it.  Of course, if the text file you do
this to doesn't contain any shell commands to execute, it won't do
anything <g>.. but if you create a shell script with your favorite
text editor, like joe, or vi, that does contain commands you can
save the file, issue the command chmod a+x filename, and then that
file becomes an executible program.

Here's an example of shell script that changes the text in
the file files.bbs from all upper case to all lower case:

==========start bash script==========

#!/bin/sh
cat files.bbs | tr A-Z a-z > files.new

========= end bash script ===========

if you save this file as say, up2low, you can then type

chmod a+x up2low

and then execute it

./up2low


By Janis Kracht
Questions/Comments: Linux-user echo, or to jkracht@aye.net