Controlling shell processes using signals

Kirsty Pargeter, 123RF

Clear Signals

HARALD ZISSLER

Instead of just killing processes, professionals cleverly catch their communication and steer the computer's tasks in the right direction using signals.

In most cases, kill is used as a last resort. It ends a process without mercy. Systems administrators sometimes kill a process with the kill -9 <PID> command – using the signal 9, which means kill even if the signal is not catchable or ignorable) and the process ID (PID) argument. You can get the PID by using the ps command.

Signals

The standard Linux signals are listed in Table 1. Related systems, such as FreeBSD, have their own variations, with the signals 1, 3, 9, and 15 being the only uniform ones throughout.

Table 1

Signals

1	SIGHUP	Separates child from parent process	Ends process
2	SIGINT	Ends process	Same as Ctrl+C
3	SIGQUIT	Ends process	Process can create core dump
4	SIGILL	Ends process	After a false call (no privileges, unknown functions)
5	SIGTRAP	Ends process	Process triggers trace/debugger
6	SIGABRT	Ends process	From process itself
7	SIGBUS	Ends process	System call after memory access errors
8	SIGFPE	Ends process	Division by 0
9	SIGKILL	Ends process	No data written, risk of data loss
10	SIGUSR1	User-defined signal	–
11	SIGSEGV	Ends process	After memory access errors
12	SIGUSR2	User-defined signal	–
13	SIGPIPE	Ends process	Pipe error
14	SIGALRM	Ends process	After timer expiration
15	SIGTERM	Ends process	Restores files
16	SIGSTKFLT	Ends process	After co-processor stack error
17	SIGCHLD	Ends child process	Triggered by parent
18	SIGCONT	Continues process
19	SIGSTOP	Pauses process	–
20	SIGTSTP	Pauses process	Same as Ctrl+Z

You can determine which of the two methods your system uses with kill -l . You can use the kill command in the format kill -<number> <PID> or kill -s <signal_name> <PID> . When using the signal name, you don't need the preceding SIG , so you can either do:

$ kill -KILL 29737

or:

$ kill -9 29737

and it will do exactly the same thing: kill the process instantly.

For some of the examples, the target for the signals will be daemon.sh that I created in Listing 1. The program shows its shell command, its own PID, and the parent process PID in a loop function.

Listing 1

daemon.sh

#! /bin/sh
while true;
do
  echo $0 $$ $PPID
  sleep 1
done

Gently Removed

The basic process-ending method is to use SIGHUP , where the system proceeds as if you had quit the terminal. Conversely, you can prevent this by using nohup at program startup. The application continues to run after you log off, such as with an SSH session.

As long as no one affects the process or shuts down the system, the script runs until it terminates by itself. You can read the output in the nohup.out protocol file by using tail -f nohup.out .

You can restart most system daemons in the same way: kill -HUP <daemon_PID> . It's still recommended, however, to use the daemon's init script or the system's start/stop mechanism instead.

If necessary, you can halt processes to allow other necessary resources to run. For example, the daemon.sh script gets a signal 19 while going through its motions, which pauses the process. You can continue the process using kill -18 <PID> (Figure 1).