On Unix-like operating systems such as Linux, signals are software interrupts. They provide a way for the user (or a process) to directly communicate with a process.
Software may be programmed to respond intelligently to a wide array of signals, and certain signals cause processes to behave in a standardized, predefined way at the kernel level.
Description
Process signals were developed as part of UNIX in the 1970s. They are used on all modern Unix-like operating systems, including Linux, BSD, and macOS X.
- Description
- Finding system-specific signals
- Signals in Linux
- Examples: sending signals
- Related commands
- Linux commands help
When a signal is sent to a process, the operating system interrupts the normal flow of the process execution and delivers the notification. If the process has previously registered a way to handle that particular signal, that routine is executed, otherwise the system executes the default signal handler.
Signals can be sent with the kill command, which is named for its default signal (SIGKILL) that instructs the OS to forcefully terminate a process before doing anything else.
Signal names are commonly abbreviated without their SIG prefix, e.g., “KILL”, including in the command arguments of kill.
Finding system-specific signals
Signals are defined in the system library signal.h. To view the signals used by your operating system, open a terminal and run man signal or man 7 signal.
Signals in Linux
Signals not supported by Linux
The following signals may be used by other systems, such as BSD, but are interpreted as SIGUNUSED in Linux.
Sending signals from the keyboard
Signals may be sent from the keyboard. Several standard defaults are listed below. Default key combinations for sending interrupt signals can be defined with the stty command.
Real-time signals
Real-time signals are a set of signals with no predefined purpose, for programmers to use as they want in their software. Two signal names, SIGRTMIN and SIGRTMAX, define the minimum and maximum signal numbers of the real-time signals. For example, the programmer may use the signal number as SIGRTMIN+3 to refer to the fourth real-time signal number.
Examples: sending signals
The kill command sends signals to processes. Your shell may have a built-in version of kill, which supersedes the version installed at /bin/kill. The two versions have slightly different options, but basic functions are the same. The following examples may run using either version of kill.
The process to be signaled is referred to by PID (process ID). If you’re not sure of the process ID, you can find it with the ps command, for example ps -aux.
kill 1234
Send the KILL signal to the process with PID 1234.
kill 123 456 789
Kill three processes: PIDs 123, 456, and 789.
kill -15 1234 5678
Send signal number 15 (TERM) to processes 1234 and 5678.
kill -TERM 1234 5678
Same as the previous command.
kill -l
List all available signals. Example output:
- SIGHUP 2) SIGINT 3) SIGQUIT 4) SIGILL 5) SIGTRAP
- SIGABRT 7) SIGBUS 8) SIGFPE 9) SIGKILL 10) SIGUSR1
- SIGSEGV 12) SIGUSR2 13) SIGPIPE 14) SIGALRM 15) SIGTERM
- SIGSTKFLT 17) SIGCHLD 18) SIGCONT 19) SIGSTOP 20) SIGTSTP
- SIGTTIN 22) SIGTTOU 23) SIGURG 24) SIGXCPU 25) SIGXFSZ
- SIGVTALRM 27) SIGPROF 28) SIGWINCH 29) SIGIO 30) SIGPWR
- SIGSYS 34) SIGRTMIN 35) SIGRTMIN+1 36) SIGRTMIN+2 37) SIGRTMIN+3
- SIGRTMIN+4 39) SIGRTMIN+5 40) SIGRTMIN+6 41) SIGRTMIN+7 42) SIGRTMIN+8
- SIGRTMIN+9 44) SIGRTMIN+10 45) SIGRTMIN+11 46) SIGRTMIN+12 47) SIGRTMIN+13
- SIGRTMIN+14 49) SIGRTMIN+15 50) SIGRTMAX-14 51) SIGRTMAX-13 52) SIGRTMAX-12
- SIGRTMAX-11 54) SIGRTMAX-10 55) SIGRTMAX-9 56) SIGRTMAX-8 57) SIGRTMAX-7
- SIGRTMAX-6 59) SIGRTMAX-5 60) SIGRTMAX-4 61) SIGRTMAX-3 62) SIGRTMAX-2
- SIGRTMAX-1 64) SIGRTMAX
kill -9 -1
The special process ID -1 refers to all processes other than kill and the system root process. This command attempts to kill (-9) every possible process (-1) on the system. For more information, see the documentation of kill, linked below.
Related commands
kill — End a process.stty — Change terminal line settings.
