The difference between the &&export statement function &&su and Su-the variable problem of the Linux shell

Let's start by looking at the variables.

[Email protected] ~]# dhh=1

[Email protected] ~]# echo $DHH
1

Open a child shell test

[Email protected] ~]# bash

[Email protected] ~]# echo $DHH

No value

Using the Export method

[[Email protected] ~]# exit----------Exit child shell
Exit
[Email protected] ~]# export DHH
[[email protected] ~]# bash-----------new child shell
[Email protected] ~]# echo $DHH
1

So we can say: Export can pass the variables defined by this shell to the child shell

But can the variables of the child shell be passed to the parent shell through the Exprot variable value??

[Email protected] ~]# bash
[Email protected] ~]# dkk=1
[Email protected] ~]# export DKK
[[Email protected] ~]# exit
Exit
[Email protected] ~]# echo $DKK

No value

The answer is: no ...

To achieve the words: There are several ways to come from http://blog.csdn.net/dreamcoding/article/details/8519689

So what can a child shell do to pass its own variables to the parent shell? The following methods can be considered:

1. Through an intermediate file:

   #!/bin/bash (subvar= "Hello Shell" echo "$subvar" > Temp.txt) Read Pvar < Temp.txtecho $pvar Run output: $ sh Subandp.shhello Shell

2. Replace with command:

   #!/bin/bashpvar= ' subvar= "Hello Shell"; Echo $subvar ' echo $pvar

Run Output:::

$./subandp.shhello Shell

The command that executes the command substitution (two anti-single quotes) is also done in the child shell.

3. Using Named pipes:

   #!/bin/bashmkfifo-m 777 Npipe (  subsend= "Hello World"  echo "$subsend" > Npipe &) Read Pread < Npipeecho "$pread" Exit 0

Run output:

[Email protected]:~/shell$./var.shhello World

About the famous pipe creation command Mkfifo can be consulted: http://www.groad.net/bbs/read.php?tid-3707.html

4. Use here Documentation:

   #!/bin/bashread pvar << here ' subvar= "Hello Shell" echo $subvar ' Hereecho $pvar

Run output:

$./subandp.shhello Shell

There should be a lot of methods, the essential principle of these methods is based on interprocess communication.

take a look at the difference between Su and Su-switching users

Su method

[Email protected] ~]# Useradd test

[Email protected] ~]# test1=1

[email protected] ~]# su test
[Email protected] root]$ echo $test 1

No value

However, it is possible to transfer variables using export

[Email protected] ~]# export test=1

[email protected] ~]# su test

[Email protected] root]$ echo $test
1

[Email protected] root]$ echo $PATH
/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin -----Environment variables are the same as root environment variables

Take a look at the Su-method

[Email protected] ~]# su-test
[Email protected] ~]$ echo $test

No value
[Email protected] ~]$ echo $test 1
No value

[Email protected] ~]$ echo $PATH

/usr/local/bin:/bin:/usr/bin:/usr/local/sbin:/usr/sbin:/sbin:/home/test/bin------------ environment variable differs from root environment variable

Comprehensive complaint: