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 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 1000
You can suspend the process
running in the foreground by typing ^Z,
i.e.hold down the [Ctrl] key and type [z]. Then to put it in the
background, type
% bg
Note: do not background programs that require user interaction
e.g. vi
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 1000
[2] Running netscape
[3] Running matlab
To restart (foreground) a
suspended processes, type
% fg %jobnumber
For example, to restart sleep
1000, type
% fg %1
Typing fg with no job number foregrounds the last
suspended process.
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
^C
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 1000
&
% ps
PID TT S TIME
COMMAND
20077 pts/5 S 0:05 sleep 1000
21563 pts/5 T 0:00 netscape
21873 pts/5 S 0:25 nedit
To kill off the process sleep 1000, 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 !!!
Summary
Command
|
Meaning
|
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
|
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