6.1 Processi e job

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.

Eseguire i processi in background

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.

Mettere in background un processo attivo

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

6.2 Elenca i processi sospesi e in background

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 %numero_del_job

For example, to restart sleep 1000, type

fg %1

Typing fg with no job number foregrounds the last suspended process.

6.3 Uccidere i processi

kill (uccide un processo)

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 %numero_del_job

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 firefox

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 !!!

Sommario

Comando

Significato

comando & 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

 

whitone@netstudent.polito.it, © 25 febbraio 2009
m.stonebank@surrey.ac.uk, © 9 ottobre 2000
Questa esercitazione è rilasciata sotto Creative Commons License.