bash "< <(command args)" 在 shell 中是什么意思？
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What does "< <(command args)" mean in the shell?
提问by stib
When looping recursively through folders with files containing spaces the shell script I use is of this form, copied from the internet:
当递归循环包含包含空格的文件的文件夹时，我使用的 shell 脚本采用这种形式，从Internet复制：
while IFS= read -r -d $'find /bar -name *foo* -print0 | while read line; do
...
done
' file; do
dosomethingwith "$file" # do something with each file
done < <(find /bar -name *foo* -print0)
I think I understand the IFS bit, but I don't understand what the '< <(...)' characters mean. Obviously there's some sort of piping going on here.
我想我了解 IFS 位，但我不明白 ' < <(...)' 字符的含义。显然这里有某种管道在进行。
It's very hard to Google "< <", you see.
谷歌“< <”很难，你看。
回答by Josh Lee
<()is called process substitutionin the manual, and is similar to a pipe but passes an argument of the form /dev/fd/63instead of using stdin.
<()在手册中称为进程替换，类似于管道，但传递形式的参数/dev/fd/63而不是使用标准输入。
<reads the input from a file named on command line.
<从命令行命名的文件中读取输入。
Together, these two operators function exactly like a pipe, so it could be rewritten as
一起，这两个运算符的功能完全像一个管道，所以它可以重写为
ls /usr/bin | more
回答by Max E.
<( command ) is process substitution. Basically, it creates a special type of file called a "named pipe," then redirects the output of the command to be the named pipe. So for example, suppose you want to page through a list of files in an extra-big directory. You could do this:
<( command ) 是进程替换。基本上，它创建一种称为“命名管道”的特殊类型的文件，然后将命令的输出重定向到命名管道。例如，假设您要翻阅一个超大目录中的文件列表。你可以这样做：
more <( ls /usr/bin )
Or this:
或这个：
more $( ls /usr/bin )
But NOT this:
但不是这个：
~$ echo $( ls /tmp )
gedit.maxtothemax.436748151 keyring-e0fuHW mintUpdate orbit-gdm orbit-maxtothemax plugtmp pulse-DE9F3Ei96ibD pulse-PKdhtXMmr18n ssh-wKHyBU1713 virtual-maxtothemax.yeF3Jo
~$ echo <( ls /tmp )
/dev/fd/63
~$ cat <( ls /tmp )
gedit.maxtothemax.436748151
keyring-e0fuHW
mintUpdate
orbit-gdm
orbit-maxtothemax
plugtmp
pulse-DE9F3Ei96ibD
pulse-PKdhtXMmr18n
ssh-wKHyBU1713
virtual-maxtothemax.yeF3Jo
The reason for this becomes clear when you investigate further:
当您进一步调查时，原因就很清楚了：
find /bar -name *foo* -print0 | \
while IFS= read -r -d $'##代码##' file; do
dosomethingwith "$file" # do something with each file
done
/dev/fd/whatever acts like a text file with the output of the command between the parenthesis.
/dev/fd/whatever 就像一个文本文件，括号之间是命令的输出。
回答by knittl
<redirects to stdin.
<重定向到标准输入。
<()seems to be some sort of a reverse pipe, as mentioned on the page:
<()似乎是某种反向管道，如页面上所述：
will not work, because the while loop will be executed in a subshell, and you'll lose changes made in the loop
将不起作用，因为 while 循环将在子 shell 中执行，并且您将丢失循环中所做的更改
