kill

kill  

kill is used to send a signal to the process, though the name suggests that it is used only to terminate a process. But, kill  helps to terminate a process more often than not. When some process  is  run unexpectedly or accidentally, it  might have to be aborted ,for which kill helps just like CTRL+ALT+DEL in windows .

Example 1)

ps command gives you the process id s  and other details of various commands, if you want to stop any command, just check the corresponding PID of the  process(the command) which you want to terminate ,then use kill  to stop that process.

Consider the output of ps,

/home/b6789$ps  -fu  b6789

UID  PID  PPID C STIME   TTY TIME  CMD

b6789   352266      1     0 05:32:38     -  0:14 java  /usr/bin/wc  –l   

b6789   368734      1     0 05:32:42     -  0:07 [wc]

------b6789   372912     120 0 05:32:43      -  5:10 java  /usr/bin/java/dev2.java            

b6789   409806      1     0 05:32:39     -  0:13 java  /usr/bin/java/start-atm.java

                                                                                          this process is consuming cpu and has to be terminated,

/home/b6789$kill  372912

This sends the signal SIGTERM to the process 372912, and terminates it.

Sometimes the process may refuse to terminate just by SIGTERM, in that case  use SIGKILL signal(kill -9)

/home/b6789$kill    -9   372912

This signal cannot be ignored . You can kill any of the process initiated by you, i:e from your  user id this way .A root user has the privilege of killing any user’s  processes .

Example 2)

You  can also  get  the specific PID  for your command without having to search it using grep and awk along with ps. Then kill the PID which you have retrieved.

If you had to kill the CMD start-atm.java above,

/home/b6789$j_pid=`ps  –fu  $LOGNAME | grep  start-atm.java | grep –v grep | awk ’{print  $2}’`

/home/b6789$echo  $ j_pid

409806

/home/b6789$kill   -9   $ j_pid

You can also use xargs  to kill process of a particular type all  at once .

Example 3)

There is a method to kill all the processes initiated by you,            

/home/b6789$kill  -9   -1

This will kill all the processes of your user id and also  closes  your login session. It is a very dangerous command ,so think thrice  and  check whether any required process is still running using ps before you press enter.

You can send many kinds  of  signals to a process(not all)   by using  kill  -<signal>  PID. The various signal numbers and their names are listed below  (for AIX) in detail .A short list can also be viewed

by using  kill  -l (lowercase  alphabet  L).

Name      num       Description

SIGHUP      1           hangup, generated when terminal disconnects

SIGINT       2           interrupt, generated from terminal special char

SIGQUIT    3           (*) quit, generated from terminal special char

SIGILL        4           (*) illegal instruction (not reset when caught)

SIGTRAP    5           (*) trace trap (not reset when caught)

SIGABRT    6          (*) abort process

SIGEMT     7           EMT instruction

SIGFPE       8           (*) floating point exception

SIGKILL     9           kill (cannot be caught or ignored)

SIGBUS     10         (*) bus error (specification exception)

SIGSEGV   11         (*) segmentation violation

SIGSYS      12         (*) bad argument to system call

SIGPIPE    13         write on a pipe with no one to read it  

SIGALRM  14         alarm clock timeout

SIGTER M  15        software termination signal

SIGURG     16         (+) urgent condition on I/O channel

SIGSTOP   17         (@) stop (cannot be caught or ignored)

SIGTSTP   18         (@) interactive stop

SIGCONT                  19         (!) continue (cannot be caught or ignored)

 SIGCHLD   20        (+) sent to parent on child stop or exit                       

 SIGTTIN   21         (@) background read attempted from control terminal            

 SIGTTOU   22        (@) background write attempted to control terminal             

 SIGIO       23         (+) I/O possible, or completed                                 

 SIGXCPU  24         cpu time limit  exceeded

 SIGXFSZ   25         file size limit exceeded

 SIGMSG    27         input data is in the ring buffer                               

 SIGWINCH  28      (+) window size changed                                        

 SIGPWR    29        (+) power-fail restart                                         

 SIGUSR1   30        user defined signal 1                                           

 SIGUSR2   31        user defined signal 2                                          

 SIGPROF   32        profiling time alarm

 SIGDANGER 33     system crash imminent; free up some page space                 

 SIGVTALRM 34     virtual time alarm

 SIGMIGRATE 35   migrate process                                                

 SIGPRE    36         programming exception                                          

 SIGVIRT   37         AIX virtual time alarm                                         

 SIGALRM1  38      m:n condition variables - RESERVED - DON'T USE                 

 SIGTALRM  38      per-thread alarm clock                                         

 SIGWAITING 39   m:n scheduling - RESERVED - DON'T USE                          

 SIGRECONFIG 58  Reserved for Dynamic Reconfiguration Operations                

 SIGCPUFAIL 59    Predictive De-configuration of Processors – (RESERVED - DON'T USE   )                   

                                 

 SIGKAP    60              keep alive poll from native keyboard                            

 SIGGRANT  SIGKAP  monitor mode granted                                          

 SIGRETRACT 61      monitor mode should be relinquished                            

 SIGSOUND  62        sound control has completed                                     

 SIGSAK    63         secure attention key                                           

Example 4)

You can  stop a running process in the midway and then continue it from that point using kill command.

Assume  you are running a ftp command and want to stop  the file transfer.

/home/b6789$ ftp   10.42.67.125

……

…….

put abigfile.txt

#####################################

Now in between if you want to suspend it. take an other session for the same user and run

kill  -17 <ftp_PID>,

where <ftp_PID>  is the PID of the ftp command. This sends the signal SIGSTOP to the command. you can observe that the file transfer is stopped.

To restart the command,simply run

Kill -19  <ftp_PID> , which sends the signal SIGCONT which is to continue  a  process. The file transfer starts from the point where it stopped.

Similarly other commands ,jobs  or utilities can be stopped and continued using kill -17 and kill -19 resp.

Example 5)
To Kill all the child processes of a particular process use
kill  -9  -<parent pid>
to kill all child processes of PID 204567
kill   -9  -204567