sh 和 Bash 的区别 [英] Difference between sh and Bash

问题描述

在编写shell程序时，我们经常使用/bin/sh和/bin/bash.我通常使用bash，但我不知道它们之间有什么区别.

Bash 和 sh 的主要区别是什么?

使用 Bash 和 sh 编程时需要注意什么?

解决方案

sh 是什么?

sh(或 Shell 命令语言)是一种由 POSIX 标准.它有许多实现(ksh88、Dash，……).Bash 也可以被认为是 sh 的实现(见下文).

因为 sh 是规范，而不是实现，/bin/sh 是大多数 POSIX 系统上实际实现的符号链接(或硬链接).

什么是 Bash?

Bash 开始时是一个与 sh 兼容的实现(尽管它比 POSIX 标准早了几年)，但随着时间的推移，它获得了许多扩展.许多这些扩展可能会改变有效 POSIX shell 脚本的行为，因此 Bash 本身不是有效的 POSIX shell.相反，它是 POSIX shell 语言的一种方言.

Bash 支持 --posix 开关，这使得它更符合 POSIX 标准.如果作为 sh 调用，它还会尝试模仿 POSIX.

sh = bash?

很长一段时间以来，/bin/sh 在大多数 GNU/Linux 系统上都用于指向 /bin/bash.结果，忽略两者之间的差异几乎变得安全.但最近这种情况开始发生变化.

/bin/sh 不指向 /bin/bash 的系统的一些流行示例(以及其中一些 /bin/bash代码>甚至可能不存在)是:

1. 现代 Debian 和 Ubuntu 系统，默认将 sh 符号链接到 dash；
2. Busybox，它通常在 Linux 系统启动时作为 的一部分运行initramfs.它使用 ash shell 实现.
3. BSD 系统，以及通常的任何非 Linux 系统.OpenBSD 使用 pdksh，KornShell.FreeBSD 的 sh 是原始 Unix Bourne shell 的后代.Solaris 有自己的 sh不符合 POSIX；可以从 Heirloom 项目 获得免费实现.

您如何找出/bin/sh 在您的系统上指向的内容?

复杂的是 /bin/sh 可以是符号链接或硬链接.如果是符号链接，便携的解决方法是:

% file -h/bin/sh/bin/sh: bash 的符号链接

如果是硬链接，试试

% find -L/bin -samefile/bin/sh/bin/sh/bin/bash

事实上，-L 标志涵盖了符号链接和硬链接，但是这种方法的缺点是不便携——POSIX 不需要 find 来支持-samefile 选项，虽然都是 GNU find 和 FreeBSD find 支持它.>

社帮线

最终，由您决定使用哪个，将 «shebang» 行写为脚本的第一行.

例如

#!/bin/sh

将使用 sh(以及任何指向的内容)，

#!/bin/bash

将使用 /bin/bash 如果它可用(如果它不可用则失败并显示错误消息).当然，你也可以指定另一种实现，例如

#!/bin/dash

使用哪个

对于我自己的脚本，我更喜欢 sh，原因如下:

• 标准化
• 它更简单，更容易学习
• 它可以跨 POSIX 系统移植——即使他们碰巧没有 bash，他们也需要有 sh

使用 bash 也有好处.它的特性使编程更加方便，类似于其他现代编程语言的编程.这些包括诸如作用域局部变量和数组之类的东西.简单的 sh 是一种非常简约的编程语言.

When writing shell programs, we often use /bin/sh and /bin/bash. I usually use bash, but I don't know what's the difference between them.

What's main difference between Bash and sh?

What do we need to be aware of when programming in Bash and sh?

解决方案

What is sh?

sh (or the Shell Command Language) is a programming language described by the POSIX standard. It has many implementations (ksh88, Dash, ...). Bash can also be considered an implementation of sh (see below).

Because sh is a specification, not an implementation, /bin/sh is a symlink (or a hard link) to an actual implementation on most POSIX systems.

What is Bash?

Bash started as an sh-compatible implementation (although it predates the POSIX standard by a few years), but as time passed it has acquired many extensions. Many of these extensions may change the behavior of valid POSIX shell scripts, so by itself Bash is not a valid POSIX shell. Rather, it is a dialect of the POSIX shell language.

Bash supports a --posix switch, which makes it more POSIX-compliant. It also tries to mimic POSIX if invoked as sh.

sh = bash?

For a long time, /bin/sh used to point to /bin/bash on most GNU/Linux systems. As a result, it had almost become safe to ignore the difference between the two. But that started to change recently.

Some popular examples of systems where /bin/sh does not point to /bin/bash (and on some of which /bin/bash may not even exist) are:

1. Modern Debian and Ubuntu systems, which symlink sh to dash by default;
2. Busybox, which is usually run during the Linux system boot time as part of initramfs. It uses the ash shell implementation.
3. BSD systems, and in general any non-Linux systems. OpenBSD uses pdksh, a descendant of the KornShell. FreeBSD's sh is a descendant of the original Unix Bourne shell. Solaris has its own sh which for a long time was not POSIX-compliant; a free implementation is available from the Heirloom project.

How can you find out what /bin/sh points to on your system?

The complication is that /bin/sh could be a symbolic link or a hard link. If it's a symbolic link, a portable way to resolve it is:

% file -h /bin/sh
/bin/sh: symbolic link to bash

If it's a hard link, try

% find -L /bin -samefile /bin/sh
/bin/sh
/bin/bash

In fact, the -L flag covers both symlinks and hardlinks, but the disadvantage of this method is that it is not portable — POSIX does not require find to support the -samefile option, although both GNU find and FreeBSD find support it.

Shebang line

Ultimately, it's up to you to decide which one to use, by writing the «shebang» line as the very first line of the script.

E.g.

#!/bin/sh

will use sh (and whatever that happens to point to),

#!/bin/bash

will use /bin/bash if it's available (and fail with an error message if it's not). Of course, you can also specify another implementation, e.g.

#!/bin/dash

Which one to use

For my own scripts, I prefer sh for the following reasons:

• it is standardized
• it is much simpler and easier to learn
• it is portable across POSIX systems — even if they happen not to have bash, they are required to have sh

There are advantages to using bash as well. Its features make programming more convenient and similar to programming in other modern programming languages. These include things like scoped local variables and arrays. Plain sh is a very minimalistic programming language.

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