20.4 Sending Signals to Processes

   

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Signals are used to send some information to a process or to handle an exception. Signals are sent to processes using the kill command explicitly. Some signals are bound to keyboard key combinations and are sent to the processes that are currently attached to the terminal. Every signal has a signal name and a number associated with it. All signals have default responses. Some signals are ignored by default. A list of signals and their default responses is shown in Table 20-2.

Table 20-2. Signals

Signal Name	Signal Number	Description
SIGNULL		Check access to PID
SIGHUP	1	Hangup
SIGINT	2	Interrupt
SIGQUIT	3	Quit
SIGILL	4	Illegal instruction (not reset when caught)
SIGTRAP	5	Trace trap (not reset when caught)
SIGABRT	6	Process abort signal
SIGIOT	SIGABRT	IOT instruction
SIGEMT	7	EMT instruction
SIGFPE	8	Floating point exception
SIGKILL	9	Kill (cannot be caught or ignored)
SIGBUS	10	Bus error
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
SIGTERM	15	Software termination signal from the kill; the default signal sent when using the kill command
SIGUSR1	16	User -defined signal 1
SIGUSR2	17	User-defined signal 2
SIGCHLD	18	Child process terminated or stopped
SIGCLD	SIGCHLD	Death of a child
SIGPWR	19	Power state indication
SIGVTALRM	20	Virtual timer alarm
SIGPROF	21	Profiling timer alarm
SIGIO	22	Asynchronous I/O
SIGPOLL	SIGIO	For HP-UX hpstreams signal
SIGWINCH	23	Window size change signal
SIGWINDOW	SIGWINCH	Added for compatibility reasons
SIGSTOP	24	Stop signal (cannot be caught or ignored)
SIGTSTP	25	Interactive stop signal
SIGCONT	26	Continue if stopped
SIGTTIN	27	Read from the control terminal attempted by a member of a background process group
SIGTTOU	28	Write to the control terminal attempted by a member of a background process group
SIGURG	29	Urgent condition on the I/O channel
SIGLOST	30	Remote lock lost (NFS)
SIGRESERVE	31	Save for future use
SIGDIL	32	DIL signal
SIGXCPU	33	CPU time limit exceeded ( setrlimit )
SIGXFSZ	34	CPU file size limit exceeded ( setrlimit )
SIGCANCEL	35	Used for pthread cancelation
SIGGFAULT	36	Graphics framebuffer fault
SIGRTMIN	37	First (highest priority) realtime signal
SIGRTMAX	44	Last ( lowest priority) realtime signal

The kill command is used to send a signal to a process. If no signal is specified on the command line, it sends SIGTERM (signal 15 ) to the process specified. The general syntax of the kill command is:

 kill [-signal] process 

To send a SIGTERM signal to a process having process an ID of 1556, use one of the following commands; all of these are equivalent.

 kill -SIGTERM 1556 kill -15 1556 kill 1556 kill -s SIGTERM 1556 kill -s 15 1556 

The SIGTERM signal is used to terminate a process gracefully if it doesn't have a disabled signal. If a process ignores this signal, you can send a SIGKILL (signal 9 ) signal to terminate it. This signal can't be ignored and it forces a process to terminate. The following command terminates a process having a process ID of 1556 .

 kill -9 1556 

However, it must be noted that a signal is not received by a process that hangs in an I/O operation and it therefore never gets scheduled after that. So it may happen that you are sending a SIGKILL to a process and it does not terminate. The kill -l command lists available signals and their respective numbers as shown here.

 #  kill -l  1) HUP                          23) WINCH  2) INT                          24) STOP  3) QUIT                         25) TSTP  4) ILL                          26) CONT  5) TRAP                         27) TTIN  6) ABRT                         28) TTOU  7) EMT                          29) URG  8) FPE                          30) LOST  9) KILL                         31) RESERVED 10) BUS                          32) DIL 11) SEGV                         33) XCPU 12) SYS                          34) XFSZ 13) PIPE                         35) The specified trap                                      syntax is not correct. 14) ALRM                         36) The specified trap                                      syntax is not correct. 15) TERM                         37) RTMIN 16) USR1                         38) RTMIN+1 17) USR2                         39) RTMIN+2 18) CHLD                         40) RTMIN+3 19) PWR                          41) RTMAX-3 20) VTALRM                       42) RTMAX-2 21) PROF                         43) RTMAX-1 22) IO                           44) RTMAX 

The nohup Command

The nohup command executes another command with the hangup and quit signals ignored. Usually a process is attached to a terminal (TTY) or a pseudoterminal (pty). If the user logs out from that terminal, the process also terminates. The nohup command stops process termination in such a case. Unless the screen output is redirected, the screen output of the executing program is stored in the nohup.out file. If a program takes a long time, such as a batch program, you may start it using the nohup command and then log out. The following command line runs a shell program myprog with the help of the nohup command.

 nohup myprog & 

The nohup command applies to all lines in the shell script. If you are using more than one command with nohup on the command line, these can be grouped as follows .

 nohup (prog1 ; prog2) 

Signals can also be sent to a processes using the SAM Process Control area as shown in Figure 20-1. Just select a process and use the Actions menu to kill it or change its priority, as discussed in the next section.

Figure 20-1. Managing processes using SAM.

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