Unix Shell: Process Control(3)

1. Trap the signal
script_1:
Use trap command to setup the customized handler for signal HUP and TERM

 #! /bin/bash  
   
 trap "echo Catch the HUP signal!" HUP  
 trap "echo Catch the terminal signal! ; rm ./temp ; exit 0" TERM  
   
 echo "" >temp  
 a=1  
 while [ $a -ne 0 ]  
 do  
   ((a++))  
   echo $a >>temp  
   sleep 5  
 done  

terminal:
1) Launch the script_1 process
2) Run ps command and script_1 process is already launched
3) Output the temp file which is generated by script_1, the process is adding text into the file
4) Send the HUP signal to script_1 process
5) script_1 process catch the HUP signal and output the text on standard output
6) Run ps command, but the script_1 process is still alive
7) List the temp file, temp file is not deleted
8) Send the TERM signal to script_1 process
9) script_1 process catch the TERM signal and output the text
10) Run the ps command, script_1 process is already removed
11) List the temp file, temp file is already deleted.

 aubinxia@aubinxia-fastdev:~/Desktop/xxdev$ ./script_1 &  
 [2] 3448  
 aubinxia@aubinxia-fastdev:~/Desktop/xxdev$ ps  
  PID TTY     TIME CMD  
  2954 pts/0  00:00:00 bash  
  3162 pts/0  00:00:09 emacs  
  3448 pts/0  00:00:00 script_1  
  3449 pts/0  00:00:00 sleep  
  3450 pts/0  00:00:00 ps  
 aubinxia@aubinxia-fastdev:~/Desktop/xxdev$ cat temp  
   
 2  
 3  
 aubinxia@aubinxia-fastdev:~/Desktop/xxdev$ kill -HUP 3448  
 aubinxia@aubinxia-fastdev:~/Desktop/xxdev$ Catch the HUP signal!  
   
 aubinxia@aubinxia-fastdev:~/Desktop/xxdev$ ps  
  PID TTY     TIME CMD  
  2954 pts/0  00:00:00 bash  
  3162 pts/0  00:00:09 emacs  
  3448 pts/0  00:00:00 script_1  
  3456 pts/0  00:00:00 sleep  
  3457 pts/0  00:00:00 ps  
 aubinxia@aubinxia-fastdev:~/Desktop/xxdev$ ls -lrt temp  
 -rw-rw-r-- 1 aubinxia aubinxia 15 Jul 26 17:17 temp  
 aubinxia@aubinxia-fastdev:~/Desktop/xxdev$ kill -TERM 3448  
 aubinxia@aubinxia-fastdev:~/Desktop/xxdev$ Catch the terminal signal!  
   
 [2]+ Done          ./script_1  
 aubinxia@aubinxia-fastdev:~/Desktop/xxdev$ ps  
  PID TTY     TIME CMD  
  2954 pts/0  00:00:00 bash  
  3162 pts/0  00:00:09 emacs  
  3462 pts/0  00:00:00 ps  
 aubinxia@aubinxia-fastdev:~/Desktop/xxdev$ ls -lrt temp  
 ls: cannot access temp: No such file or directory  

2. Other Signals -EXIT
EXIT signal is caught before the exit() system call is called.
./script_1:

 #! /bin/bash  
   
 trap "echo Catch the EXIT signal" EXIT  
   
 a=1  
 while [ $a -ne 0 ]  
 do  
   date >/dev/null  
   sleep 5  
 done  

terminal:
1) Launch the script_1 process
2) kill the process script_1 and it catch the EXIT signal

 aubinxia@aubinxia-fastdev:~/Desktop/xxdev$ ./script_1 &  
 [2] 3524  
 aubinxia@aubinxia-fastdev:~/Desktop/xxdev$ kill -HUP 3524  
 Catch the EXIT signal  

3. Other Signals -DEBUG
script_1:

 #! /bin/bash  
   
 trap "echo Catch the DEBUG signal" DEBUG  
   
 a=2  
 echo "Hello world! a=" $a  
 #output:  
 #Catch the DEBUG signal:  
 #Catch the DEBUG signal:  
 #Hello world! a= 2  
 #  
 #This indicates that DEBUG signal handler is launched  
 #before each statement  
   
 ((a++))  
 echo "Hello world! a="$a  
 #output:  
 #Catch the DEBUG signal  
 #Catch the DEBUG signal  
 #Hello world! a=3  
 #  
 #This also indicates that DEBUG signal handler is launched  
 #before each statement.  

4. Other signal -ERR
script_1:

 #! /bin/bash  
   
 trap "echo Catch the ERR signal" ERR  
   
 echo "file information: " $(ls no-such-file)  
 #output:  
 #ls: cannot access no-such-file: No such file or directory  
 #file information:   
   
 ls no_such_file  
 #output:  
 #ls: cannot access no_such_file: No such file or directory  
 #Catch the ERR signal  

It indicates that only the 2nd "ls" command catch the ERR signal.