用途说明
trap是一个shell内建命令,它用来在脚本中指定信号如何处理。比如,按Ctrl+C会使脚本终止执行,实际上系统发送了SIGINT信号给脚本进程,SIGINT信号的默认处理方式就是退出程序。如果要在Ctrl+C不退出程序,那么就得使用trap命令来指定一下SIGINT的处理方式了。trap命令不仅仅处理Linux信号,还能对脚本退出(EXIT)、调试(DEBUG)、错误(ERR)、返回(RETURN)等情况指定处理方式。
常用参数
trap [-lp] [[arg] sigspec ...]
格式:trap "commands" signals
当shell接收到signals指定的信号时,执行commands命令。(The command arg is to be read and executed when the shell receives signal(s) sigspec. )
格式:trap signals
如果没有指定命令部分,那么就将信号处理复原。比如 trap INT 就表明恢复Ctrl+C退出。(If arg is absent (and there is a single sigspec) or -, each specified signal is reset to its original disposition (the value it had upon entrance to the shell). )
格式:trap "" signals
忽略信号signals,可以多个,比如 trap "" INT 表明忽略SIGINT信号,按Ctrl+C也不能使脚本退出。又如 trap "" HUP 表明忽略SIGHUP信号,即网络断开时也不能使脚本退出。(If arg is the null string the signal specified by each sigspec is ignored by the shell and by the commands it invokes. )
格式:trap -p
格式:trap -p signal
把当前的trap设置打印出来。(If arg is not present and -p has been supplied, then the trap commands associated with each sigspec are displayed. If no arguments are supplied or if only -p is given, trap prints the list of commands associated with each signal.)
格式:trap -l
把所有信号打印出来。(The -l option causes the shell to print a list of signal names and their corresponding numbers. Each sigspec is either a signal name defined in <signal.h>, or a signal number. Signal names are case insensitive and the SIG prefix is optional.)
格式:trap "commands" EXIT
脚本退出时执行commands指定的命令。(If a sigspec is EXIT (0) the command arg is executed on exit from the shell.)
格式:trap "commands" DEBUG
在脚本执行时打印调试信息,比如打印将要执行的命令及参数列表。(If a sigspec is DEBUG, the command arg is executed before every simple command, for command, case command, select command, every arithmetic for command, and before the first command executes in a shell function (see SHELL GRAMMAR above). Refer to the description of the extdebug option to the shopt builtin for details of its effect on the DEBUG trap.)
格式:trap "commands" ERR
当命令出错,退出码非0,执行commands指定的命令。(If a sigspec is ERR, the command arg is executed whenever a simple command has a non-zero exit status, subject to the following conditions. The ERR trap is not executed if the failed command is part of the command list immediately following a while or until keyword, part of the test in an if statement, part of a && or ┅Ι│ list, or if the command’s return value is being inverted via !. These are the same conditions obeyed by the errexit option.)
格式:trap "commands" RETURN
当从shell函数返回、或者使用source命令执行另一个脚本文件时,执行commands指定的命令。(If a sigspec is RETURN, the command arg is executed each time a shell function or a script executed with the . or source builtins finishes executing. Signals ignored upon entry to the shell cannot be trapped or reset. Trapped signals that are not being ignored are reset to their original values in a child process when it is created. The return status is false if any sigspec is invalid; otherwise trap returns true.)
使用示例
示例一
[root@new55 ~]# trap -p
[root@new55 ~]# trap "echo hello" INT
[root@new55 ~]# trap -p
trap -- 'echo hello' SIGINT
[root@new55 ~]# trap -p INT
trap -- 'echo hello' SIGINT
[root@new55 ~]# trap -p QUIT
[root@new55 ~]# Ctrl+C
[root@new55 ~]# hello
[root@new55 ~]#
示例二
[root@new55 ~]# trap -l
1) SIGHUP 2) SIGINT 3) SIGQUIT 4) SIGILL
5) SIGTRAP 6) SIGABRT 7) SIGBUS
![](/images/smiles/icon_cool.gif)
9) SIGKILL 10) SIGUSR1 11) SIGSEGV 12) SIGUSR2
13) SIGPIPE 14) SIGALRM 15) SIGTERM 16) SIGSTKFLT
17) SIGCHLD 18) SIGCONT 19) SIGSTOP 20) SIGTSTP
21) SIGTTIN 22) SIGTTOU 23) SIGURG 24) SIGXCPU
25) SIGXFSZ 26) SIGVTALRM 27) SIGPROF 28) SIGWINCH
29) SIGIO 30) SIGPWR 31) SIGSYS 34) SIGRTMIN
35) SIGRTMIN+1 36) SIGRTMIN+2 37) SIGRTMIN+3 38) SIGRTMIN+4
39) SIGRTMIN+5 40) SIGRTMIN+6 41) SIGRTMIN+7 42) SIGRTMIN+8
43) SIGRTMIN+9 44) SIGRTMIN+10 45) SIGRTMIN+11 46) SIGRTMIN+12
47) SIGRTMIN+13 48) SIGRTMIN+14 49) SIGRTMIN+15 50) SIGRTMAX-14
51) SIGRTMAX-13 52) SIGRTMAX-12 53) SIGRTMAX-11 54) SIGRTMAX-10
55) SIGRTMAX-9 56) SIGRTMAX-8 57) SIGRTMAX-7 58) SIGRTMAX-6
59) SIGRTMAX-5 60) SIGRTMAX-4 61) SIGRTMAX-3 62) SIGRTMAX-2
63) SIGRTMAX-1 64) SIGRTMAX
[root@new55 ~]#
信号说明
Linux支持的信号列表如下。很多信号是与机器的体系结构相关的
信号值 默认处理动作 发出信号的原因
SIGHUP 1 A 终端挂起或者控制进程终止
SIGINT 2 A 键盘中断(如break键被按下)
SIGQUIT 3 C 键盘的退出键被按下
SIGILL 4 C 非法指令
SIGABRT 6 C 由abort(3)发出的退出指令
SIGFPE 8 C 浮点异常
SIGKILL 9 AEF Kill信号
SIGSEGV 11 C 无效的内存引用
SIGPIPE 13 A 管道破裂: 写一个没有读端口的管道
SIGALRM 14 A 由alarm(2)发出的信号
SIGTERM 15 A 终止信号
SIGUSR1 30,10,16 A 用户自定义信号1
SIGUSR2 31,12,17 A 用户自定义信号2
SIGCHLD 20,17,18 B 子进程结束信号
SIGCONT 19,18,25 进程继续(曾被停止的进程)
SIGSTOP 17,19,23 DEF 终止进程
SIGTSTP 18,20,24 D 控制终端(tty)上按下停止键
SIGTTIN 21,21,26 D 后台进程企图从控制终端读
SIGTTOU 22,22,27 D 后台进程企图从控制终端写
示例三 在脚本中使用
下面的脚本用于检查和启动java程序。
第7行:找出正在运行的符合指定特征的进程;
第11行:如果找到了这样的进程,就杀掉;
第22行:以后台方式启动java程序;
第24行:得到刚启动的程序的pid;
第28行:对SIGTERM信号设置处理方式:结束启动的java程序;
第30行:等待后台进程结束。
Bash代码
#!/bin/sh #2007.05.06/07 # 增加了杀掉LAST_PID功能 # 增加了脚本退出时杀掉THIS_PID功能 LAST_PID=$(ps -ef|grep 'java.*Zhenjiang'|grep -v grep|awk '{print $2}') echo "LAST_PID=$LAST_PID" if [ -n "$LAST_PID" ] && [ "$LAST_PID" -gt 0 ]; then echo "LAST PROCESS NOT EXIT, NOW KILL IT!" kill $LAST_PID sleep 1 fi if ! cd ../opt/zhenjiang; then echo "CHANGE DIRECTORY FAILED" exit 1 fi java -classpath .:./cpinterfaceapi.jar:./log4j-1.2.14.jar:./hyjc.jar:./smj.client.jar Zhenjiang & THIS_PID=$! echo "THIS_PID=$THIS_PID" trap "kill $THIS_PID" TERM wait