Shell variables

define

• A variable is, as the name suggests, a quantity that can be changed
• A variable must start with a letter or underscore. The name must contain only digits, letters, or underscores
• The variable name contains a maximum of 255 characters
• The variable name must be unique within the valid range
• The default variable type is string

classification

• string
• The integer
• Floating point Numbers
• The date type

User-defined variables

• The values of these variables are self-defined
• The variable name cannot start with a number
• No Spaces on either side of the equal sign

Define variables

name=wanghaoyu
age=23
Copy the code

The output variable

echo name # wanghaoyu
Copy the code

Note that variable types are defined as strings by default.

x=1
y=2
z=3
$o=$x+$y+$z
echo $o # 1+2+3
Copy the code

Reference variables during assignment

There are two ways to refer to a variable during assignment

• m="$x"2

• m=${x}2

Both methods mean assigning a value to the variable m, which is the value of the variable x plus the string 2.

setCommand to view the variables that have been set

• setCommand to query all variables that have taken effect by default, including system variables and user-defined variables.

set | grep name

unset

• unsetDelete variable

unset a

The environment variable

• Environment variables are global, while custom variables are local
• Custom variables will only be currentshellWhile environment variables can take effect in the currentshellAnd the sonshellIn effect
• These variables mainly hold data related to the operating system environment
• The variable can be defined by itself, but the name and function of the variable in effect for the system are fixed

Subshells can share environment variables, but different Windows cannot access environment variables of another shell, also temporary, other shell Windows cannot access.

Customize environment variables

• useexportThe command defines environment variables

Export Variable name = variable valueCopy the code

env

• envandsetThe difference is,setYou can view all the variablesenvOnly set environment variables can be viewed.

env | grep name

Common environment variables

PATHSystem search path

echo $PATH
/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/root/bin
Copy the code

The role of $PATH was previously mentioned in the PATH environment variable.

If we enter an ls command, the system will search for the corresponding command in the $PATH directory from left to right, and then execute it. The command does not exist if it is not found.

When we want to make a script executable via global commands, we can do so in the following ways:

The same is true with nodeJS.

1. Add executable permission to the corresponding file.

The beginning of the file indicates the environment in which the file is run. For example, the shell needs to start with #! /bin/bash, node file #! /usr/bin/node tells the system that this is a node to file.

2. We can add our corresponding executable file to any directory in the environment variable, and then we can use it directly through the command. (bashYou can execute the script directly by calling the command with the file name.

Add the command script to the directory of the existing environment variable $PATH.

3. And of course we can be here$PATHAnd put the command script into this new file. Pay attention to:Partition, this time can also achieve the same effect.

By redefining the environment variable export $PATH=”$PATH:[new directory PATH]”.

Linguistic environment variable

• Query current language

• In Linux, you can use local to set up the different languages in which your program will run. Local support is provided by ANSI C.

• The local command rule is < language >_< locale >.< string encoding set >, for example, zh_CN. Utf-8.

• The LANG system defines variables in the host language

Position parameter variable

• This type of variable is used to pass parameters or data to the script, the variable name cannot be customized, the function of the variable is fixed.

• $nThe example test. Sh

#! /bin/bash
echo "$1+$2+$3"
Copy the code

sh test.sh 1 2 3

Output 1 + 2 + 3

• $@For example the for sh

#! /bin/bash
for i in "$@"
    do
        echo "i=$i"
    done
Copy the code

sh for.sh 1 2 3
Copy the code

i=1
i=2
i=3
Copy the code

The concept of $@ is similar to arguments in JS, which can be obtained one by one through a loop.

• $*For example the for sh

#! /bin/bash
for i in "$*"
    do
        echo "i=$i"
    done
Copy the code

i=3 4 5
Copy the code

The difference between $@ and $* is that each parameter is treated separately, or all parameters are treated as a whole.

• $#Number of parameters

echo $#
Copy the code

sh hello.sh 1 2 3 4 5
Copy the code

5 # The parameter length is 5Copy the code

Predefined variable

• Predefined variables are defined in scripts. They cannot be customized and have fixed functions.

When we need to determine a branch condition to run based on the return of the command, then $? It’s very useful.

readThe command

• Read [option] [variable name]

Indicates that the value entered by the user can be read and stored in the variable name.

#! /bin/bash read -p 'Your name is' -t 5 name echo -e '\n' read -p' Your gender is :' -n 1 gender echo -e '\n' read -p 'Your password is' -s password Echo $name,$gender,$password #Copy the code

The read command is often seen in things like logging in to mysql.