问题

当我们在linux shell（如bash)中使用管道执行多个命令时，$?只能获取到最后一条命令的返回码，那么应该怎么获取管道前面的命令的返回码呢，请看下文：

当执行一条管道命令时，linux会使用PIPESTATUS这个数组变量来保存每个管道的返回码。PIPESTATUS[0]表示管道中第一个命令的返回码，PIPESTATUS[1]表示第二个命令的返回值，这是一个共享的变量，为避免别的命令改变它的内容，可以将它复制到一个新的数组。

#!/bin/bash
echo "tftf"
true | false | true | false
RC=( "${PIPESTATUS[@]}" )
echo "RC[0] = ${RC[0]}"        # true = 0
echo "RC[1] = ${RC[1]}"        # false = 1
echo "RC[2] = ${RC[2]}"        # true = 0
echo "RC[3] = ${RC[3]}"        # false = 1

echo "ftft"
false | true | false | true
RC=( "${PIPESTATUS[@]}" )
echo "RC[0] = ${RC[0]}"        # false = 1
echo "RC[1] = ${RC[1]}"        # true = 0
echo "RC[2] = ${RC[2]}"        # false = 1
echo "RC[3] = ${RC[3]}"        # true = 0

echo "fftt"
false | false | true | true
RC=( "${PIPESTATUS[@]}" )
echo "RC[0] = ${RC[0]}"        # false = 1
echo "RC[1] = ${RC[1]}"        # false = 1
echo "RC[2] = ${RC[2]}"        # true = 0
echo "RC[3] = ${RC[3]}"        # true = 0

执行上面的bash文件，输出如下所示：

jagitch@jagitch-MS-7B93:~$ source pipeline.sh
tftf
RC[0] = 0
RC[1] = 1
RC[2] = 0
RC[3] = 1
ftft
RC[0] = 1
RC[1] = 0
RC[2] = 1
RC[3] = 0
fftt
RC[0] = 1
RC[1] = 1
RC[2] = 0
RC[3] = 0

一个具体的实例

#!/bin/bash
WEB_SERVER=steve-parker.org
curl -# -f -u ${USERNAME}:${PASSWORD} http://${WEB_SERVER}/ | grep "SomeMessage"
RC=( "${PIPESTATUS[@]}" )
if [ "${RC[0]}" -eq "22" ]; then
  # curl returned 22, indicating some error above 400,
  # such as: 404 Not Found, 401 Unauthorized, etc.
  echo "Invalid credentials"
  exit 1
fi

# PIPESTATUS has gone, but we can still inspect RC
if [ "${RC[1]}" -eq "0" ]; then
  echo "Grep succeeded"
  echo "Web server reported SomeMessage"
else
  echo "Web server didn't report SomeMessage"
fi

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