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This article introduces the knowledge of "how to understand the process state variables of special variables in Shell programming". In the operation of actual cases, many people will encounter such a dilemma, so let the editor lead you to learn how to deal with these situations. I hope you can read it carefully and be able to achieve something!

Process state variable

1. $$gets the process number (PID) of the current shell

2. $! The PID that executed the previous instruction

3. $? Gets the return value of the execution of the last command (0 for success, non-zero for failure, which is common)

4. $_ the last argument to a command or script executed before that

The code is as follows:

Cat > test$.sh

Echo'$$='$$

Echoing sounds and sounds!

Echo'$?='$?

Echo'$@='$@

Echo'$_='$_

# the output is as follows

Sh test\ $.sh 1 2 3

$= 2556

$! =

$? = 0

$@ = 1 2 3

The difference between $* and $@

$* treat all parameters as a single string, which is equivalent to "$1 $2 $3"

$@ treats each parameter as a single string, preserving any white space characters on the command line

The code is as follows:

Set--'I am'jane lee

Fori in$*; doecho$i; done

I

Am

Jane

Lee

Fori in$@; doecho$i; done

I

Am

Jane

Lee

Fori in "$@"; doecho$i; done

I am

Jane

Lee

Fori in "$*"; doecho$i; done

I am jane lee

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