linux shell脚本中处理信号的方法
- trap是一个shell内建命令，它用来在脚本中指定信号如何处理。
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.）
应用实例
捕获信号
trap commands signals
现在通过一个例子，展示了如何用trap命令来忽略SIGINT信号，并控制脚本的行为：
#!/bin/bash#测试信号捕获trap "echo ' Sorry! I have trapped Ctrl-C'" SIGINTecho "This is a test script"count=1
while [ $count -le 10 ]
do echo "Loop #$count"sleep 1count=$[ $count + 1 ]
doneecho "This is the end of the test script"结果
上面的例子中用到了trap命令会在每次检测到SIGINT信号时显示一行简单的文本信息。捕获这些信号会阻止用户用bash shell组合键Ctrl + C来停止程序。每次按下Ctrl + c组合键，脚本都会执行trap命令中指定的echo语句，而不是处理该信号并允许shell停止该脚本。
捕获脚本退出
除了在shell脚本中捕获信号，可以在shell脚本退出时进行捕获。要想捕获shell脚本的退出。只要在trap命令后加上EXIT信号就行。
#!/bin/bash#捕获脚本退出信号trap "echo \"Goodbye"\" EXITcount=1
while [ $count -le 5 ]
doecho "Loop #$count"sleep 1count=$[ $count +1 ]
done
当脚本运行时，正常退出位置时，会捕获就被触发了，shell会执行在trap命令行指定的命令。如果提前退出脚本，同样能够捕获到EXIT
移出捕获
要想在脚本中不同的位置进行不同的捕获处理，只需要重新使用带有新选项的trap命令
trap -- SIGINT #终止脚本运行
可以在删除已设置好的捕获。只需要在trap命令与希望默认行为的信号列表之间加上两个破折号就行了。例如：
#!/bin/bash#Modifying a set traptrap "echo ' Sorry... Ctrl-C is trapped.'" SIGINTcount=1
while [ $count -le 5 ]
doecho "Loop #$count"sleep 1count=$[ $count + 1 ]
donetrap -- SIGINT
echo "I just removed the trap"
count=1
while [ $count -le 5 ]
doecho "Second Loop #$count"sleep 1count=$[ $count + 1 ]
done
在移除信号捕获后，脚本按照默认的行为来处理SIGINT信号，也就是终止脚本的运行。但如果信号是在捕获移除前接收到的，那么脚本会按照原先的trap命令中设置进行处理。
因为信号在捕获被移除前已经接收到了，脚本会按照旧的执行trap中指定的命令。捕获随后被移除，再按下Ctrl +c 就能够提前终止脚本。
总结
Linux 系统运行利用信号来控制shell脚本。当bash shell接受信号，并将它们传递给运行在改=该shell进程中的所有进程。可以在脚本中用trap语句来捕获信号并执行特定命令。