Shell variables and variable types

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The shell is a weak type into a language

variable assignment : var_name=value

undo Variable : unset var_name

Bash:

variable type 1, environment variable

Export Var_name=value

The script starts a child shell process when it executes:

Scripts that are started on the command line inherit the environment variables of the current shell

Program scripts that are automatically executed by the system (not command-line startup) require self-defined environment variables

2. Local variables (local variables)

Var_name=value acts on the entire bash process.

The local Var_name=value acts on the current code snippet.

3. Position variable

$1,$2,.....

4. Special variables

$?: Save Last Command execution status return value

There may be two return values after program execution

1. Program execution Results

2. Program Execution status return code (0-255)

0 is performed correctly

1-255: Error execution 1,2,127 system retention

How does bash achieve conditional judgment?

condition test Type :

1. Integer test

2. Character test

3. File test

Expressions for conditional tests :

[expression] Two spaces or grammatical error

[[expression]] Ibid.

Tset expression

integer comparison :

-EQ: Tests whether two integers are equal such as $A-eq $B

-NQ: Test whether two integers are unequal to true

-GT: Test whether a number is greater than another number

-LT: Tests whether one number is less than the other

-ge: greater than or equal to

-le: Less than or equal to

logical relationship of the command :

Logic and:&&

Logical OR: | |

#!/bin/bash!  ID user1 &>/dev/null && useradd user1 && echo "user1" |passwd--stdin user1 &>/dev/null | | echo "User already exists"

Conditional judgment, control structure

if judgment condition; then

Statement1

Statement1

.....

Else

Statement3

Statement4

....

Fi

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Shell variables and variable types