(1) Shell has no spaces on both sides when assigning values

(2) When the right side of the shell is a word group, there must be double quotation marks

(3) The format of the variable is the same as that of the C language

2. Types of variables

3. Examples

hello.sh中
echo "参数个数："$#                       //3
echo "所有参数@" $@                      //123 hi hhh
echo "所有参数*" $*                        //123 hi hhh
echo "上一条shell命令的返回值*" $？        // 0
echo "第三个参数*" $3                      //hhh

output on the command line

./hello.sh 123 hi hhh

3. Special symbols

1. Single quotes, double quotes, backticks

2. Pipeline

ls -l | wc, ls -l collects the information of the files in the current directory, and then sends it to the pipeline, wc reads it from the pipeline, calculates the number of lines, the number of words and the total characters

3. Greater than > and less than < (redirection)

(1) common sense

When the process is running, the system will open three files, which are standard input (0), standard output (1) (the display is read from descriptor 1), standard error (2)

(2) Application (when the descriptor is on the right, the & symbol should be added)

ls -l 1>a.txt	Originally, the command was to be output to descriptor 1, and then output to a.txt after redirection
echo 0<b.doc	Originally read files from the keyboard, but now read data from b.doc
echo “hello world” 1>&2:	Hello world was originally supposed to be input to descriptor 1, but it has now been output to descriptor 2. That is, output a sentence to the standard error device
cat > name	Get information from standard input and write directly to the file name
Cat >name <a.txt	Take the content of a.txt as input, then > output to name

(3) The difference between > and >>

>, the file exists, delete and recreate

>>, append if the file exists

Four, string operation

1. Calculate the number of characters

time="today is `date`"

echo " Number of characters in time: ${#time }"

2. Delete the characters on the left side of the string

path="/etc/rc0.d/Z20openbsd-inetd"

level=${ path#/etc/rc[0-9].d/[AZJ]}

echo $level (output: 20openbsd-inetd )

3. Delete the characters on the right side of the string

path="/etc/rc0.d/Z20openbsd-inetd"

vel = $ { level %% [ a - zA - Z ] * }

echo $vel (output: /etc/rc0.d/Z20 ) (special symbols are not considered, such as "-")

5. Test statement

Six, logic statement

1、if-elif

Are the strings equal	if [ "$var1" = "$var2" ]
whether the file exists and is readable	if test -e name && test -r name
whether the file exists and is readable	if [ -e name ] && [ -r name ]
if [ -e file ] && [ -r file ] then cat file # If the file exists and is readable, display the contents of the file elif [ - e file ] #judging whether the file exists then chmod u + r file #Let the user's permission add readable cat file # If the file exists but cannot be read, then display its content after adding read permission else touch file # If the file does not exist, create the empty file fi

2. Multiple branches

read VAR #Receivea user input from the keyboard

case $ VAR in #judgingthe value entered by the user$VAR

1 ) echo "one" #Ifthe value of$VARis1, then display one

;; #Eachbranch must end with a double semicolon (except the last branch) similar tobreak

2) echo "two"

;;

* ) echo "unknown" #Theasterisk*isa wildcard in the Shell

esac

Precautions:

1. The value of the variable VAR is actually a string, so 1) in the above code can also be written as "1")

2. The entire case structure must end with esac.

Problem: "\r": command not found

There are two types of editor end symbols

Line break\n(LF)
结束符\r＋换行符\n（CRLF）

3、for循环

files=`ls` # 在当前目录下执行 ls，将所有的文件名保存在变量 files 中

for a in $files # 循环地将 files 里面的每个单词赋给 a，赋完则退出循环

if [ -f $a ] # 如果文件$a 是一个普通文件，那么就计算他的行数

then

wc -l $a

done

4、ifeq

ifeq ($(OS), Linux)
$(info Running on Linux) //如果成立，则输出Running on Linux)
else
$(warning This Makefile is intended for Linux)

//如果不成立，则输出警告：This Makefile is intended for Linux
endif

.PHONY: all

all:
@echo "Building target..."
# 其他构建命令和规则

1. Basic knowledge of shell scripts - variables, strings, loops

1. Basic knowledge

2. Shell variables and assignments

1. Pay attention