"$(cat file)"，"$(< file)"和"$(cat file)"之间的区别是什么?和“阅读...<文件"对于只有一行的文件?

问题描述

我有一个仅包含一行的输入文件:

I have an input file that contains only one line:

$ cat input
foo bar

我想在我的脚本中使用这一行，我知道有3种方法来获取它:

I want to use this line in my script and there are 3 ways to get it that I know of:

line=$(cat input)

line=$(<input)

IFS= read -r line < input

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例如，使用命令替换意味着我产生了一个子shell，而对于read我却没有，对吗?还有什么其他区别，有没有一种方法比其他方法更可取?我还注意到(使用strace)由于某些原因，只有read会触发系统调用openat.其他人怎么可能不这样做?

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For example, using command substitution means I spawn a subshell, whereas with read I do not, correct? What other differences are there and is one way preferred over the others? I also noticed (with strace) that only read triggers the syscall openat for some reason. How is it possible that the others don't?

$ strace ./script |& grep input
read(3, "#!/usr/bin/env bash\n\ncat > input"..., 80) = 80
read(255, "#!/usr/bin/env bash\n\ncat > input"..., 167) = 167
read(255, "\nline=$(cat input)\nline=$(<input"..., 167) = 60
read(255, "line=$(<input)\nIFS= read -r line"..., 167) = 41
read(255, "IFS= read -r line < input\n", 167) = 26
openat(AT_FDCWD, "input", O_RDONLY)     = 3

推荐答案

• line=$(cat input)是读取整个文件的POSIX方法.它需要一个叉子.

  • line=$(cat input) is the POSIX way of reading the entire file. It requires a fork.

    line=$(< input)是读取整个文件的效率稍高的Bashism.它也可以分叉，但不必执行.

    line=$(< input) is a marginally more efficient Bashism for reading the entire file. It also forks, but doesn't have to execve.

    未提及，但mapfile/readarray是将行中的整个文件逐行读取到数组中的高效Bashisms.没有叉子.

    Not mentioned but mapfile/readarray are significantly more efficient Bashisms for reading the entire file line-by-line into arrays. No forks.

    IFS= read -r line < input是POSIX方式，无需子shell即可读取单行.没有叉子.

    IFS= read -r line < input is the POSIX way of reading a single line without a subshell. No forks.

    之所以只看到后者打开文件，是因为其他人在子shell中进行了操作，而您未指定-f来跟踪子进程.

    The reason why you only see the latter opening the file is simply that the others do it in a subshell, and you haven't specified -f to trace child processes.

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