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Subshell

Subshell

Subshell 嘅定義喺一啲書本或者資料中模糊唔清，畀出嘅解釋往往同某啲例子自相矛盾，令人捉摸唔透。 所以，為咗避免呢種語義同邏輯上嘅問題，呢度唔會畀出佢嘅定義（建議參考官方 BASH 手冊）， 亦唔會用 “子 Shell” 呢個翻譯，只係從佢嘅實際表現去理解佢嘅定義。 下面列舉嘅變量同 Subshell 概念有密切嘅關係。

Bash Reference Manual

Bash 參考手冊

BASH_SUBSHELL

Shell 變量，非環境變量，官方手冊解釋。

Incremented by one within each subshell or subshell environment when the shell begins executing in that environment. The initial value is 0. If BASH_SUBSHELL is unset, it loses its special properties, even if it is subsequently reset.

亦可以用 man 命令，內容可能會有啲唔同。

man bash | grep -A 3 'BASH_SUBSHELL' | head -n 4
echo $BASH_SUBSHELL
0

SHLVL

環境變量，官方手冊解釋。

Incremented by one each time a new instance of Bash is started. This is intended to be a count of how deeply your Bash shells are nested.

用 man 命令。

man bash | grep 'SHLVL'

呢個值係從 1 開始嘅。

echo $SHLVL
1

命令分組

全稱 Command Grouping，Bash 提供兩種方法創建命令分組。

括號：()

呢個方法會創建一個 Subshell 環境去處理命令分組。

(pwd; echo $BASH_SUBSHELL)
/home/kuga
1
(pwd; (echo $BASH_SUBSHELL))
/home/kuga
2 
(pwd; (echo $SHLVL))
1

可以得出以下結論。

  • BASH_SUBSHELL：每創建一個 Subshell 就加 1。
  • SHLVL：無論創建幾多個 Subshell，都唔變。

花括號：{}

呢個方法唔會創建 Subshell，命令分組係喺當前 Shell 嘅上下文中處理嘅。 喺語法上，花括號同命令之間嘅空格唔可以省略，每個命令結尾嘅分號亦係必須嘅。

{ pwd; { echo $BASH_SUBSHELL; } }
/home/kuga
0
{ pwd; { echo $SHLVL; } }
/home/kuga
1

Shell PID

可以通過 BASHPID 或者 $$ 睇 Shell 嘅 PID，但佢哋係有區別嘅。

BASHPID

Shell 變量，非環境變量，官方解釋。

Expands to the process ID of the current Bash process. This differs from $$ under certain circumstances, such as subshells that do not require Bash to be re-initialized. Assignments to BASHPID have no effect. If BASHPID is unset, it loses its special properties, even if it is subsequently reset.
echo $BASHPID
56414

用 () 睇 BASHPID。

(ps -f --forest; echo $BASHPID)
UID          PID    PPID  C STIME TTY          TIME CMD
kuga       56414   56412  0 10:11 pts/0    00:00:00 -bash
kuga       57325   56414  0 15:13 pts/0    00:00:00  \_ -bash
kuga       57326   57325  0 15:13 pts/0    00:00:00      \_ ps -f --forest
57325

可以睇到，BASHPID 輸出咗 Subshell 嘅 PID。

特殊參數 $$

官方解釋。

($$) Expands to the process ID of the shell. In a subshell, it expands to the process ID of the invoking shell, not the subshell.

喺 Subshell 中，$$ 表示嘅係 invoking shell 嘅 PID。

(pwd; (ps -f --forest; echo $$))
/home/kuga
UID          PID    PPID  C STIME TTY          TIME CMD
kuga       56414   56412  0 10:11 pts/0    00:00:00 -bash
kuga       57347   56414  0 15:20 pts/0    00:00:00  \_ -bash
kuga       57348   57347  0 15:20 pts/0    00:00:00      \_ -bash
kuga       57349   57348  0 15:20 pts/0    00:00:00          \_ ps -f --forest
56414

可以睇到，無論有幾多個 Subshells，$$ 始終表示頂層 Bash 嘅 PID。

創建 Bash 實例

喺 Bash 中輸入 bash 就可以創建一個全新嘅 Bash 實例。

bash
ps -f --forest
UID          PID    PPID  C STIME TTY          TIME CMD
kuga       56414   56412  0 10:11 pts/0     00:00:00 -bash
kuga       57359   56414  0 15:29 pts/0     00:00:00  \_ bash
kuga       57402   57359  0 15:30 pts/0     00:00:00      \_ ps -f --forest

呢個時候再觀察一下上面提到嘅變量。

echo $BASH_SUBSHELL $SHLVL $BASHPID $$
0 2 57359 57359
  • BASH_SUBSHELL：冇變。
  • SHLVL：從 1 -> 2。
  • BASHPID：新 Bash 實例嘅 PID。
  • $$：新 Bash 實例嘅 PID。

如果話呢種創建 Bash 嘅方式都係 Subshell 嘅話，語義同表現上就會自相矛盾。

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