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
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.
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-^?;
Send signal to process from command line
@EugeneSh. I’d say a command that would run a function in the code would be perfect. I’d be abble to create a function to send the signal without a problem. I’d still want to know what kill does though
3 Answers 3
As far as I understood your question you want to signal a process by its name, not by its PID. This can easily be achieved by combining the two commands:
kill -s signal $(ps -C executable) Does it kill the process that signals?
kill can kill. It doesn’t necessarily.
The command kill sends the specified signal to the specified processes or process groups.
That means, the kill command is used to **send any signal in general.
If it kills a process it means that it’s similar to do exit(0) , or does the process resume after the signal is sent back?
The SIGKILL signal is used to cause immediate program termination. It cannot be handled or ignored, and is therefore always fatal. It is also not possible to block this signal.
If a process receives the SIGKILL signal, it terminates immediately (no destructors called, no cleanup done). The only processes that do not terminate are uninterruptible processes.
A full list of signals available on Linux is found here.
so I can use kill without a problem, and it will not pause or kill any process, as long as I don’t use the SIGKILL signal?
However, I have 3 processes, so I think your answer proposal would send the signal to all 3 of them, not 1 specific
@AndréAlmeida kill only sends signals. There are other signals such as SIGSTOP or SIGTERM , which stop/terminate programs. You need to look each signal’s function up to be sure what it does.
Here is a concrete example I use from time to time:
I’d like to send a signal to a specific proccess from the command line. I’d like to not need to print the PID of the process I want to signal, but rather use a way that the code understands which process should be signaled.
The killall command meets those criteria, if you have it available to you. It allows you to specify the process(es) to signal based on their names. All of the following comments apply equally to the effects of delivering a signal via killall and delivering one via kill .
Other than that I’d like to understand exacly what the kill command does.
Does it kill the process that signals?
It delivers the specified signal. That’s it. Some signals it can deliver will have the effect of killing the process.
Does it kill the process where it’s called?
It delivers a signal only to the process you specify.
If it kills a process it means that it’s similar to do exit(0), or does the process resume after the signal is sent back?
Processes can handle some signals without terminating. Exactly what happens when a process receives one of those signals depends on the process. If a process does not provide a handler for a signal whose default action is to terminate the process, then the process will die, never to resume. The effect is somewhat like that of calling the exit() function or returning from main() , but the termination is abrupt, without calling exit handlers or such.