使用陷阱并不总是一个选择。例如，如果您正在编写某种需要错误处理的可重用函数，并且可以从任何脚本调用(在使用helper函数源文件之后)，则该函数不能假设外部脚本的退出时间，这使得使用trap非常困难。使用陷阱的另一个缺点是糟糕的可组合性，因为您可能会覆盖之前可能在调用者链中较早设置的陷阱。
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。正如你可能已经从其他答案中知道的，如果你在命令后使用||操作符，set -e在命令中不起作用，即使你在子shell中运行它们;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。wait内置函数将返回内部命令的退出码，现在你在wait后使用||，而不是内部函数，所以set -e在后者中正常工作:
#!/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
下面是基于这个思想的泛型函数。如果你删除本地关键字，它应该在所有posix兼容的shell中工作，即用x=y替换所有本地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
}
用法示例:
#!/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
在使用此方法时，您需要注意的唯一一件事是，您传递给运行的命令对Shell变量所做的所有修改都不会传播到调用函数，因为该命令运行在子Shell中。
