通过管道进入while循环获取流程替换的退出代码 [英] Get exit code of process substitution with pipe into while loop

问题描述

以下脚本调用另一个程序在while循环中读取其输出(请参见

The following script calls another program reading its output in a while loop (see Bash - How to pipe input to while loop and preserve variables after loop ends):

while read -r col0 col1; do
    # [...]
done < <(other_program [args ...])

如何检查other_program的退出代码以查看循环是否正确执行?

How can I check for the exit code of other_program to see if the loop was executed properly?

推荐答案

^{注意:ls -d / /nosuch在下面用作示例命令，因为它仍然失败(退出代码1)，同时仍生成标准输出(</)(除了stderr输出).}

^{Note: ls -d / /nosuch is used as an example command below, because it fails (exit code 1) while still producing stdout output (/) (in addition to stderr output).}

ccarton的有用答案原则上效果很好，但是默认情况下，while循环在子外壳中运行，这意味着在循环中创建或修改的任何变量对 current 外壳都是不可见的.

ccarton's helpful answer works well in principle, but by default the while loop runs in a subshell, which means that any variables created or modified in the loop will not be visible to the current shell.

在 Bash v4.2 + 中，您可以通过打开 lastpipe选项来更改此设置，这将使 last 部分成为在 current 外壳中运行的管道；
就像在ccarton的答案中一样，必须将 pipefail选项设置为使$?反映管道中第一个 failing 命令的退出代码:

In Bash v4.2+, you can change this by turning the lastpipe option on, which makes the last segment of a pipeline run in the current shell;
as in ccarton's answer, the pipefail option must be set to have $? reflect the exit code of the first failing command in the pipeline:

shopt -s lastpipe  # run the last segment of a pipeline in the current shell
shopt -so pipefail # reflect a pipeline's first failing command's exit code in $?

ls -d / /nosuch | while read -r line; do 
  result=$line
done

echo "result: [$result]; exit code: $?"

以上结果(省略了stderr输出):

The above yields (stderr output omitted):

result: [/]; exit code: 1

如您所见，在while循环中设置的$result变量可用，并且ls命令的(非零)退出代码反映在$?中.

As you can see, the $result variable, set in the while loop, is available, and the ls command's (nonzero) exit code is reflected in $?.

ikkachu的有用答案效果很好，并且显示了先进的技术，但这有点麻烦.
这是一个更简单的选择:

ikkachu's helpful answer works well and shows advanced techniques, but it is a bit cumbersome.
Here is a simpler alternative:

while read -r line || { ec=$line && break; }; do   # Note the `|| { ...; }` part.
    result=$line
done < <(ls -d / /nosuch; printf $?)               # Note the `; printf $?` part.

echo "result: [$result]; exit code: $ec"

• 通过将ls命令的退出代码$?的值附加到输出而没有尾随的\n (printf $?)，read读取它在最后一次循环操作中，但指示失败(退出代码1)，通常会退出循环.

  • By appending the value of $?, the ls command's exit code, to the output without a trailing \n (printf $?), read reads it in the last loop operation, but indicates failure (exit code 1), which would normally exit the loop.

    我们可以使用||检测到这种情况，然后将退出代码(仍被读入$line的代码)分配给变量$ec，然后退出循环.

    We can detect this case with ||, and assign the exit code (that was still read into $line) to variable $ec and exit the loop then.

    在命令输出没有尾随\n 的偶然机会上，需要做更多的工作:

    On the off chance that the command's output doesn't have a trailing \n, more work is needed:

    while read -r line || 
      { [[ $line =~ ^(.*)/([0-9]+)$ ]] && ec=${BASH_REMATCH[2]} && line=${BASH_REMATCH[1]};
        [[ -n $line ]]; }
    do
        result=$line
    done < <(printf 'no trailing newline'; ls /nosuch; printf "/$?")
    
    echo "result: [$result]; exit code: $ec"
    

    以上结果(省略了stderr输出):

    The above yields (stderr output omitted):

    result: [no trailing newline]; exit code: 1
    

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