$?

範例

執行

$ true

接著執行

$ echo $?

顯示

0

執行

$ false

接著執行

$ echo $?

顯示

1

文件說明

$ man bash

可以找到下面這一段說明

Special Parameters
    The shell treats several parameters specially.  These parameters may only be referenced; assignment to them  is
    not allowed.
...略...
    ?      Expands to the exit status of the most recently executed foreground pipeline.

...略...

或是在

Bash Reference Manual / 3 Basic Shell Features / 3.4 Shell Parameters / 3.4.2 Special Parameters

可以找到下面這一段說明

?

    ($?) Expands to the exit status of the most recently executed foreground pipeline.

更多範例和說明請參考

• help :
• help true
• help false
• help echo