shell语言打印异常栈

bash shell 能不能打印出类似Java的异常栈(Stack Trace)呢？

Java的异常栈会打印出调用方法链的具体位置，具体信息是<文件，函数，行号>三元组。类似下面：

StudentException: Error finding students
        at StudentManager.findStudents(StudentManager.java:13)
        at StudentProgram.main(StudentProgram.java:9)
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BASH_SOURCE变量

BASH_SOURCE是一个数组变量，存储元素文件名。
FUNCNAME也是一个数组变量，存储元素是函数名。

这两个数组是按索引对应的，${BASH_SOURCE[0]}代表的是异常栈底部当前文件名，${FUNCNAME[0]}代表的异常栈底部的当前函数。

BASH_SOURCE
An array variable whose members are the source filenames where the corresponding shell function names in the FUNCNAME array variable are defined. The shell function ${FUNCNAME[$i]} is defined in the file ${BASH_SOURCE[$i]} and called from ${BASH_SOURCE[$i+1]}

FUNCNAME
An array variable containing the names of all shell functions currently in the execution call stack. The element with index 0 is the name of any currently-executing shell function. The bottom-most element (the one with the highest index) is “main”.

例子

打印异常栈小demo

写个demo验证一下

• test.sh

source test2.sh

function fun_b() {
    echo "Info in ${BASH_SOURCE[0]}:${FUNCNAME[0]}:${BASH_LINENO[0]} "
    fun_c
}

function fun_a() {
    echo "Info in ${BASH_SOURCE[0]}:${FUNCNAME[0]}:${BASH_LINENO[0]} "
    fun_b
}
fun_a
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• test.sh

function fun_c() {
  echo "Info in ${BASH_SOURCE[0]}:${FUNCNAME[0]}:${BASH_LINENO[0]} "
}
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其中${BASH_LINENO[0]}代表函数调用语句的行号

执行test.sh查看异常栈内容：

Info in test.sh:fun_a:14 
Info in test.sh:fun_b:11 
Info in test2.sh:fun_c:5 
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成熟的异常栈函数

把上面那个demo稍微润一下，可以实现一个很好的生产级函数：

• test2.sh

function errexit() {
  local err=$?
  set +o xtrace
  local code="${1:-1}"
  echo "Error in ${BASH_SOURCE[1]}:${BASH_LINENO[0]}. '${BASH_COMMAND}' exited with status $err"
  # Print out the stack trace described by $function_stack  
  if [ ${#FUNCNAME[@]} -gt 2 ]
  then
    echo "Call tree:"
    for ((i=1;i<${#FUNCNAME[@]}-1;i++))
    do
      echo " $i: ${BASH_SOURCE[$i+1]}:${BASH_LINENO[$i]} ${FUNCNAME[$i]}(...)"
    done
  fi
  echo "Exiting with status ${code}"
  exit "${code}"
}
function errexit() {
  local err=$?
  set +o xtrace
  local code="${1:-1}"
  echo "Error in ${BASH_SOURCE[1]}:${BASH_LINENO[0]}. '${BASH_COMMAND}' exited with status $err"
  # Print out the stack trace described by $function_stack  
  if [ ${#FUNCNAME[@]} -gt 2 ]
  then
    echo "Call tree:"
    for ((i=1;i<${#FUNCNAME[@]}-1;i++))
    do
      echo " $i: ${BASH_SOURCE[$i+1]}:${BASH_LINENO[$i]} ${FUNCNAME[$i]}(...)"
    done
  fi
  echo "Exiting with status ${code}"
  exit "${code}"
}

function func_c() {
    cat hello  # some danger
    res=$?
    if [ $res -ne 0 ]
    then
      errexit $res
    fi
}
# trap ERR to provide an error handler whenever a command exits nonzero
#  this is a more verbose version of set -o errexit
# If  a  sigspec  is  ERR, the command arg is executed whenever a simple command has a non-zero exit status
trap 'errexit' ERR
# setting errtrace allows our ERR trap handler to be propagated to functions,
#  expansions and subshells
set -o errtrace
function func_c() {
    cat hello  # bad command exit with code 1
}
function func_a() {
    func_c
}
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• test.sh

source test2.sh

function func_b() {
    func_a
}
func_b
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测试一下：

调用链是 func_b ->func_a -> func_c ，打印出的内容函数前的文件名是调用语句所在的文件名，行号是调用语句所在的行号

[root@localhost utiltest]# bash test.sh 
cat: hello: No such file or directory
Error in test2.sh:21. 'cat hello' exited with status 1
Call tree:
 1: test2.sh:33 func_c(...)
 2: test.sh:5 func_a(...)
 3: test.sh:7 func_b(...)
Exiting with status 1

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分析

上面有几个符号还是注释一下

• 关闭debug信息

# set +o 代表关闭选项，set -o 代表启动选项
# set -x (long notation: set -o xtrace) traces commands before executing them.
set +o xtrace
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这个是关闭debug信息，要不然会打印出类似这种(这是bash shell的运行时值计算)：

Call tree:
+ (( i=1 ))
+ (( i<5-1 ))
+ echo ' 1: test2.sh:28 func_c(...)'
 1: test2.sh:28 func_c(...)
+ (( i++ ))
+ (( i<5-1 ))
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• 数组长度

${#FUNCNAME[@]}代表FUNCNAME数组长度。

• 条件赋值

# code赋值为函数第一个参数（占位符），否则赋值为1.（前缀用了'-'字符，这里不是-1）
 local code="${1:-1}"
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• 异常处理句柄

trap机制是当发生异常时，也就是命令以非0状态退出时，需要调用异常处理程序。

trap defines and activates handlers to run when the shell receives signals or other special conditions.

# 当有命令非零退出时，执行errexit句柄（函数）
trap 'errexit' ERR 
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执行errexit时，要打印出哪个命令是异常退出了，使用了BASH_COMMAND变量。这个变量在trap条件下读取时，值为异常退出语句，也就是我们想要的。

BASH_COMMAND
    The  command currently being executed or about to be executed, unless the shell is executing a command as the result of a trap, in which case it is the command executing at the time of the trap
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另外为了能够将这个触发机制传播到子shell（bash每次执行函数也是执行一个子shell）:

-E, equal to set -o errtrace   
If  set,  any trap on ERR is inherited by shell functions, command substitutions, and commands executed 
in a subshell environment.  The ERR trap is normally not inherited in such cases.
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否则trap机制要在每个可能引起错误的shell函数中定义一次，才能实现相同的效果。比如：

function func_c() {
    trap 'errexit' ERR
    cat hello  # some danger
}
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