As can be seen from the output of kill, all supported signals on the platform are shown; the first 31 of these (on your typical Linux box) are called the standard or Unix signals. Unlike the real-time signals that follow, each standard/Unix signal has a very specific name, and, as you might guess, purpose.
(Worry not; we shall discuss the real-time signals, numbers 34 to 64, in the next chapter).
The table you will see shortly, essentially reproduced from the man page on signal(7), summarizes the standard (Unix) signals in the following column order: the signal's symbolic name, integer value(s), the default action taken upon delivery to a process, and a comment describing the signal.
The default action column has the following types: the default action of the signal handler is to:
- Terminate: Terminate the process.
- Term&Core: Terminate the process and emit a core dump. (A core dump is, essentially, a snapshot of the process's dynamic segments, the data and stack segments, at the time when the (fatal) signal was delivered). This terminate and core dump action occurs when the kernel sends a fatal signal to a process. The implication is that the process has done something illegal (buggy); an exception is the SIGQUIT signal: we get a core dump when SIGQUIT is delivered to a process.
- Ignore: Ignore the signal.
- Stop: Process enters the stopped (frozen/suspended) state (represented by T in the output of ps -l).
- Continue: Continue execution of a previously stopped process.
Refer to the table Standard or Unix signals:
|
Signal |
Integer value |
Default action |
Comment |
|
SIGHUP |
1 |
Terminate |
Hang up detected on controlling terminal or death of controlling process |
|
SIGINT |
2 |
Terminate |
Interrupt from keyboard : ^C |
|
SIGQUIT |
3 |
Term&Core |
Quit from keyboard : ^ |
|
SIGILL |
4 |
Term&Core |
Illegal Instruction |
|
SIGABRT |
6 |
Term&Core |
Abort signal from abort(3) |
|
SIGFPE |
8 |
Term&Core |
Floating-point exception |
|
SIGKILL |
9 |
Terminate |
(Hard) kill signal |
|
SIGSEGV |
11 |
Term&Core |
Invalid memory reference |
|
SIGPIPE |
13 |
Terminate |
Broken pipe: write to pipe with no readers; see pipe(7) |
|
SIGALRM |
14 |
Terminate |
Timer signal from alarm(2) |
|
SIGTERM |
15 |
Terminate |
Termination signal (soft kill) |
|
SIGUSR1 |
30,10,16 |
Terminate |
User-defined signal 1 |
|
SIGUSR2 |
31,12,17 |
Terminate |
User-defined signal 2 |
|
SIGCHLD |
20,17,18 |
Ignore |
Child stopped or terminated |
|
SIGCONT |
19,18,25 |
Continue |
Continue if stopped |
|
SIGSTOP |
17,19,23 |
Stop |
Stop process |
|
SIGTSTP |
18,20,24 |
Stop |
Stop typed at terminal : ^Z |
|
SIGTTIN |
21,21,26 |
Stop |
Terminal input for background process |
|
SIGTTOU |
22,22,27 |
Stop |
Terminal output for background process |
What if the system admin needs to urgently kill a process? Yes, its quite possible that, while logged into an interactive shell, time is very precious and an extra couple of seconds may make a difference. In such cases, typing kill -9 is better than kill -SIGKILL, or even kill -KILL. (The previous point is with regard to writing source code).
Passing the signal number to kill -l causes it to print the signal's symbolic name (albeit in a shorthand notation). For example:
$ kill -l 11
SEGV
$
The preceding table (and, as a matter of fact the following table as well) reveal that, with two exceptions, all the signals have a special purpose. Scanning the comment column reveals it. The exceptions are SIGUSR1 and SIGUSR2 these are general purpose signals; their use is left entirely to the imagination of the application designers.
Further, the man page informs us that the following signals (shown in this table) are newer and included in the SUSv2 and POSIX.1-2001 standards:
| Signal | Integer Value |
Default Action |
Comment |
| SIGBUS | 10,7,10 | Term&Core | Bus error (bad memory access) |
| SIGPOLL | Terminate | Pollable event (Sys V). Synonym for SIGIO | |
| SIGPROF | 27,27,29 | Terminate | Profiling timer expired |
| SIGSYS | 12,31,12 | Term&Core | Bad system call (SVr4); see also seccomp(2) |
| SIGTRAP | 5 | Term&Core | Trace/breakpoint trap |
| SIGURG | 16,23,21 | Ignore | Urgent condition on socket (4.2BSD) |
| SIGVTALRM | 26,26,28 | Terminate | Virtual alarm clock (4.2BSD) |
| SIGXCPU | 24,24,30 | Term&Core | CPU time limit exceeded (4.2BSD); see prlimit(2) |
| SIGXFSZ | 25,25,31 | Term&Core | File size limit exceeded (4.2BSD); see prlimit(2) |
A few remaining (not so common) signals are further mentioned by the same man page (signal(7)). Take a look if you are interested.
It's important to note that, out of all the signals mentioned, only two of them cannot be caught, ignored or blocked: the SIGKILL and the SIGSTOP. This is because the OS must guarantee a way to kill and/or stop a process.