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trap
Capture signals and other events and execute commands.
overview
trap [-lp] [[arg] signal_spec ...]
The main purpose
Used to specify the action to be taken when a signal is received.
Perform cleanup work when the script is interrupted.
options
-l: Print the signal name and the number corresponding to the signal name.-p: Displays the trap command associated with each signal.
parameter
arg: The command to be executed when the signal is received.signal_spec: The signal name or the number corresponding to the signal name.
return value
Returns 0 if the expression execution result is successful, and returns 1 when the parameter signal_spec does not specify a valid value.
About signals
Signal is an inter-process communication mechanism that provides an asynchronous software interrupt to the application program, giving the application program the opportunity to accept commands (i.e. signals) sent by other programs or terminals. After the application receives the signal, there are three ways to handle it: ignore, default, or catch. After a process receives a signal, it checks the handling mechanism for the signal. If it is SIG_IGN, the signal will be ignored; if it is SIG_DFT, the system's default processing action will be adopted, usually terminating the process or ignoring the signal; if a processing function (capture) is specified for the signal, the current process will be interrupted. The task being executed is turned to execute the signal processing function, and then the interrupted task is continued after returning.
In some cases, we do not want our shell script to be interrupted while running. For example, if we write a shell script and set it as the default shell for a certain user, the user can only do a certain job after entering the system, such as For database backup, we don't want users to use methods such as Ctrl+C to enter the shell state and do things we don't want to do. This uses signal processing.
Here are some common signs you may encounter:
| Signal name | Number of signals | describe |
|---|---|---|
| SIGHUP | 1 | This signal is issued when the user terminal connection (normal or abnormal) ends, usually when the terminal's control process ends, to notify various jobs in the same session. At this time, they are no longer associated with the controlling terminal. When logging in to Linux, the system will assign a terminal (Session) to the logged-in user. All programs running in this terminal, including the foreground process group and background process group, generally belong to this Session. When the user logs out of Linux, the foreground process group and the background process that outputs to the terminal will receive the SIGHUP signal. The default action of this signal is to terminate the process, so the foreground process group and the process with terminal output in the background will be terminated. For daemons that are disconnected from the terminal, this signal is used to notify it to re-read the configuration file. |
| SIGINT | 2 | Program termination (interrupt) signal, issued when the user types Ctrl+C. |
| FOLLOWS | 3 | Similar to SIGINT, but controlled by the QUIT character (usually Ctrl /). The core file is generated when the process exits due to receiving SIGQUIT, which is similar to a program error signal in this sense. |
| SIGFPE | 8 | Emitted when a fatal arithmetic error occurs. Including not only floating-point arithmetic errors, but also all other arithmetic errors such as overflow and division by 0. |
| SIGKILL | 9 | Used to end the program immediately. This signal cannot be blocked, processed or ignored. |
| SIGALRM | 14 | A clock timing signal that calculates actual time or clock time. The alarm function uses this signal. |
| TARGET TERM | 15 | The program ends (terminate) signal, which is different from SIGKILL in that this signal can be blocked and processed. Usually used to require a program to exit normally by itself; the kill command generates this signal by default. |
example
When the shell receives commands such as HUP, INT, PIPE, QUIT, TERM, the currently executing program will be executed exit 1.
trap "exit 1" HUP INT PIPE QUIT TERM
Clean temporary files
Here's how to delete the file and then exit if someone tries to kill the program from the terminal:
trap "rm -f $WORKDIR/work1 $WORKDIR/dataout; exit" 2
Execute the shell program. If the program receives a signal of 2, then these two files (work1 and dataout) will be automatically deleted.
ignore signal
If the command listed in the trap is empty, the specified signal will be ignored when received:
trap '' 2
Ignore multiple signals:
trap '' 1 2 3 15
reset trap
When you change the action to take when a signal is received, you can omit the first parameter to reset to the default behavior.
trap 1 2
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