 |
|
Unix Tutorial Five
| 5.1
File system security (access rights) |
In your unixstuff
directory, type
%
ls -l (l for
long listing!)
You will see that you
now get lots of details about the contents of your directory, similar to
the example below.
Each file (and directory)
has associated access rights, which may be found by typing ls
-l. Also, ls
-lg gives additional information as to which group owns the file
(beng95 in the following example):
-rwxrw-r--
1 ee51ab beng95 2450 Sept29 11:52 file1
In the left-hand column
is a 10 symbol ?string? consisting of the symbols d, r, w,
x, -, and, occasionally, s or S. If d
is present, it will be at the left hand end of the string, and indicates
a directory: otherwise ?-? will be the starting symbol of the string.
The 9 remaining symbols
indicate the permissions, or access rights, and are taken as three groups
of 3.
-
The left group of
3 gives the file permissions for the user that owns
the file (or directory) (ee51ab in the above example);
-
the middle group
gives the permissions for the group of people to whom the file
(or directory) belongs (eebeng95 in the above example);
-
the rightmost group
gives the permissions for all others.
The symbols r, w, etc.,
have slightly different meanings depending on whether they refer to a simple
file or to a directory.
Access rights on files.
- r (or -),
indicates read permission (or otherwise), that is, the presence or absence
of permission to read and copy the file
- w (or -),
indicates write permission (or otherwise), that is, the permission (or
otherwise) to change a file
- x (or -),
indicates execution permission (or otherwise), that is, the permission
to execute a file, where appropriate
Access rights on directories.
- r allows users
to list files in the directory;
- w means that
users may delete files from the directory or move files into it;
- x means the
right to access files in the directory. This implies that you may read
files in the directory provided you have read permission on the individual
files.
So, in order to read
a file, you must have execute permission on the directory containing that
file, and hence on any directory containing that directory as a subdirectory,
and so on, up the tree.
Some examples
-rwxrwxrwx a
file that everyone can read, write and execute (and delete).
-rw------- a
file that only the owner can read and write - no-one else
can read or write and no-one has execution rights (e.g. your
mailbox file).
| 5.2
Changing access rights |
chmod (changing a file
mode)
Only the owner of a
file can use chmod to change the permissions of a file. The options of chmod
are as follows
| Symbol |
Meaning
|
| u |
user |
| g |
group |
| o |
other |
| a |
all |
| r |
read |
| w |
write (and delete) |
| x |
execute (and access
directory) |
| + |
add permission |
| - |
take away permission |
For example, to remove read write and execute permissions on the file biglist
for the group and others, type
%
chmod go-rwx biglist
This will leave the
other permissions unaffected.
To give read and write
permissions on the file biglist to all,
%
chmod a+rw biglist
Exercise 5a
Try changing access
permissions on the file science.txt and on the directory backups
Use ls
-l to check that the permissions have changed.
A process is an executing
program identified by a unique PID (process identifier). To see information
about your processes, with their associated PID and status, type
%
ps
A process may be in
the foreground, in the background, or be suspended. In general the shell
does not return the UNIX prompt until the current process has finished executing.
Some processes take
a long time to run and hold up the terminal. Backgrounding a long process
has the effect that the UNIX prompt is returned immediately, and other tasks
can be carried out while the original process continues executing.
Running background
processes
To background a process,
type an & at the end of the command line. For example, the command
sleep waits a given number of seconds before continuing. Type
%
sleep 10
This will wait 10 seconds
before returning the command prompt %. Until the command prompt is returned,
you can do nothing except wait.
To run sleep in the
background, type
%
sleep 10 &
[1] 6259
The & runs the
job in the background and returns the prompt straight away, allowing you
do run other programs while waiting for that one to finish.
The first line in the
above example is typed in by the user; the next line, indicating job number
and PID, is returned by the machine. The user is be notified of a job number
(numbered from 1) enclosed in square brackets, together with a PID and is
notified when a background process is finished. Backgrounding is useful
for jobs which will take a long time to complete.
Backgrounding a current
foreground process
At the prompt, type
%
sleep 100
You can suspend the
process running in the foreground by holding down the [control] key and
typing z (written as ^Z) Then to put it in the background, type
%
bg
Note: do not background
programs that require user interaction e.g. pine
| 5.4
Listing suspended and background processes |
When a process is running,
backgrounded or suspended, it will be entered onto a list along with a job
number. To examine this list, type
%
jobs
An example of a job
list could be
[1] Suspended sleep 100
[2] Running netscape
[3] Running nedit
To restart (foreground)
a suspended processes, type
%
fg %jobnumber
For example, to restart
sleep 100, type
%
fg %1
Typing fg
with no job number foregrounds the last suspended process.
kill (terminate or
signal a process)
It is sometimes necessary
to kill a process (for example, when an executing program is in an infinite
loop)
To kill a job running
in the foreground, type ^C (control c). For example, run sleep
100 then kill it with ^C
To kill a suspended
or background process, type
%
kill %jobnumber
For example, run sleep
100 & then type jobs
to see its job number. If it is job number 4, type
%
kill %4
To check whether this
has worked, examine the job list again to see if the process has been removed.
ps (process status)
Alternatively, processes
can be killed by finding their process numbers (PIDs) and using kill PID_number.
Run sleep
100 & again, then type
%
ps
PID TT S TIME COMMAND
20077 pts/5 S 0:05 sleep 100
21563 pts/5 T 0:00 netscape
21873 pts/5 S 0:25 nedit
To kill off the process
sleep 100, type
%
kill 20077
and then type ps
again to see if it has been removed from the list.
If a process refuses
to be killed, uses the -9 option, i.e. type
%
kill -9 20077
Note: It is not
possible to kill off other users? processes !!!
| ls -lag |
list access rights
for all files |
| chmod [options]
file |
change access
rights for named file |
| command
& |
run command in
background |
| ^C |
kill the job running
in the foreground |
| ^Z |
suspend the job
running in the foreground |
| bg |
background the
suspended job |
| jobs |
list current jobs |
| fg %1 |
foreground job
number 1 |
| kill %1 |
kill job number
1 |
| ps |
list current processes |
| kill 26152 |
kill process number
26152 |
|
