Study Notes shell 1: shell variables in the method and common symbols

variable

That statement by

a=2
b="123"

Call
$ {varName} or $ varName

echo $b
echo ${a}

Common variable

• The $ ?: determine whether a statement success
• $ 0: execute the script file name
• $ 1: The first argument
• $ N: n th argument
• $ *: What are the parameters
• $ #: The number of parameters

Examples

demo1.sh

echo "脚本文件名称:$0"
echo "第1个参数:$1"
echo "所有的参数:$*"
echo "参数个数:$#"

Given execute permissions

chmod u+x demo1.sh

carried out

./demo1.sh param2 1 2 3

Output

脚本文件名称:./demo1.sh
第1个参数:param2
所有的参数:param2 1 2 3
参数个数:4

symbol

Common Symbols

> : Write to target file

• It will overwrite the original content

• Examples: hello world string write to the file 1.txt

    # 直接在bash中可运行
    echo hello world > 1.txt 

>> : Insert the end of the file content to the target

• In the end of the file additional content source

• Example: add a line to the file 1.txt string hello world

    # 直接在bash中可运行
    echo hello world >> 1.txt 

; : Execute multiple instructions

• Example: an additional line of text and then displays the current contents of the file

    # 直接在bash中可运行
    echo hello world >> 1.txt ; cat 1.txt

| : Pipe symbol

• Result of the processing on a command passed to the next command processing

• Example: cat results to statistics wc command line

    # 直接在bash中可运行
    # 最后一个字母为小写的L
    cat 1.txt | wc -l

&& : Execute multiple commands

• Before a command error, block the subsequent command

• Example: Append a single line to view the number of rows and then 1.txt

    # 直接在bash中可运行
    echo str >> 1.txt && wc -l 1.txt

|| : Execute multiple commands

• The previous command will execute the subsequent instruction error

• Example: Append content to 1.txt line error, and then view its contents

    # 直接在bash中可运行
    echo3 str >> 1.txt || cat 1.txt 

"" : Output variable values ​​can be

• Able to parse the string variable $ varName

• Example: Output hello

  # 编写demo3.sh
  
    a="hello"
    echo "$a" 

'' : Output variable values ​​can not be

• Not enough to resolve $ varName variable string

• Examples: can not output hello

  # 编写demo3.sh
  
    a="hello"
    echo '$a' 

  The output is: $ a

`` : Output command result

• Example: output current date

• date command to get the current date

  echo `date`

2>/dev/null : Error output to the bottomless pit

• Command execution error does not occur error
• The implementation will have correct output

• example

  # 执行
  echo1 123
  # 正常错误输出
  bash: echo1: 未找到命令
  # 使用无底洞
  echo1 123 2>/dev/null
  # 没有任何内容输出

1>/dev/null : Correct output to the bottomless pit

• Command execution results are not displayed correctly
• Execution errors will have an error message content

• example

  # 执行
  echo 123
  # 正常错误输出
  123
  # 使用无底洞
  echo 123 1>/dev/null
  # 没有任何内容输出