UNIX / Linux: 3 Ways to Send Signal to Processes

Question: How do I send signal to another process? Can you explain me all available options to send signals to a process in UNIX / Linux environment?

Answer: You can send various signals to processes using one of the methods explains in this article.

1. Send Signal to a Process Using Kill

Use kill command to send a signal to a process. For example, if you want to send USR1 signal to the process “a.out”, do the following.

$ ps -C a.out
  PID TTY          TIME CMD
 3699 pts/1    00:00:00 a.out

$ kill -s USR1 3699

Note: Refer to 4 Ways to Kill a Process – kill, killall, pkill, xkill.

2. Send Signal to a Process from Another Process

You can use the UNIX system call kill (from a C program) to send signal from one process to another. The following C code snippet shows how to use the kill command.

Kill system call takes two arguments: 1) the PID (process id) of the process that needs to be signalled 2) the signal that needs to be send to the process. Kill command returns 0 when it is successful.

int send_signal (int pid)
{
        int ret;
        ret = kill(pid,SIGHUP);
        printf("ret : %d",ret);

}

3. Send Signal to a Process from Keyboard

When a process is running on the terminal, you can send signal to that process from the keyboard by using some specific combination of keys. The following are couple of examples.

• SIGINT (Ctrl + C) – You know this already. Pressing Ctrl + C kills the running foreground process. This sends the SIGINT to the process to kill it.
• You can send SIGQUIT signal to a process by pressing Ctrl + \ or Ctrl + Y

You can view the key mappings that sends specific signal to a process using the “stty -a” command as shown below.

$ stty -a | grep intr
intr = ^C; quit = ^\; erase = ^?; kill = ^U; eof = ^D; eol = M-^?; eol2 = M-^?;