Shell theory Study (iii)

37. Adjust the properties of a variable

• Read-only variable: use the readonly command

  ReadOnly variable Name

• ReadOnly Other uses

  • ReadOnly or readonly-p: Displays a list of variables that currently have read-only properties

  • readonly-f function Name: Set the function cannot be modified

  • READONLY-A array Variable: Set-a followed by the variable name is a read-only array

• Read-only variable: using declare

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38. Custom Environment Variables

• When you log on as a normal user

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• When working in the role of the system administrator, there are three environment variable profiles

  • /etc/profile

  • /etc/bash.bashrc

  • All files under the/etc/skel directory

39.Here Document

Basic usage:

[[email protected] ~]# cat >>123.txt<< end> Hello > My name is ' Jack ' > Welcome to bj> Endyou has New mail In/var/spool/mail/root[[email protected] ~]# cat 123.txt Hello my name is "Jack" Welcome to BJ

• Here document also supports variable substitution. In the input, if there is a variable, bash will replace the variable value before turning.

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• How to use in variables

[Email protected] ~]# wow= ' wow,great! ' [Email protected] ~]# m1=$ (cat <<HERE> Line 1 was good.> they are jack,marry and john.> $WoW > Here>) [Email protected] ~]# echo $m 1line 1 is good. They is Jack,marry and John.

• How to close a variable

[Email protected] ~]# m1=$ (cat << "Here" > Line 1 is good.> they be jack,marry and john.> $WoW > here> ) [[email protected] ~]# echo $m 1line 1 is good. They is Jack,marry and John. $WoW

• Multi-line annotations

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40. How to test if a variable exists

Syntax: ${variables to be tested-default value}

Explanation: To determine whether the variable being tested exists, if the variable exists, it returns the value of the variable, if not it will pass the default value to the variables to be tested

• Determine if a variable exists
  

[[email protected] ~]# unset myname[[email protected] ~]# r=${myname-' basher '}[[email protected] ~]# echo $rbasher

[Email protected] ~]# myname= "Jack" [[email protected] ~]# r=${myname-' basher '}[[email protected] ~]# echo $rJack

• Determine if the variable does not exist or is empty

[[email protected] ~]# myname=[[email protected] ~]# r=${myname:-' basher '}[[email protected] ~]# echo $rbasher [[Email Pro Tected] ~]# unset myname[[email protected] ~]# r=${myname:-' basher '}[[email protected] ~]# echo $rbasher

• Use if to judge
  

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• Test whether the variable "does not exist" or its value is empty: Prompt for error message

Syntax: ${variables to be tested:? Prompt message}

Explanation: Determine whether the variable "does not exist" or "empty", if the condition is true, then display: after the prompt message. and immediately stop executing the script

To disassemble an expression:

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Example:

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This article is from the "Linux Revolution" blog, so be sure to keep this source http://kaibinyuan.blog.51cto.com/7304008/1612017

Shell theory Study (iii)