What do these Terminal Signals mean?

Question

When I start to use a command to terminate an ongoing process, I can include a signal.

What is the difference between the signal 9 and the signal 15?

Answer 1

There is a manual page here - the ones I think you are referring to are:

Signal     Value    Action       Comment
-------------------------------------------------------------------------
SIGKILL          9       Term    Kill signal
SIGTERM         15       Term    Termination signal

Here are some other signals:

• Ctrl+C on a running process in terminal sends the Keyboard Interupt signal:
  SIGINT2 Term Interrupt from keyboard. This does not always stop the program straight way, often it will just finish that part of what it is doing and then stop.

• Ctrl+\ on a running process in terminal sends the Quit signal:
  SIGQUIT3 Core Quit from keyboard

• The default signal sent by the killall PROCESS-NAME is
  SIGTERM15 Term Termination signal. This is also the default one sent from top when pressing k and entering the process PID

To make sure a process is stopped you can use SIGKILL (or alternatively, SIGSTOP) as that cannot be caught, blocked, or ignored by the program.

Here is a longer list from the above link, for reference:

   Signal     Value     Action   Comment
   -------------------------------------------------------------------------
   SIGHUP        1       Term    Hangup detected on controlling terminal
                 or death of controlling process
   SIGINT        2       Term    Interrupt from keyboard
   SIGQUIT       3       Core    Quit from keyboard
   SIGILL        4       Core    Illegal Instruction
   SIGABRT       6       Core    Abort signal from abort(3)
   SIGFPE        8       Core    Floating point exception
   SIGKILL       9       Term    Kill signal
   SIGSEGV      11       Core    Invalid memory reference
   SIGPIPE      13       Term    Broken pipe: write to pipe with no readers
   SIGALRM      14       Term    Timer signal from alarm(2)
   SIGTERM      15       Term    Termination signal
   SIGUSR1   30,10,16    Term    User-defined signal 1
   SIGUSR2   31,12,17    Term    User-defined signal 2
   SIGCHLD   20,17,18    Ign     Child stopped or terminated
   SIGCONT   19,18,25    Cont    Continue if stopped
   SIGSTOP   17,19,23    Stop    Stop process
   SIGTSTP   18,20,24    Stop    Stop typed at tty
   SIGTTIN   21,21,26    Stop    tty input for background process
   SIGTTOU   22,22,27    Stop    tty output for background process

   The  signals SIGKILL and SIGSTOP cannot be caught, blocked, or ignored.

   Next the signals not in the  POSIX.1-1990  standard  but  described  in
   SUSv2 and POSIX.1-2001.

   Signal       Value     Action   Comment
   -------------------------------------------------------------------------
   SIGBUS      10,7,10     Core    Bus error (bad memory access)
   SIGPOLL             Term    Pollable event (Sys V). Synonym of SIGIO
   SIGPROF     27,27,29    Term    Profiling timer expired
   SIGSYS      12,-,12     Core    Bad argument to routine (SVr4)
   SIGTRAP        5        Core    Trace/breakpoint trap
   SIGURG      16,23,21    Ign     Urgent condition on socket (4.2BSD)
   SIGVTALRM   26,26,28    Term    Virtual alarm clock (4.2BSD)
   SIGXCPU     24,24,30    Core    CPU time limit exceeded (4.2BSD)
   SIGXFSZ     25,25,31    Core    File size limit exceeded (4.2BSD)

   Up  to and including Linux 2.2, the default behaviour for SIGSYS, SIGX-
   CPU, SIGXFSZ, and (on architectures other than SPARC and  MIPS)  SIGBUS
   was  to  terminate  the  process (without a core dump).  (On some other
   Unices the default action for SIGXCPU and SIGXFSZ is to  terminate  the
   process  without  a core dump.)  Linux 2.4 conforms to the POSIX.1-2001
   requirements for these signals, terminating the  process  with  a  core
   dump.

   Next various other signals.

   Signal       Value     Action   Comment
   --------------------------------------------------------------------
   SIGIOT         6        Core    IOT trap. A synonym for SIGABRT
   SIGEMT       7,-,7      Term

   SIGSTKFLT    -,16,-     Term    Stack fault on coprocessor (unused)
   SIGIO       23,29,22    Term    I/O now possible (4.2BSD)
   SIGCLD       -,-,18     Ign     A synonym for SIGCHLD
   SIGPWR      29,30,19    Term    Power failure (System V)
   SIGINFO      29,-,-         A synonym for SIGPWR
   SIGLOST      -,-,-      Term    File lock lost
   SIGWINCH    28,28,20    Ign     Window resize signal (4.3BSD, Sun)
   SIGUNUSED    -,31,-     Term    Unused signal (will be SIGSYS)

   (Signal 29 is SIGINFO / SIGPWR on an alpha but SIGLOST on a sparc.)

   SIGEMT  is  not  specified in POSIX.1-2001, but nevertheless appears on
   most other Unices, where its default action is typically  to  terminate
   the process with a core dump.

   SIGPWR (which is not specified in POSIX.1-2001) is typically ignored by
   default on those other Unices where it appears.

   SIGIO (which is not specified in POSIX.1-2001) is ignored by default on
   several other Unices.