Question

跟进

鉴于coproc的明显使用并不像我预期的那样有效，如下所示：

$ cat test.sh
coproc cat auto/etc/build.cfg
while read -u ${COPROC[0]} BRANCH TARGET SVNSRC SVNTAG BUILDTYPE DISTTYPE DISTARGS
do
    echo hello
done

$ bash -x test.sh
+ read -u 63 BRANCH TARGET SVNSRC SVNTAG BUILDTYPE DISTTYPE DISTARGS
+ cat auto/etc/build.cfg
+ echo hello
hello
+ read -u BRANCH TARGET SVNSRC SVNTAG BUILDTYPE DISTTYPE DISTARGS
test.sh: line 2: read: BRANCH: invalid file descriptor specification

问题：为什么在脚本读取一行输出后coproc会消失？

Answer 1

我无法重现：

bash-4.1 $ cat infile 
one
two
three
four
five

bash-4.1 $ cat s.sh 
coproc cat infile
while read -u ${COPROC[0]} v; do
  echo "$v"
done

bash-4.1 $ bash -x s.sh 
+ read -u 63 v
+ cat infile
+ echo one
one
+ read -u 63 v
+ echo two
two
+ read -u 63 v
+ echo three
three
+ read -u 63 v
+ echo four
four
+ read -u 63 v
+ echo five
five
+ read -u 63 v
+ echo ''

+ read -u 63 v

编辑：我确实像这样重现：

bash-4.1 $ cat s.sh 
coproc cat infile

sleep 1

while read -u ${COPROC[0]} v; do
  echo "$v"
done

bash-4.1 $ bash  -x s.sh 
+ sleep 1
+ cat infile
+ read -u v
s.sh: line 5: read: v: invalid file descriptor specification

编辑：请参阅以下评论。

似乎协同进程很快就会超时...... 可能是你的系统很慢:)

不，作为协同进程执行的命令太快了，如果你放慢速度，它会起作用：

bash-4.1 $ cat s.sh 
coproc while read -r; do
  printf '%s\n' "$REPLY"
  sleep 1
done < infile

sleep 1

while read -u ${COPROC[0]} v; do
  echo "$v"
done

bash-4.1 $ bash s.sh 
one
two
three
four
five

无论如何，我认为这个测试用例并不合适。当你需要一个双向管道时，你需要一个协同处理（即你需要与协同进程聊天。您可以使用单个数据库连接（数据库连接资源昂贵）并来回查询您的查询和shell代码。

编辑（见下面的评论）。与 stdin 缓冲相关的问题可以解决使用一些非标准工具（在这种情况下使用 stdbuf （我相信 GNU coreutils 的最新版本的一部分）：

~/t$ cat s
coproc stdbuf -oL -i0 mysql

printf '%s;\n' 'show databases' >&${COPROC[1]}

printf '\n\nshowing databases, fisrt time ...\n\n\n'

while read -t3 -u${COPROC[0]}; do
  printf '%s\n' "$REPLY"
  [[ $REPLY == test ]] && {
    printf '%s\n' 'test found, dropping it ...'
    printf '%s;\n' 'drop database test' >&${COPROC[1]}
    }
done

printf '\n\nshowing databases, second time ...\n\n\n'


printf '%s;\n' 'show databases' >&${COPROC[1]}

while read -t3 -u${COPROC[0]}; do
  printf '%s\n' "$REPLY"
done


printf '%s\n' quit >&${COPROC[1]}

输出：

~/t$ bash s


showing databases, fisrt time ...


Database
information_schema
mysql
sakila
test
test found, dropping it ...
world


showing databases, second time ...


Database
information_schema
mysql
sakila
world

我意识到这种方法有许多缺点......

bash coproc - 出乎意料的行为

1 个答案: