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Bash variable Category:

Local variable: Also called local variable, valid only for the current shell process

Environment variables: current shell and its child shell, child shell ....

Special variables: $? The status of the last command execution, and 0 indicates that execution succeeded. There are a lot of failed code.

[Email protected]:/'cm'config-manager'  Package [ Email protected]echo $? 127 [email protected] Echo $? 0

Variable assignment:

[Email protected]:/$ age=[email protected]echo  $age

Assign the value of a variable to another variable:

[Email protected]:/$ age=[email protected]-virtual-machine:/$ age1=$age [email protected]  echo  $age 1

Now there is a question, what is the value of age and age1 after executing the above code? Will the value of age1 change with age? Here is a new value for age to see

[Email protected]:/$ age=[email protected]echo  $age[email Protected]echo  $age 1

Now look at the following code, when the value of age is present, assign the value of age to a variable, and there is no value assigned to the variable

[Email protected]:/$ result=${age-}[email protected]echo  $result [email protected]-virtual-machine:/$ result=${age2-}[email protected]echo $result  +

An error message is returned when the age value is present when it is assigned and does not exist:

[Email protected]:/$ result=${age?' Error ' }[email protected]echo  $result[email protected]-virtual-machine:/$ Result=${age2?' Error ' }bash:age2:error

Think about what the above code does? Try it.

[Email protected]:/$ result=${age3?' Age3 does not exist ' }bash:age3:age3 does not exist

Output:

Echo $age Echo " Hello " #输出后不换行 Echo ' Hello '    #让转义符生效 echo'hello\tworld '   Echo " Hello\nworld "

Single quotes: Strong references, strings in quotes no matter what, intact

Double quotation marks: Weak references, the variables in the quotation marks are replaced with the values of the variables

Anti-quote: is the character on the keyboard ~ below, the command replaces

[Email protected]:/$ age= at[email protected]-virtual-machine:/$Echo 'I am $age'I am $age [email protected]-virtual-machine:/$Echo "I am $age"I am at[email protected]-virtual-machine:/$Echo "today is ' date +%f '"Today is .- on- -

A reference to a variable

${varname}

Curly braces can sometimes be omitted, sometimes not, take a look

[Email protected]:/$ age=[email protected]echo  $age[email Protected]-virtual-machine:/$ animal=pig[email protected]echo"there is some ${ Animal}s"there is some pigs[email protected]echo"there is Some $animals"there is some

Why does the last output display variable have no value? Because the animals is recognized as a variable.

Revocation of variables:

[Email protected]:/$ age=[email protected]echo  $age[email Protected]-virtual-machine:/$ unset age[email protected]echo $age

Read-only variable: cannot be undone, cannot be modified

[Email protected]:/$ readonly animal=pigbash:animal: Read-only variable [email protected]-virtual-machine:/$ unset Animalbash:unset:animal: Unable to reset: read-only variable

Declaration of environment variables:

[Email protected]:/$ export Age

You can also assign a value at the time of declaration

[Email protected]:/$ export Age

environment variable name generally all uppercase

Here are a few commands:

#查看所有环境变量的命令 (3) Env Printenvexport

New environment variable Path

Export Path=/usr/local/apache/bin: $PATH

Command substitution:

Echo " Today is ' date +%f ' "  -protected [email]echo 'date +%f-%h:%m: %S-(+)-£ º

Generating sequences

 [email protected]:/$ seq  3  Span style= "COLOR: #800080" >1  2  3  Span style= "COLOR: #000000" >[email protected] -virtual-machine:/$ seq  3  6   3  4  5  6  [email protected] -virtual-machine:/$ seq  1  3    1  4  7  10  

The environment variables generated by export will be lost after the shell restarts or restarts the system, then we can save the environment variables through the configuration file

#全局文件/etc/profile,   /etc/profile.d/*   ,/etc/bashrc# personal files ~/.bash_profile   ~/. BASHRC

Configuration file load Order

#交互式/etc/profile     /etc/profile.d/*     /bash_profile    ,     ~/.BASHRC/   etc /bashrc# non-interactive ~/.BASHRC     /ETC/BASHRC    /etc/profied/* #如果同一个环境变量存在多个配置文件中, whichever is the last loaded configuration file

Configuration file Categories

Profile class: Setting environment variables, running commands or scripts (before users log on or before bash starts)

BASHRC class: Setting local variables, setting command aliases

After the configuration file modification is complete, you need to use the following command to make it work, such as:

Souce ~/.bash_profile

Command aliases:

vi=vimalias cdnet='cd/etc/sysconfig/network-scripts/'   VI    #撤消别名

Enter redirection:

Command < file

Output redirection

Command > File

1 is the standard output 2 is the error output

Command 2>file

Correct and Error Redirect:

Command > File 2>&1 #都输出到file

Command > File 2 > File2 #输出到不同的文件

Command &>file Merge output stream

Pipeline:

Command | Command2 #把第一个命令的输出当作第二个命令的输入

Command | Command2 | Command3 | ... | CommandN

It is important to note that the pipeline only passes the correct execution result, and it terminates when an error is encountered.

[Email protected]:/$Echo "ABC"|TR "A- z" "A- z"Abc[email protected]-virtual-machine:/$Echo 'AbCd'|TR 'ABCD' 'ABCD'ABCD

Record a command

Cut–d ' –f2 the string with a space and outputs a second

Cut–d:–f2 to: Cut

Uniq to move a duplicate row

There are times when you encounter this situation:

ls /var/log | file  file [-bchikllnnprsvz0] [--apple] [--mime-encoding] [--mime-type]            [file  ...        file -C [-m magicfiles       ]file [--help]

Because file must be followed by a document, there is no way, in which case we can use-instead of

ls /var/log | file -/dev/stdin:ascii text

File is the type of view files

and record a command tee.

Tee /tmp/tee. Out |command2    #将command的结果保存到tee. Out and then passed to Command2

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