Stop and terminate hang program in Linux

There are two types of hang programs in Linux. The usual hang is in application level and the unusual hang is in kernel level which programmers called crash. We are not going to talk about crash now, perhaps in the future.

Why program hangs?

When you run or open a big database program, or maybe surf the Internet with lynx, one page to another, suddenly it stop responding. You wait for a moment, press an arrow keys, Enter, Esc, Ctrl+C, Ctrl+Z but to no avail, the program just freezes. What? Is the program hang?

Program hang happens in Linux when a process get stuck waiting for something. It could be waiting for a process or resources to be free. What makes a program hang is the process or resource it is waiting is not available or busy.

Some resources take some times to be free. The process is running slowly. The program is not hanged. Hang is real-time problem that one can analyze live the process. Another hang problem is deadlock. This happens when process A is waiting for process B to be free and process B is waiting for process A to be free.

Using Linux ps command

The Linux ps command which means process status is used to display live running processes on the system. What is a process? A process is called daemon in Linux. Every program that is running in the Linux system is a process. More information about ps command can be found in Linux administration - Manage Linux process and daemons tutorial. Let's have some more practice with ps command to solve the Linux hang program.

Login as a normal user. Run this command:

Welcome to Linux 2.6.27.7-smp (tty1)

slackware login: bill
Password:
Linux 2.6.27.7-smp
Last login: Sun Jun 21 11:42:55 +0800 2009 on tty4.
No mail.
[email protected]:~$ vim &
[1] 551
[email protected]:~$ find / *.mpeg

The first command, vim & means we want to run the program in the background. Type the second command. When you press Enter, the result is running quite fast on the screen. Don't worry if you don't like this. Switch to second terminal by pressing Ctrl+Alt+F2. Login as another normal user and su to root. Let's try different ps command's option:

slackware login: luzar
Password:
Linux 2.6.27.7-smp
Last login: Sun Jun 21 11:43:16 +0800 2009 on tty4.
No mail.
[email protected]:~$ su -
Password:
[email protected]:~# ps
PID TTY TIME CMD
585 tty2 00:00:00 bash
619 tty2 00:00:00 ps
[email protected]:~# ps -a
PID TTY TIME CMD
463 tty5 00:00:00 bash
551 tty1 00:00:00 vim
585 tty2 00:00:00 bash
629 tty2 00:00:08 updatedb
639 tty1 00:00:04 find
640 tty2 00:00:00 ps
[email protected]:~# ps au
USER PID %CPU %MEM VSZ RSS TTY STAT START TIME COMMAND
root 355 0.0 0.3 1392 476 tty3 Ss+ 09:14 0:00 /sbin/agetty 384
root 356 0.0 0.3 1392 476 tty4 Ss+ 09:14 0:00 /sbin/agetty 384
luzar 357 0.0 1.2 2704 1624 tty5 Ss 09:14 0:00 -bash
root 359 0.0 0.3 1392 476 tty6 Ss+ 09:14 0:00 /sbin/agetty 384
root 463 0.0 1.2 2704 1620 tty5 S+ 09:57 0:00 -su
bill 541 0.0 1.2 2720 1632 tty1 Ss+ 10:57 0:00 -bash
bill 551 0.0 2.3 10560 2952 tty1 T 11:01 0:00 vim
luzar 575 0.0 1.2 2700 1604 tty2 Ss 11:06 0:00 -bash
root 585 0.0 1.2 2704 1612 tty2 S 11:06 0:00 -su
root 629 1.5 0.6 1652 808 tty2 T 11:35 0:08 updatedb
bill 639 86.8 0.5 1652 692 tty1 D+ 11:43 0:06 find / *.mpg
root 641 0.0 0.6 2408 840 tty2 R+ 11:44 0:00 ps au
[email protected]:~#

The first command, ps with no option prints out running processes in current terminal. The PID column is the Process ID. The TTY column indicates which terminal the process is running on. The TIME column indicates how much CPU time the process has been running and the CMD column shows the command used.

The second command ps -a, shows more details than ps alone. You can see currently running process in all terminals.

The final command, ps au gives us more details plus some new columns. This time the USER column, %CPU column, %MEM column, VSZ column, RSS column, STAT column and START column added. Maybe you can guess the meaning of some of the new columns here. The important column is the STAT column which shows the status of the process. S stands for sleeping which means the process is waiting for something to happen. Z stands for a zombie process. A zombie process is one whose the parent process has died, leaving the child processes behind. You can stop these zombies processes.

Check out detail information page with info ps command running in the Linux terminal.

Stop hang program with kill command

The commonly used command to stop a process is kill. You are going to use this command a lot in the future, yes, kill the misbehaved program, memory and cpu eating processes, hang programs, etc. Don't worry you're just a process killer ;-)

We just learn how to view running process with ps. We are going to use all the informations above to kill off a process. Let's kill some processes in TTY1.

[email protected]:~# ps au | grep tty1
bill 541 0.0 1.2 2720 1632 tty1 Ss 10:57 0:00 -bash
bill 551 0.0 2.3 10560 2952 tty1 T 11:01 0:00 vim
bill 713 69.0 0.5 1652 648 tty1 R+ 12:47 0:02 find / *.mpg
root 715 0.0 0.0 196 28 tty2 R+ 12:47 0:00 grep tty1
[email protected]:~#

The grep command is used to grab processes from tty1. To kill find process, check it's PID and issue the kill command with the following format:

kill <PID>

[email protected]:~# kill 713
[email protected]:~# ps au | grep tty1
bill 541 0.0 1.2 2720 1640 tty1 Ss+ 10:57 0:00 -bash
bill 551 0.0 2.3 10560 2952 tty1 T 11:01 0:00 vim
root 715 0.0 0.4 1688 596 tty2 S+ 12:57 0:00 grep tty1
[email protected]:~#

There's no more find process in the list, right?

To force terminate a process, use kill with -9 option.

[email protected]:~# kill -9 551
[email protected]:~# ps au | grep tty1
bill 541 0.0 1.2 2720 1640 tty1 Ss+ 10:57 0:00 -bash
root 731 0.0 0.4 1688 596 tty2 S+ 13:04 0:00 grep tty1
[email protected]:~#

Switch to terminal 1 and check whether the process is still there:

/usr/lib/libgdbm.la
/usr/lib/libgdbm.so
/usr/lib/gdk-pixbuf
Terminated
[email protected]:~$ fg
-bash: fg: job has terminated
[1]+ Killed vim
[email protected]:~$

The program has been terminated. Ok that's good. Now switch back to terminal 2 and issue this command:

[email protected]:~# killall -9 bash

Welcome to Linux 2.6.27.7-smp (tty2)

slackware login:

You can use killall instead of kill to terminate all processes. The command killall kills all processes which share the same name. As you can see from the example above, program's name is used instead of pid.

The basic syntax for Linux killall command is:

killall [options] program_name(s)

That's it. Like always, you can find more detail information with man and info commands.

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