How to understand the process state variables of special variables in Shell programming

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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