使用陷阱并不总是一种选择。例如，如果您正在编写某种需要错误处理并且可以从任何脚本调用的可重用函数（在使用辅助函数获取文件之后），则该函数不能假设外部脚本的退出时间，这使得使用陷阱非常困难。使用陷阱的另一个缺点是可组合性差，因为您可能会覆盖之前可能在调用者链中设置的陷阱。
有一个小技巧可以用来在没有陷阱的情况下进行正确的错误处理。正如您可能已经从其他答案中知道的那样，如果您在命令后面使用 || 运算符，set -e 在命令中不起作用，即使您在子shell 中运行它们也是如此；例如，这行不通：
#!/bin/sh
# prints:
#
# --> outer
# --> inner
# ./so_1.sh: line 16: some_failed_command: command not found
# <-- inner
# <-- outer
set -e
outer() {
echo '--> outer'
(inner) || {
exit_code=$?
echo '--> cleanup'
return $exit_code
}
echo '<-- outer'
}
inner() {
set -e
echo '--> inner'
some_failed_command
echo '<-- inner'
}
outer
但是需要|| 操作符来防止在清理之前从外部函数返回。诀窍是在后台运行内部命令，然后立即等待它。内置的wait 将返回内部命令的退出代码，现在您在wait 之后使用||，而不是内部函数，因此set -e 在后者内部正常工作：
#!/bin/sh
# prints:
#
# --> outer
# --> inner
# ./so_2.sh: line 27: some_failed_command: command not found
# --> cleanup
set -e
outer() {
echo '--> outer'
inner &
wait $! || {
exit_code=$?
echo '--> cleanup'
return $exit_code
}
echo '<-- outer'
}
inner() {
set -e
echo '--> inner'
some_failed_command
echo '<-- inner'
}
outer
这里是基于这个想法的通用函数。如果您删除 local 关键字，它应该可以在所有 POSIX 兼容的 shell 中工作，即将所有 local x=y 替换为 x=y：
# [CLEANUP=cleanup_cmd] run cmd [args...]
#
# `cmd` and `args...` A command to run and its arguments.
#
# `cleanup_cmd` A command that is called after cmd has exited,
# and gets passed the same arguments as cmd. Additionally, the
# following environment variables are available to that command:
#
# - `RUN_CMD` contains the `cmd` that was passed to `run`;
# - `RUN_EXIT_CODE` contains the exit code of the command.
#
# If `cleanup_cmd` is set, `run` will return the exit code of that
# command. Otherwise, it will return the exit code of `cmd`.
#
run() {
local cmd="$1"; shift
local exit_code=0
local e_was_set=1; if ! is_shell_attribute_set e; then
set -e
e_was_set=0
fi
"$cmd" "$@" &
wait $! || {
exit_code=$?
}
if [ "$e_was_set" = 0 ] && is_shell_attribute_set e; then
set +e
fi
if [ -n "$CLEANUP" ]; then
RUN_CMD="$cmd" RUN_EXIT_CODE="$exit_code" "$CLEANUP" "$@"
return $?
fi
return $exit_code
}
is_shell_attribute_set() { # attribute, like "x"
case "$-" in
*"$1"*) return 0 ;;
*) return 1 ;;
esac
}
使用示例：
#!/bin/sh
set -e
# Source the file with the definition of `run` (previous code snippet).
# Alternatively, you may paste that code directly here and comment the next line.
. ./utils.sh
main() {
echo "--> main: $@"
CLEANUP=cleanup run inner "$@"
echo "<-- main"
}
inner() {
echo "--> inner: $@"
sleep 0.5; if [ "$1" = 'fail' ]; then
oh_my_god_look_at_this
fi
echo "<-- inner"
}
cleanup() {
echo "--> cleanup: $@"
echo " RUN_CMD = '$RUN_CMD'"
echo " RUN_EXIT_CODE = $RUN_EXIT_CODE"
sleep 0.3
echo '<-- cleanup'
return $RUN_EXIT_CODE
}
main "$@"
运行示例：
$ ./so_3 fail; echo "exit code: $?"
--> main: fail
--> inner: fail
./so_3: line 15: oh_my_god_look_at_this: command not found
--> cleanup: fail
RUN_CMD = 'inner'
RUN_EXIT_CODE = 127
<-- cleanup
exit code: 127
$ ./so_3 pass; echo "exit code: $?"
--> main: pass
--> inner: pass
<-- inner
--> cleanup: pass
RUN_CMD = 'inner'
RUN_EXIT_CODE = 0
<-- cleanup
<-- main
exit code: 0
使用此方法时唯一需要注意的是，从您传递给run 的命令完成的所有Shell 变量修改都不会传播到调用函数，因为该命令在子shell 中运行。