linux变量$ BASH_SUBSHELL与$ SHLVL之间的区别 [英] Difference between linux variables $BASH_SUBSHELL vs $SHLVL

问题描述

我对两者感到困惑.

虽然$ BASH_SUBSHELL内部变量指示子外壳的嵌套级别，但是$ SHLVL变量显示子外壳内没有任何变化.

While the $BASH_SUBSHELL internal variable indicates the nesting level of a subshell, the $SHLVL variable shows no change within a subshell.

这到底是什么意思?如果我在另一个外壳中打开一个外壳，则$ SHLVL的值会增加.那不是子壳吗?

What does it exactly mean? If i open a shell within another shell, the value of $SHLVL increments. Isn't that a subshell?

推荐答案

否，在这种情况下，手动运行新外壳程序(通过/bin/sh或/bin/bash等)不是 子外壳程序

No, manually running a new shell (via /bin/sh or /bin/bash etc.) is not a subshell in this context.

子外壳程序是指外壳程序自己生成新的外壳程序实例 来处理某些工作的情况.

A subshell is when the shell spawns a new shell instance on its own to handle some work.

使用命令替代(即$(command) )是一个子shell(较早的backticks调用也是如此).

Using Command Substitution (i.e. $(command)) is a subshell (as is the older backticks invocation).

使用管道(即echo '5.1+5.3' | bc -l)创建管道每个组件的子壳.

Using a pipeline (i.e. echo '5.1+5.3' | bc -l) creates subshells for each component of the pipeline.

使用流程替代(即<(command) )创建一个子外壳.

Using Process Substitution (i.e. <(command)) creates a subshell.

分组命令(即(declare a=5; echo $a))创建一个子外壳.

Grouping commands (i.e. (declare a=5; echo $a)) creates a subshell.

在背景(即sleep 1 & )创建一个子外壳.

Running commands in the background (i.e. sleep 1 &) creates a subshell.

也许还有其他事情，但这是常见的情况.

There may be other things as well but those are the common cases.

测试起来很简单:

$ printf "Outside: $BASH_SUBSHELL , $SHLVL\nInside: $(echo $BASH_SUBSHELL , $SHLVL)\n"
Outside: 0 , 1
Inside: 1 , 1
$ (printf "Outside: $BASH_SUBSHELL , $SHLVL\nInside: $(echo $BASH_SUBSHELL , $SHLVL)\n")
Outside: 1 , 1
Inside: 2 , 1
$ bash -c 'printf "Outside: $BASH_SUBSHELL , $SHLVL\nInside: $(echo $BASH_SUBSHELL , $SHLVL)\n"'
Outside: 0 , 2
Inside: 1 , 2
$ bash -c '(printf "Outside: $BASH_SUBSHELL , $SHLVL\nInside: $(echo $BASH_SUBSHELL , $SHLVL)\n")'
Outside: 1 , 2
Inside: 2 , 2

您的报价来源(通常较差，通常最好避免使用， ABS )甚至对此进行了一点演示(并且以一种非常不清楚的方式，只是该高级"指南中普遍缺乏严格性和质量的另一例):

The source of your quote (the generally poor, and often better avoided, ABS) even demonstrates this a little bit (and in a rather unclear manner, just another instance of the general lack of rigor and quality in that "Advanced" guide):

echo " \$BASH_SUBSHELL outside subshell       = $BASH_SUBSHELL"           # 0
  ( echo " \$BASH_SUBSHELL inside subshell        = $BASH_SUBSHELL" )     # 1
  ( ( echo " \$BASH_SUBSHELL inside nested subshell = $BASH_SUBSHELL" ) ) # 2
# ^ ^                           *** nested ***                        ^ ^

echo

echo " \$SHLVL outside subshell = $SHLVL"       # 3
( echo " \$SHLVL inside subshell  = $SHLVL" )   # 3 (No change!)

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