Capture signal of shell scripts
trap, trap translates to mean, shell scripts in the trap is designed to capture the signal. What signal do? Such as frequently used kill -9, kill -15, CTRL + C belong signal
1, view all available signals
trap -l or to kill -l
[root@linux1 ~]# kill -l
63) SIGRTMAX-1 64) SIGRTMAX
[root@linux1 ~]# trap -l
1) SIGHUP 2) SIGINT 3) SIGQUIT 4) SIGILL 5) SIGTRAP
6) SIGABRT 7) SIGBUS 8) SIGFPE 9) SIGKILL 10) SIGUSR1
11) SIGSEGV 12) SIGUSR2 13) SIGPIPE 14) SIGALRM 15) SIGTERM
......
2, common signals are as follows:
Value the Comment Signal
─────────────────────────────
SIGHUP 1 to terminate the process, especially when the terminal exit, within this process will end termination
SIGINT 2 interrupt the process, almost equivalent to sigterm, will release as much as possible the implementation of clean-up, the release of resources, conservation status (CTRL + C)
SIGQUIT signal sent from the keyboard kill (terminate) processes 3
SIGKILL 9 forced to kill the process, the signal can not be caught and ignored, does not perform any clean-up process behavior after receiving the signal, so the resources are not released, the state will not save
SIGTERM 15 kill (terminate) processes, almost equivalent to sigint signal, will release the implementation of clean-up as much as possible, the release of resources, conservation status, etc.
SIGSTOP 19 The signal is captured and can not be ignored in the process of stopping the information, after receiving the signal will enter the stopped state
SIGTSTP 20 is the signal process can be ignored a stop signal (CTRL + Z)
The real signal name is not SIGXXX, but after the removal of the word SIG, each signal as well as the corresponding code
For example, initiating a signal for the process to the PID 12345
kill -1 12345
kill -HUB 12345
kill -SIGHUB 12345
3, trap option
trap -l list the current list of supported signal system, has been used above, the same root kill -l
trap -p equivalent to the trap, the trap has been arranged to view the shell
You can see there are three default shell traps, omit 20,21,22 signal
[root @ linux1 ~] # stair
staircase - 'SIGTSTP
stage -' SIGTTIN
stage - 'SIGTTOU
4, after trapping the signal why
• ignore signals
• capture the signal corresponding processing. The main is to clean up temporary files created by the script, and then exit.
5, is provided a trap CTRL + C can be ignored and the signal 15
CTRL signal corresponds to the signal corresponding to the SIGINT 15 SIGTERM
[root @ linux1 ~] # trap '' SIGINT SIGTERM
[root @ linux1 ~] # trap
trap - '' SIGINT
trap - '' SIGTERM
trap - '' SIGTSTP
trap - '' SIGTTIN
trap - '' SIGTTOU
In this way, the current shell can not be killed by kill -15
6, set a trap to capture the signal -15, print "I caught you anyway ..."
[root @ linux1 ~] # trap 'echo " I caught you anyway ~"' TERM
[root @ linux1 ~] # Trap
Trap - '' SIGINT
Trap - 'echo "I caught you anyway ~"' SIGTERM
Trap - '' the SIGTSTP
Trap - '' the SIGTTIN
Trap - '' the SIGTTOU
Effect, when I launch bash kill -15 signal current is printed out
[root @ linux1 ~] # echo $$
8827
[root @ linux1 ~] # the kill -15 8827
I caught you anyway ~
[root @ linux1 ~] # the kill -15 8827
I caught you anyway ~
[root @ linux1 ~ ] # kill -15 8827
I caught you anyway ~
7, is provided in a script can ignore a script CTRL + C and CTRL + Z signal
CTRL + C 2 signal, i.e. SIGINT
CTRL + Z signal 20, i.e. SIGTSTP
script:
Script sleeping 10s, then print success, the script ignores signals INT and TSTP
[root@linux1 ~]# cat trap.sh
#!/bin/bash
trap '' SIGINT SIGTSTP
sleep 10
echo success
effect:
<Font color = red> CTRL + C can not prevent me from sleep </ font>
[root @ Linux 1 ~] # bash trap.sh
^ C ^ C ^ Z ^ Z ^ C ^ C ^ Z ^ CCC ^ Z ^ Z ^ Z ^ C ^ C ^ C
success
8, arranged a terminal when the script is able to clean up the garbage and exit the trap script immediately
Script is as follows:
[root@linux1 ~]# cat trap1.sh
#!/bin/bash
trap 'echo trap handing...;rm -rf /tmp/$BASHPID;echo TEMP files cleaned;exit' SIGINT SIGTERM SIGQUIT SIGHUP
mkdir -p /tmp/$$/
touch /tmp/$$/{a..c}.txt
sleep 10
echo first sleep success
sleep 10
echo second sleep success
In this way, the script except SIGKILL signal (kill -9), always clean out temporary garbage
effect
It has been unable to terminate the beginning, and later performed in front of shell found himself under the trap set up a trap to ignore the CTRL + C to exit the shell to re-enter
[root@linux1 ~]# bash trap1.sh
^Ctrap handing...
TEMP files cleaned
9, trap guardian objects
Trap guard the object is the shell process itself, not the guardian of the child within the shell environment. But if it is to ignore the signal type traps, the entire shell daemon process group will be to ignore the given signal.
[root@linux1 ~]# cat trap2.sh
#!/bin/bash
trap 'echo trap_handle_time: $(date +"%F %T")' SIGINT SIGTERM
echo time_start: $(date +"%F %T")
sleep 10
echo time_end1: $(date +"%F %T")
sleep 10
echo time_end2: $(date +"%F %T")
# After executing the script, open a new terminal to kill it using kill -15
[root @ linux1 ~] # killall -s SIGTERM trap2.sh
# View the output situation
[root @ linux1 ~] # ./trap2.sh
TIME_START: 2019-08-27 10:43:48
trap_handle_time: 2019-08-27 10:43:58
time_end1: 2019-08-27 10:43 : 58
time_end2: 2019-08-27 10:44:08
Can be found, kill executed, the screen does not print trap_handle immediately, but only in 10 runs sleep after printing. sleep guardian of the process have been ignored type trap
As long as a signal is sent to the shell process will wait after the end of the current running process command signal, and then continue to run the script down. (In fact, only when the operation shell script being executed is safe signal system call, the signal can not interrupt the process appear, in various commands in the shell, we can not know directly what the command is system call is executed system call).
But sleep command initiated sleep () call, a signal is safe, so sleep during the execution of the script above, the signal does not directly interrupt their operation, but wait for it to finish after running the command again to perform signal processing.
10, if the shell set a trap for a signal, the shell process receives the signal, <font color = red> will wait for command within a running start before the end of the process traps </ font>
11, CTRL + C and SIGINT not equivalent. When a time pressing CTRL + C, which is transmitted to the entire SIGINT signal set currently running process. Shell script for it, SIGINT not only sent to the shell script process, also sent to the process currently running script
[root@linux1 ~]# cat trap2.sh
#!/bin/bash
trap 'echo trap_handle_time: $(date +"%F %T")' SIGINT SIGTERM
echo time_start: $(date +"%F %T")
sleep 10
echo time_end1: $(date +"%F %T")
sleep 10
echo time_end2: $(date +"%F %T")
# After executing the script, immediately + C CTRL
[root @ linux1 ~] # bash trap2.sh
TIME_START: 2019-08-27 10:20:53
^ Ctrap_handle_time: 2019-08-27 10:20:54
time_end1: 2019-08 10:20:54 -27
time_end2: 2019-08-27 10:21:04
It can be found after the CTRL + C, not only the trap processing, sleep also flew over; CTRL + C illustrate not only the script process receives a SIGINT signal, let the current process receives a SIGINT signal
A bit difficult to understand, come again and look.