module Signal

Many operating systems allow signals to be sent to running processes. Some signals have a defined effect on the process, while others may be trapped at the code level and acted upon. For example, your process may trap the USR1 signal and use it to toggle debugging, and may use TERM to initiate a controlled shutdown.

pid = fork do
  Signal.trap("USR1") do
    $debug = !$debug
    puts "Debug now: #$debug"
  Signal.trap("TERM") do
    puts "Terminating..."
  # . . . do some work . . .


# Controlling program:
Process.kill("USR1", pid)
# ...
Process.kill("USR1", pid)
# ...
Process.kill("TERM", pid)


Debug now: true
Debug now: false

The list of available signal names and their interpretation is system dependent. Signal delivery semantics may also vary between systems; in particular signal delivery may not always be reliable.

Public Class Methods

list → a_hash click to toggle source

Returns a list of signal names mapped to the corresponding underlying signal numbers.

Signal.list   #=> {"EXIT"=>0, "HUP"=>1, "INT"=>2, "QUIT"=>3, "ILL"=>4, "TRAP"=>5, "IOT"=>6, "ABRT"=>6, "FPE"=>8, "KILL"=>9, "BUS"=>7, "SEGV"=>11, "SYS"=>31, "PIPE"=>13, "ALRM"=>14, "TERM"=>15, "URG"=>23, "STOP"=>19, "TSTP"=>20, "CONT"=>18, "CHLD"=>17, "CLD"=>17, "TTIN"=>21, "TTOU"=>22, "IO"=>29, "XCPU"=>24, "XFSZ"=>25, "VTALRM"=>26, "PROF"=>27, "WINCH"=>28, "USR1"=>10, "USR2"=>12, "PWR"=>30, "POLL"=>29}
static VALUE
sig_list(VALUE _)
    VALUE h = rb_hash_new();
    const struct signals *sigs;

    FOREACH_SIGNAL(sigs, 0) {
        rb_hash_aset(h, rb_fstring_cstr(sigs->signm), INT2FIX(sigs->signo));
    return h;
signame(signo) → string or nil click to toggle source

Convert signal number to signal name. Returns nil if the signo is an invalid signal number.

Signal.trap("INT") { |signo| puts Signal.signame(signo) }
Process.kill("INT", 0)


static VALUE
sig_signame(VALUE recv, VALUE signo)
    const char *signame = signo2signm(NUM2INT(signo));
    if (!signame) return Qnil;
    return rb_str_new_cstr(signame);
trap( signal, command ) → obj click to toggle source
trap( signal ) {| | block } → obj

Specifies the handling of signals. The first parameter is a signal name (a string such as “SIGALRM”, “SIGUSR1”, and so on) or a signal number. The characters “SIG” may be omitted from the signal name. The command or block specifies code to be run when the signal is raised. If the command is the string “IGNORE” or “SIG_IGN”, the signal will be ignored. If the command is “DEFAULT” or “SIG_DFL”, the Ruby’s default handler will be invoked. If the command is “EXIT”, the script will be terminated by the signal. If the command is “SYSTEM_DEFAULT”, the operating system’s default handler will be invoked. Otherwise, the given command or block will be run. The special signal name “EXIT” or signal number zero will be invoked just prior to program termination. trap returns the previous handler for the given signal.

Signal.trap(0, proc { puts "Terminating: #{$$}" })
Signal.trap("CLD")  { puts "Child died" }
fork && Process.wait


Terminating: 27461
Child died
Terminating: 27460
static VALUE
sig_trap(int argc, VALUE *argv, VALUE _)
    int sig;
    sighandler_t func;
    VALUE cmd;

    rb_check_arity(argc, 1, 2);

    sig = trap_signm(argv[0]);
    if (reserved_signal_p(sig)) {
        const char *name = signo2signm(sig);
        if (name)
            rb_raise(rb_eArgError, "can't trap reserved signal: SIG%s", name);
            rb_raise(rb_eArgError, "can't trap reserved signal: %d", sig);

    if (argc == 1) {
        cmd = rb_block_proc();
        func = sighandler;
    else {
        cmd = argv[1];
        func = trap_handler(&cmd, sig);

    if (rb_obj_is_proc(cmd) &&
        !rb_ractor_main_p() && !rb_ractor_shareable_p(cmd)) {
        cmd = rb_proc_isolate(cmd);

    return trap(sig, func, cmd);