击：头和放大器;与bash脚本尾部行为 [英] Bash: Head & Tail behavior with bash script

问题描述

假设我有以下脚本： -

Suppose I have following script:-

test.sh

#!/bin/bash
command1  #prints 5 lines
command2  #prints 3 lines

我运行 test.sh脚本|头-n5

将在此情况下，会发生什么？它运行两个命令？还是会命令1后停下来？
如果我把它叫做 -n1

What will happen in this case? Will it run both the commands? or will it stop after command1? What if I call it with -n1?

背景：我可能会问一个很基本的问题，但实际上我注意到一些有趣的事情。我的脚本（不同的）正在处理7000文件，每个文件产生1路输出。它需要7分钟完全但这样做运行脚本的头-n1 给我的提示立刻像脚本只处理第一个文件后终止

Background: I might be asking a very basic question, but I actually noticed something interesting. My script(different one) was processing 7,000 files and each file produces 1 line of output. It takes 7 minutes to run the script completely but doing head -n1 gave me prompt immediately like the script has terminated after processing first file only

编辑：
    下面是我的脚本

Following is my script

for i in $(ls filepath);do
     echo "$i" # issue here
    python mySript "$i" > "/home/user/output/""$i"".out"
  fi
done

删除回声上面使脚本是头运行完整7分钟-n1 ，但与它呼应只是打印第一行然后退出。

Removing echo above enables the script to run full 7 minute with head -n1, but with echo it just prints first line then exit.

推荐答案

这是一个相当有趣的问题！感谢张贴了！

This is a fairly interesting issue! Thanks for posting it!

我认为出现这种情况的头处理第几行，所以 SIGPIPE 信号后退出被送到的bash 运行脚本时它会尝试回声$ X 下一次。我用RedX的脚本来证明这个理论：

I assumed that this happens as head exits after processing the first few lines, so SIGPIPE signal is sent to the bash running the script when it tries to echo $x next time. I used RedX's script to prove this theory:

#!/usr/bin/bash
rm x.log
for((x=0;x<5;++x)); do
    echo $x
    echo $x>>x.log
done

这工作，因为你描述的！使用 t.sh |头-n 2 写入只有2线将屏幕和x.log。但是诱捕SIGPIPE这种行为改变...

This works, as You described! Using t.sh|head -n 2 it writes only 2 lines to the screen and to x.log. But trapping SIGPIPE this behavior changes...

#!/usr/bin/bash
trap "echo SIGPIPE>&2" PIPE
rm x.log
for((x=0;x<5;++x)); do
    echo $x
    echo $x>>x.log
done

输出：

$ ./t.sh |head -n 2
0
1
./t.sh: line 5: echo: write error: Broken pipe
SIGPIPE
./t.sh: line 5: echo: write error: Broken pipe
SIGPIPE
./t.sh: line 5: echo: write error: Broken pipe
SIGPIPE

作为标准输出发生的写入错误的管的另一端是封闭已经关闭。而任何试图写入封闭管道引起SIGPIPE信号，默认终止程序（请参阅人7信号）。该x.log现在包含5行。

The write error occurs as stdout is already closed as the other end of the pipe is closed. And any attempt to write to the closed pipe causes a SIGPIPE signal, which terminates the program by default (see man 7 signal). The x.log now contains 5 lines.

这也解释了为什么 /斌/回声解决了这个问题。见下面的脚本：

This also explains why /bin/echo solved the problem. See the following script:

rm x.log
for((x=0;x<5;++x)); do
    /bin/echo $x
    echo "Ret: $?">&2
    echo $x>>x.log
done

输出：

$ ./t.sh |head -n 2
0
Ret: 0
1
Ret: 0
Ret: 141
Ret: 141
Ret: 141

十进制141 =六角8D。十六进制80表示被接收的信号，六角0D为SIGPIPE。因此，当 /斌/回声试图写入stdout它得到一个SIGPIPE并终止（默认行为），而不是的庆典运行脚本。

Decimal 141 = hex 8D. Hex 80 means a signal was received, hex 0D is for SIGPIPE. So when /bin/echo tried to write to stdout it got a SIGPIPE and it was terminated (as default behavior) instead of the bash running the script.

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