Question

在bash中，如果我在反引号中执行一些管道连接的命令，我怎样才能找到第一个命令的退出状态？

即。在这种情况下，我试图得到＆＃34; 1＆＃34;。如果我不使用反引号，我可以通过PIPESTATUS [0]获得，但是当我想保存输出时它似乎不起作用：

## PIPESTATUS[0] works to give me the exit status of 'false':
$ false | true;
$ echo $? ${PIPESTATUS[0]} ${PIPESTATUS[1]};
0 1 0

## doesn't work:
$ a=`false | true`;
$ echo $? ${PIPESTATUS[0]} ${PIPESTATUS[1]};
0 0

更一般地说，我试图完成：将某个程序输出的最后一行保存到变量，但是能够判断程序是否失败：

$ myvar=` ./someprogram | tail -1 `;
$ if [ "what do i put here" ]; then echo "program failed!"; fi

理想情况下，我也想了解发生了什么，而不仅仅是答案是什么。

感谢。

Answer 1

尝试设置pipefail选项。它返回失败的管道的最后一个命令。一个例子：

首先我禁用它：

set +o pipefail

创建一个perl脚本（script.pl）来测试管道：

#!/usr/bin/env perl

use warnings;
use strict;

if ( @ARGV ) { 
    die "Line1\nLine2\nLine3\n";
}
else {
    print "Line1\nLine2\nLine3\n";
}

在命令行中运行：

myvar=`perl script.pl | tail -1`
echo $? "$myvar"

产量：

0 Line3

看起来是正确的，让我们看一下pipefail启用：

set -o pipefail

运行命令：

myvar=`perl script.pl fail 2>&1 | tail -1`
echo $? "$myvar"

产量：

255 Line3

Answer 2

我的解决方案是使用fifos和bash“coproc”内置来获取管道中每个命令的消息和状态。我之前从未使用过fifos。（哦，小伙，下次我在Fedora上使用BashEclipse）。它变成了管理任何管道命令的通用机制。我解决了这个问题，但不是10或20行代码。更像是一个200 强大的可重复使用的解决方案（花了我三天的时间）。

我分享我的笔记：

* stderr for all pipe commands goes to the fifos.  
  If you want to get messages from stdout, you must redirect '1>&2', like this:  
  PIPE_ARRAY=("cat ${IMG}.md5" "cut -f1 -d\" \" 1>&2")  
  You must put "2>/fifo" first. Otherwise it won\'t work. example:  
    cat ${IMG}.md5 | cut -f1 -d' ' 1>&2  
  becomes:  
    cat ${IMG}.md5 2>/tmp/fifo_s0 | cut -f1 -d" " 2>/tmp/fifo_s1 1>&2 ; PSA=( "${PIPESTATUS[@]}" )

* With more tha one fifo, I found that you must read each fifo in turn.  
  When "fifo1" gets written to, "fifo0" reads are blocked until you read "fifo1"  
  I did\'nt use any special tricks like "sleep", "cat", or extra file descriptors  
  to keep the fifos open.  

* PIPESTATUS[@] must be copied to an array immediately after the pipe command returns.  
  _Any_ reads of PIPESTATUS[@] will erase the contents. Super volatile !  
  "manage_pipe()" appends '; PSA=( "${PIPESTATUS[@]}" )' to the pipe command string  
  for this reason. "$?" is the same as the last element of "${PIPESTATUS[@]}",  
  and reading it seems to destroy "${PIPESTATUS[@]}", but it's not absolutly verifed.

run_pipe_cmd() {  
  declare -a PIPE_ARRAY MSGS  
  PIPE_ARRAY=("dd if=${gDEVICE} bs=512 count=63" "md5sum -b >${gBASENAME}.md5")  
  manage_pipe PIPE_ARRAY[@] "MSGS"  # (pass MSGS name, not the array) 
}  
manage_pipe () {
  # input  - $1 pipe cmds array, $2 msg retvar name
  # output - fifo msg retvar
  # create fifos, fifo name array, build cnd string from $1 (re-order redirection if needed)
  # run coprocess 'coproc execute_pipe FIFO[@] "$CMDSTR"'
  # call 'read_fifos FIFO[@] "M" "S"' (pass names, not arrays for $2 and $3)
  # calc last_error, call _error, _errorf
  # set msg retvar values (eval ${2}[${i}]='"${Msg[${i}]}"')
}
read_fifos() {  
  # input  - $1 fifo array, $2 msg retvar name, $3 status retvar name  
  # output - msg, status retvars  
  # init local fifo_name, pipe_cmd_status, msg arrays  
  # do read loop until all 'quit' msgs are received
  # set msg, status retvar values (i.e. eval ${3}[${i}]='"${Status[${i}]}"' 
}
execute_pipe() {  
  # $1 fifo array, $2 cmdstr, $3 msg retvar, $4 status retvar   
  # init local fifo_name, pipe_cmd_status arrays
  # execute command string, get pipestaus  (eval "$_CMDSTR" 1>&2)
  # set fifo statuses from copy of PIPESTATUS
  # write 'status', 'quit' msgs to fifo  
}

Answer 3

问题是反引号启动了一个子shell。您的子shell有自己的${PIPESTATUS[@]}数组，但不会持久存储到父shell中。这是将其推送到输出变量$a然后将其检索到名为${PIPESTATUS2[@]}的新数组中的技巧：

## PIPESTATUS[0] works to give me the exit status of 'false':
$ false | true
$ echo $? ${PIPESTATUS[0]} ${PIPESTATUS[1]}
0 1 0

## Populate a $PIPESTATUS2 array:
$ a=`false | true; printf :%s "${PIPESTATUS[*]}"`
$ ANS=$?; PIPESTATUS2=(${a##*:})
$ [ -n "${a%:*}" ] && a="${a%:*}" && a="${a%$'\n'}" || a=""
$ echo $ANS ${PIPESTATUS2[0]} ${PIPESTATUS2[1]};
0 1 0

这会将子shell的${PIPESTATUS[@]}数组保存为$a末尾的以空格分隔的值列表，然后使用shell变量substring removal将其提取出来（请参阅我给this similar question的更长的例子和描述。只有在您确实要保存$a的值而没有额外状态时才需要第三行（就像在此示例中它以false | true运行一样）。

bash pipestatus在backticked命令？

3 个答案: