在字符串上运行sed，使用“echo”的好处+“管道” over"<<<<< [英] Running sed on string, benefit of using "echo" + "pipe" over "<<<"

问题描述

通常我看到人们使用sed操纵字符串如下：

Commonly I see people manipulating strings using sed as follows:

echo "./asdf" | sed -n -e "s%./%%p"

我最近了解到我也可以做：

I recently learned I can also do:

sed -n -e "s%./%%p" <<< "./asdf"

有理由避免后者吗？
例如，它是特定于bash的行为吗？

Is there a reason to avoid the latter? For instance, is it bash-specific behaviour?

推荐答案

我应该如何修剪。 / 从路径的开头（或执行其他简单的字符串操作）？

Bash的内置语法称为参数扩展。 $ {s＃。/} 将使用任何前导扩展 $ s ./ 在shell内部修剪，没有子进程或其他开销。 BashFAQ＃100 涵盖了许多其他字符串操作操作。

How should I trim ./ from the beginning of a path (or perform other simple string manipulations)?

Bash's built-in syntax for this is called parameter expansion. ${s#./} will expand $s with any leading ./ trimmed internal to the shell, with no subprocess or other overhead. BashFAQ #100 covers many additional string manipulation operations.

1. 便携性

1. Portability

如您所知，<<< 在POSIX sh中不可用;这是一个ksh扩展，也可以在bash和zsh中使用。

As you've noted, <<< is not available in POSIX sh; this is a ksh extension also available in bash and zsh.

这就是说，如果你需要可移植性，那么多行等价物并不遥远：

That said, if you need portability, the multiline equivalent is not far away:

... <<EOF
$s
EOF

磁盘使用情况

Disk usage

目前由bash实现（以及作为实施细节可能会有变化） ，<<<< 创建一个临时文件，填充，重新定向并重定向。如果您的 TEMPDIR 不在内存文件系统中，这可能会更慢，或者可能会产生客户流失。

As currently implemented by bash (and as an implementation detail subject to change), <<< creates a temporary file, populates, it, and redirects from it. If your TEMPDIR is not on an in-memory filesystem, this may be slower, or may generate churn.

流程开销

管道，如 echo foo | ... ，创建一个子shell - 它会激活一个全新的进程，负责运行 echo 然后退出。当你运行 result = $（echo$ s| ...）时，那个管道本身就在你的父shell的子shell中，并且那个 shell的输出是由父级读取的。

A pipeline, as in echo foo | ..., creates a subshell -- it forks off a completely new process, responsible for running echo and then exiting. When you're running result=$(echo "$s" | ...), then that pipeline is itself in a subshell of your parent shell, and that shell has its output read by the parent.

现代unixlikes付出了巨大努力使 fork（）尽可能地降低子进程的低开销，但即使这样，它也可以在循环中完成的操作中加起来 - 而在Cygwin等平台上它可能更加重要。

Modern unixlikes go to significant effort to make fork()ing off a subprocess low-overhead to the extent possible, but even then it can add up when in an operation done in a loop -- and on platforms such as Cygwin it can be even more significant.

echo 错误

最后但并非最不重要 - <<<$ s将精确地表示变量 s 的任何内容，但它除外可以添加尾随换行符。相比之下， echo 在其指定的行为中有很大的余地：它可以遵循反斜杠扩展或不符合标准的可选XSI扩展的符合性（和存在或缺少 -e 的广泛但完全不合规的扩展，和/或禁用它的运行时标志）;标准不保证能够避免使用 -n 添加尾随换行符;和C。即使您使用的是管道，最好使用 printf ：

Last but not least -- <<<"$s" will represent any contents of the variable s precisely, with the exception that it can add a trailing newline. By contrast, echo has a great deal of leeway in its specified behavior: It can honor backslash expansions or not depending on compliance with the optional XSI extensions to the standard (and presence or lack of the widespread but entirely noncompliant extension of -e, and/or runtime flags that disable it); the ability to avoid addition of trailing newlines with -n is not guaranteed by the standard; &c. Even if you're using a pipeline, it's better to use printf:

# emit *exactly* the contents of "$s", with no newline added
printf '%s' "$s" | ...

# emit the contents of "$s", with an added trailing newline
printf '%s\n' "$s" | ...

# emit the contents of "$s", with '\t', '\n', '\b' &c replaced, and no added newline
printf '%b' "$s" | ...

这篇关于在字符串上运行sed，使用“echo”的好处+“管道” over"<<<<<的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！