如何使用变量来指示 bash 中的文件描述符？
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原文地址: http://stackoverflow.com/questions/8295908/
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How to use a variable to indicate a file descriptor in bash?
提问by WH's HeV
I want to use a bash variable to indicate a file descriptor, like this:
我想使用 bash 变量来指示文件描述符，如下所示：
id=6
file=a
exec $id<>$file
But the usage is wrong:
但是用法是错误的：
-bash: exec: 6: not found
So, how to use a variable to indicate a file descriptor in exec command?
那么，如何在 exec 命令中使用变量来表示文件描述符呢？
采纳答案by eduffy
You have to use evaland put the entire expression in quotes.
您必须使用eval整个表达式并将其放在引号中。
eval "exec $id<>$file"
And do that every time you want to use $id.
每次你想使用$id.
回答by Paul Tobias
The accepted answer is correct, but as of bash 4.1, you can use automatic file descriptor allocation, and in that case you don't need eval:
接受的答案是正确的，但从bash 4.1 开始，您可以使用自动文件描述符分配，在这种情况下，您不需要eval：
file=a
exec {id}<>"$file"
Then you can use it like this:
然后你可以像这样使用它：
echo test >&${id}
or:
或者：
fsck -v -f -C ${id} /dev/something
回答by jan
I found the discussion in the answer of tobias.pal very interesting: https://stackoverflow.com/a/32689974/1184842
我发现tobias.pal的回答中的讨论很有趣：https://stackoverflow.com/a/32689974/1184842
for (( FD=3 ; FD < 100 ; FD++ )) ; do exec {FD}> file.$FD ; echo $FD >&${FD}; done
This would not work, since exec {FD}> file.${FD}would be the same descriptor over all values of $FD, right? (Denio Mariz)
这是行不通的，因为exec {FD}> file.${FD}$FD 的所有值都是相同的描述符，对吗？（德尼奥·马里兹）
I solved this by using an array as stated by Drew Chapin:
我通过使用 Drew Chapin 所述的数组解决了这个问题：
#!/bin/bash
# global variables for temp file handling
declare -a TEMPORARY_FILES_WRITE;
declare -a TEMPORARY_FILES_READ;
function createTempFile() {
local usecase=""
local id=""
local tmpfile=$(mktemp) # Create a temporal file in the default temporal folder of the system
# Lets do some magic for the tmpfile to be removed when this script ends, even if it crashes
exec {TEMPORARY_FILES_WRITE[$id]}>"$tmpfile"
exec {TEMPORARY_FILES_READ[$id]}<"$tmpfile"
rm "$tmpfile" # Delete the file, but file descriptors keep available for this script
}
for (( FD=3 ; FD < 100 ; FD++ )) ; do
TEMP_FILE_COUNTER=$((TEMP_FILE_COUNTER + 1))
createTempFile "Iteration $FD" $FD ;
echo $FD >&${TEMPORARY_FILES_WRITE[$FD] ;
done
example=$(cat <&${TEMPORARY_FILES_READ[50]})
echo $example
This will output 50.
这将输出 50。
