variable=$(...) 是否将命令或命令的结果存储在 POSIX sh 中 [英] Does variable=$(...) store the command or the result of the command in POSIX sh

问题描述

我想知道:当我例如打电话

I was wondering: When I e.g. call

list=$(ps -ax)

是 ps -ax 执行一次，我可以多次读取该值还是每次调用 $list 时执行的命令

is ps -ax executed once and I can read the value multiple times or is the command executed every time I call $list

推荐答案

POSIX sh 保证命令不会被重新执行，见下文.

POSIX sh guarantees that the command will not be re-executed, see below.

在通过经验测试得出有关 POSIX sh 的结论时要小心.例如，如果您计时 sleep 60 |真的，你会发现 bash、ksh、zsh 和 dash 都需要 60 秒.但是，POSIX sh 仍然允许它立即完成.

Be careful about concluding things about POSIX sh through empirical tests. For example, if you time sleep 60 | true, you'd find that bash, ksh, zsh and dash all take 60 seconds. However, POSIX sh still allows it to finish immediately.

$() 的规范:

shell 应通过在子 shell 环境中执行命令来扩展命令替换(请参阅 Shell 执行环境)并将命令替换(命令文本加上封闭的$()"或反引号)替换为标准输出的命令.

The shell shall expand the command substitution by executing command in a subshell environment (see Shell Execution Environment) and replacing the command substitution (the text of command plus the enclosing "$()" or backquotes) with the standard output of the command.

和应用时:

当需要执行给定的简单命令时[...]，以下扩展、赋值和重定向都应从命令文本的开头到结尾执行:

When a given simple command is required to be executed [...] the following expansions, assignments, and redirections shall all be performed from the beginning of the command text to the end:

[...]

1. 在赋值之前，每个变量赋值都应针对波浪号扩展、参数扩展、命令替换、算术扩展和引号删除进行扩展.

[...]

变量赋值应如下执行:

如果没有命令名结果，变量赋值将影响当前的执行环境.

If no command name results, variable assignments shall affect the current execution environment.

由于 $() 被替换为命令的文本，并且这发生在变量被赋值之前，所以保证不会在符合 POSIX sh 的实现中重新执行该命令.

Since $() is replaced with the text of the command, and this happens before the variable is assigned, the command is guaranteed not to be re-executed in a conforming POSIX sh implementation.

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