UNIX Tutorial Five

5.1 File system security (access rights)

In your unixstuff directory, type

$ ls -l

You will see that you now get lots of details about the contents of your directory, similar to the example below (l argument is for long listing!)

File and directory access rights

Each file (and directory) has associated access rights, which may be found by typing ls -l.

-rwxrw-r-- 1 guest guest 16665 Aug01 20:12 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.

NOTE: In this specific case, the user and the group names are exactly the same (guest), but many times that is not the case. A user can belong to many groups and a group can have many users.

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.

Access rights on directories.

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
write (and delete)
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 code-challenge.txt and on the directory backups

Use ls -l to check that the permissions have changed.

5.3 Processes and Jobs

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 going to 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

5.4 Listing suspended and backgrounded 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 firefox
[3] Running vim

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.

5.5 Killing a 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

To kill a suspended or background process, type

$ kill %jobnumber

For example, run

$ sleep 100 &
$ jobs

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

$ sleep 100 &
$ ps | grep -v root

 48 guest 0:02 sh
157 guest 0:00 sleep 100
158 guest 0:00 ps

The process list is pipped to grep, which due to the usage of the v argument filters all processes owned by the root user, which is just one possible way of printing all guest user processes (there are no other users in the system..)

To kill off the process sleep, type

$ kill 157

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 -9 157

Note: It is not possible to kill off other users' processes !!!


ls -l 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