Basic series of SHEL script (I) variables

One 、 background

         Although I am a java The programmer , Do the back end , But recently, I often need to read scripts written by others to run projects , So as a back-end programmer , We also need to know shell grammar , Can read some basic shell Script .

Two 、 Variable correlation

1、 Variable assignment

（1） Assignment of common variables

a="hello world"

（2） Assign values to variables through commands , Two ways .

a=$(pwd)

b=`uname`

2、 Cancel variables

unset A

3、 Get the length of the variable

${#b}

4、 Deletion of variable content

（1） Delete Syntax

%	Means to remove the first matching from right to left
%%	Remove all matching from right to left
#	Means to remove the first matching from left to right
##	Means to remove all matching from left to right

eg:

# return  text
a=a.txt
echo ${a##*.}

4、 Replacement of variable contents

（2） Substitution grammar

 ${ Variable name / The old value / The new value }

eg：

# return b.txt
a=a.txt
echo ${a/a/b}

5、 Print variables

（1） Mode one

echo $a

（2） Mode two

echo ${a}

6、 Query environment variables

（1） View the current environment variables

env

（2） Filter environment variables

env | grep Name

（3） Support filtering common variables or environment variables

set | grep Name

7、 Define an environment variable

(1) Mode one

declare -x BBB=jiankang

(2) Mode two

export DDD=jiankang

8、 Global variable profile

（1） Required after change source Let it take effect

Configure environment variables globally	/etc/profile
Current user's environment variables	home/.bash_profile
Current user's bash Information	/home/.bashrc
Global effective for all users bash Information	/etc/bashrc

9、 System variables

$?	Whether the last command is executed normally 0 Is normal （ really ） Not 0 Indicates an execution exception
$0	Name of the currently executing program or script
$#	The number of parameters passed after the script
$*	All the parameters behind the script , Comma separated
[email protected]	All the parameters behind the script
$1	The first parameter following the script
$9	The ninth parameter following the script
${10}	The tenth parameter following the script
$$	The process number of the current process
$!	The last process number running in the background （ Current terminal ）
!$	Call the parameters in the last command history

3、 ... and 、 summary

         The above is about shell Everything about syntax variables , Hopefully that helped , You can refer to it , If you think it's good , Welcome to wechat search java Basic notes , Relevant knowledge will be continuously updated later , Make progress together .