在变量上保存命令输出并检查退出状态

Question

以下命令将解析google ip

> ip=`dig +short google.com`
> echo $ip
> 216.58.210.238

有时（特别是当互联网连接丢失时）此命令失败并显示此错误

> ;; connection timed out; no servers could be reached

当命令失败并且我使用$#时，输出为0用于赋值

> ip=`dig +short google.com`
> echo $#
> 0
> echo $ip     # Command failed
> ;; connection timed out; no servers could be reached

如何在变量中保存命令输出，同时检查命令是否成功

Answer 1

你应该使用$?而不是$#。

$? - contains the return value from the last script.
$# - contains the total number of arguments passed to a script or function

做类似下面的事情：

if $?
then
echo "Success" # Do something here
else
echo "Fail" # Fallback mode
fi

编辑：回复this评论回答标题中的问题：

在变量上保存命令输出并检查退出状态

正如@chepner在this评论中指出的那样，任务不会干涉命令的退出状态。所以：

ip=$(dig +short google.com) # Just changed the legacy backticks to $()
[ $? -eq 0 ] && echo "dig succeeded"

Answer 2

您可以避免访问$?，只需：

if ip=$(dig +short google.com); then
    # Success.
else
    # Failure.
fi

Example:

以下功能将打印“失败”并返回1。

print_and_fail() { printf '%s' fail; return 1; }

因此，如果我们执行以下操作：

if foo=$(print_and_fail); then printf '%s\n' "$foo";fi

我们没有输出，但是将print_and_fail输出存储到$foo - 在这种情况下，“失败”。

但是，看看下面的函数，它将打印“success”并返回0。

print_and_succeed() { printf '%s' success; return 0; }

让我们看看现在发生了什么：

$ if foo=$(print_and_succeed); then printf '%s\n' "$foo";fi
$ success

Answer 3

您可以检查返回或使用命令替换并检查结果变量。例如

$ ip=$(dig +short google.com)
$ [ -n "$ip" ] && echo "all good, ip = $ip"

（你可以用-z反向检查失败

Answer 4

由于你使用的是Bash，你可以使用类似下面的脚本来捕获stdout，stderr和返回代码https://gist.github.com/jmmitchell/c4369acb8e9ea1f984541f8819c4c87b

为了便于参考，我在这里复制了脚本：

# #!/bin/bash
# 
# based on posts from stackexchange:
# http://stackoverflow.com/a/26827443/171475
# http://stackoverflow.com/a/18086548/171475
# http://stackoverflow.com/a/28796214/171475

function example_function {
    printf 'example output to stdout %d\n' {1..10}
    echo >&2 'example output to stderr'
    return 42
}



##############################
### using the dot operator ###

if [ "${BASH_VERSINFO}" -lt 4 ]; then
    printf '%s\n' "The source version of this script requires Bash v4 or higher."
else

    # stdout & stderr only
    source <({ cmd_err=$({ mapfile -t cmd_out < <(example_function); } 2>&1; declare -p cmd_out >&2); declare -p cmd_err; } 2>&1)

    printf "\n%s\n" "SOURCE VERSION : STDOUT & STDERR ONLY"
    printf "%s\n" "${cmd_out[@]}"
    printf "%s\n" "${cmd_err}"

    unset cmd_out
    unset cmd_err


    # stdout & stderr only as well as return code:
    source <({ cmd_err=$({ mapfile -t cmd_out< <( \
        example_function \
      ; cmd_rtn=$?; declare -p cmd_rtn >&3); } 3>&2 2>&1; declare -p cmd_out >&2); declare -p cmd_err; } 2>&1)


    printf "\n%s\n" "SOURCE VERSION : STDOUT, STDERR & RETURN CODE"
    printf '%s\n' "${cmd_out[@]}"
    # alternative version
    # declare -p cmd_out 
    printf '%s\n' "${cmd_err}"
    printf '%s\n' "${cmd_rtn}"

    unset cmd_out
    unset cmd_err
    unset cmd_rtn

fi

##############################
######### using exec #########

# stdout & stderr only
eval "$({ cmd_err=$({ cmd_out=$( \
    example_function \
  ); } 2>&1; declare -p cmd_out >&2); declare -p cmd_err; } 2>&1)"

printf "\n%s\n" "EVAL VERSION : STDOUT & STDERR ONLY"
printf '%s\n' "${cmd_out}"
printf '%s\n' "${cmd_err}"
printf '%s\n' "${cmd_rtn}"

unset cmd_out
unset cmd_err

# stdout & stderr only as well as return code:
eval "$({ cmd_err=$({ cmd_out=$( \
    example_function \
  ); cmd_rtn=$?; } 2>&1; declare -p cmd_out cmd_rtn >&2); declare -p cmd_err; } 2>&1)"


printf "\n%s\n" "EVAL VERSION : STDOUT, STDERR & RETURN CODE"
printf '%s\n' "${cmd_out}"
printf '%s\n' "${cmd_err}"
printf '%s\n' "${cmd_rtn}"


unset cmd_out
unset cmd_err
unset cmd_rtn