Linux命令trap - 在脚本中处理信号 例子

     在Linux中，trap命令主要用于接收信号并采取行动，信号是异步发送到一个程序的事件，在默认情况下，可以终止一个程序，trap命令原型如下：

trap command signal

signal是指接收到的信号，command是接收到该信号采取的行动。如下为两种简单的信号。

信号

说明

INT(2)

Ctrl + C

QUIT(3)

Ctrl + \

trap命令常见的用途在于脚本程序中断时完成清理工作，比如临时文件等。例子程序如下：

#!/bin/sh

trap 'rm -f /tmp/my_tmp_file_$$' INT

echo "creating file /tmp/my_tmp_file_$$"

date > /tmp/my_tmp_file_$$

echo "press interrupt (CTRL-C) to interrupt ......"

while [ -f /tmp/my_tmp_file_$$ ]; do

       echo "File exists"

       sleep 3

done

echo" we never get here"

exit 0

注：$$表示当前的shell进程号。运行该脚本，首先在tmp下创建临时文件my_tmp_file_$$，如果遇到INT中断，即Ctrl+c,即删除该文件。

    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.）