The difference between the anti-quote and $ () in a shell script

The difference between the anti-quote and $ ():

Let's first look at an example:

Create a file named Test.sh, enter the following two instructions in the file

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The results of the operation are as follows:

650) this.width=650; "Src=" Http://s3.51cto.com/wyfs02/M01/82/DC/wKiom1ditFqQf72EAAAMMSzbC7o195.png-wh_500x0-wm_3 -wmp_4-s_3539604763.png "title=" 2.PNG "alt=" Wkiom1ditfqqf72eaaammszbc7o195.png-wh_50 "/>

Why are the two statements running differently?

This is a very interesting question, but to explain the problem is quite difficult, I would like to change a simple example, a popular explanation.

For example, we have a shell script named test1.sh with two lines

650) this.width=650; "Src=" Http://s1.51cto.com/wyfs02/M00/82/DC/wKiom1ditG_yzqAvAAALnqJppRU204.png-wh_500x0-wm_3 -wmp_4-s_3476569554.png "title=" 3.PNG "alt=" Wkiom1ditg_yzqavaaalnqjppru204.png-wh_50 "/>

We can use Sh-x test1.sh to analyze how bash interprets this script, and the information returned is as follows:

650) this.width=650; "Src=" Http://s4.51cto.com/wyfs02/M00/82/DB/wKioL1ditaDyUJqQAAAW1KMq5vg285.png-wh_500x0-wm_3 -wmp_4-s_2155111376.png "title=" 4.PNG "alt=" Wkiol1ditadyujqqaaaw1kmq5vg285.png-wh_50 "/>

We can draw two conclusions:

1. \$ does not convert the special meaning of $ in the inverted quotation mark contains the contents of ECHO \ $hostname is still interpreted as an echo $HOSTNAME takes the value of this variable and outputs so the value returned by the inverse quotation mark is Localdomain

$ () on the contrary, $ is obviously escaped to a normal character, so it does not take the value of the variable, but returns the meaning of the string itself, and returns the $hostname

2. This indicates that the inverted quotation marks have a special conversion to the backslash (\), and at least we see that when we use 1 \ directly, it does not itself escape. Now let's make some changes to the contents of the script:

650) this.width=650; "Src=" Http://s3.51cto.com/wyfs02/M01/82/DC/wKiom1ditJqDv6tYAAALWaveLlA588.png-wh_500x0-wm_3 -wmp_4-s_3819798427.png "title=" 5.PNG "alt=" Wkiom1ditjqdv6tyaaalwavella588.png-wh_50 "/>

We've added 1 more backslashes, this time we'll look at the output:

650) this.width=650; "Src=" Http://s1.51cto.com/wyfs02/M02/82/DB/wKioL1ditdDBxPldAAAnkLM7jqM457.png-wh_500x0-wm_3 -wmp_4-s_2408731810.png "title=" 6.PNG "alt=" Wkiol1ditddbxpldaaanklm7jqm457.png-wh_50 "/>

This time it looks like a reversal:

1. Counter-quotes instead output the literal meaning of $hostname

2.$ () as we wish to take the value of the variable, and rightfully output an escaped string \

Let's do another experiment and build a a.sh with a foot name:

650) this.width=650; "Src=" Http://s2.51cto.com/wyfs02/M00/82/DC/wKiom1ditNTDS7U4AAAIG_97TGk595.png-wh_500x0-wm_3 -wmp_4-s_679179395.png "title=" 7.PNG "alt=" Wkiom1ditntds7u4aaaig_97tgk595.png-wh_50 "/>

Note: The last inverted quotation mark and) are preceded by a space, otherwise the last anti-quote and) will be commented, and will not be used as the ending substitution symbol.

The results of using sh-x a.sh are as follows:

650) this.width=650; "Src=" Http://s5.51cto.com/wyfs02/M01/82/DB/wKioL1ditg_Th5MBAAAYBkNQGxM929.png-wh_500x0-wm_3 -wmp_4-s_3690905200.png "title=" 8.PNG "alt=" Wkiol1ditg_th5mbaaaybknqgxm929.png-wh_50 "/>

From what we can see:

1.4 \ is output as 1 \ In the inverted quotation mark

2. $ () 4 \ are output to 2 \

We have modified the script to 8 \:

650) this.width=650; "Src=" Http://s1.51cto.com/wyfs02/M00/82/DB/wKioL1ditiqhiQgTAAAKagCrwLw120.png-wh_500x0-wm_3 -wmp_4-s_2318970954.png "title=" 9.PNG "alt=" Wkiol1ditiqhiqgtaaakagcrwlw120.png-wh_50 "/>

Then look at the output:

650) this.width=650; "Src=" Http://s5.51cto.com/wyfs02/M01/82/DC/wKiom1ditSXCsVgCAAAeaCmLzkc875.png-wh_500x0-wm_3 -wmp_4-s_1304316199.png "title=" 10.PNG "alt=" Wkiom1ditsxcsvgcaaaeacmlzkc875.png-wh_50 "/>

Thus, we can conclude that:

1. The anti-quotation mark itself is escaped by itself, preserving the meaning of the alignment itself, if we want to play the special meaning in the anti-quotation marks, we must use 2 \ to represent.

2. So we can simply imagine the inverted quotation mark: \ \ = \

3. $ () does not need to consider the problem, as we normally use: \ = \

Note: Anti-quotes are old usage, $ () is a new usage, whether in the learning test, or in the actual work, the usage of $ () is recommended.

The difference between the anti-quote and $ () in a shell script