Signals and traps in Unix

In this article, we will discuss the overview of Signals and traps in Unix and understand it with the help of an example. And finally will conclude different scenarios for signals and traps in Unix. Let’s discuss it one by one.

Overview :

• Signals are software interrupts that are sent to a program to notify it of an important occurrence. User requests to unlawful memory access errors are examples of occurrences. Some signals, such as the interrupt signal, indicate that a user has asked the program to do an action that is not normally part of the program’s flow of control.
• Trap specifies and activates handlers that will be executed when the shell receives signals or other exceptional circumstances.
• When the shell gets the signal(s) SIGNAL SPEC, it will read and execute ARG. If ARG is not present (and only a single SIGNAL SPEC is provided), or if ARG is a dash (“-”), each given signal is reset to its default value. If ARG is the null string, the shell and the commands it calls disregard each SIGNAL SPEC.
• If you hit Ctrl-C or Ctrl- while your application is running, it will terminate as soon as the signal comes. There are occasions when you don’t want the application to stop immediately once the signal arrives. You might choose to ignore the signal and continue running, or you could conduct some type of cleaning before quitting the script. The trap command lets you to regulate how a program responds to a signal.
• A signal is an asynchronous communication consisting of a number that can be delivered from one process to another or from the operating system to a process if particular keys are pushed or if anything unusual occurs.

Example –
If you’ve written any amount of bash code, you’ve almost certainly encountered the trap command. Trap enables you to capture signals and run code when they occur. Signals are asynchronous notifications delivered to your script when specific events occur. The majority of these alerts are for situations that you hope never occur, such as an incorrect memory access or a poor system call. However, there are one or two occurrences that you may wish to address. There are additional “user” events that are never created by the system and may be used to signal your script. Bash also has a pseudo-signal named “EXIT” that is performed when your script terminates.

Implementation in python :

Python3

#GeeksforGeeks
trap itsatrap 1 2 3 7

#cleanup method
cleanup()
{
  echo "We got the signal to clean, beginning"
  rm -rf /tmp/temp_*.$$
  echo "Done! ... exiting now!."
  exit 1
}
# for loop
for i in *
do
  sed s/FOO/BAR/g $i > /tmp/temp_${i}.$$ && mv /tmp/temp_${i}.$$ $i
done

Caution –
DO NOT RUN THE ABOVE SCRIPT, AS IT’s A TRAP!

Typical Unix/Linux activities :

• Each signal is linked with a default procedure. The operation that a script or program does when it receives a signal is known as the default operation with a signal.
• One of the possible default actions is:
• Put an end to the process
• Signal for skipping
• Dump of core memory When it gets the signal, it generates a file called the core that contains the process’s memory image.
• Put an end to the procedure.
• Continue the procedure till it comes to an end.

Different Scenarios for Signals and traps in Unix :

1. If there are several commands set to trap, they must be surrounded by citations. You’ll also note that the command line shell scans when the trap command is executed and again when one of the mentioned signals is received.
2. If you disregard a signal, all the additional shells will do the same. However, if you specify an action to be done when the signal is received, all sub-shells will still conduct the action when the signal is received.
3. There may be times when you don’t want users of your scripts to quit prematurely by utilizing keyboard abort sequences, such as when input is required or cleaning is required. The trap statement intercepts these sequences and may be designed to execute a set of commands when they are intercepted.
4. When Bash receives a signal for which a trap has been set while waiting for a command to finish, the trap is not performed until the command is finished.
5. When Bash is waiting for an asynchronous command using the wait built-in, the arrival of a signal for which a trap has been set causes the wait built-in to return immediately with an exit status larger than 128, and the trap to be performed.
6. When one or more specified signals are received, this command defines the action to be done. When the shell gets a certain signal, it performs the action indicated.
7. Specifies the signal to be trapped. You have the option of specifying a signal number or a signal name. When you specify a signal name, you must omit the leading SIG (for example, specify INT for SIGINT). The requirements for each signal may be found in the documentation for the operating system being utilized. 
8. It should be noted that SIGTERM cannot be specified.