Getting Started with shell scripting--variables

First, the basic
View all shells
Cat/etc/shells or Chsh–l
1. Examples of scripts.
Linux does not differentiate files, for the convenience of memory, here with. Sh end.
(1) Vim first.sh
#!/bin/bash
Mkdir/root/shell
Ifconfig
(2) Add Execute permissions chmod +x first.sh The file will turn green at this point.
(3) Execution: 5 ways:
①./first.sh
② Absolute Path execution
③sh + script name (no execute permission required)
④source + script name (does not require EXECUTE permission) recommended use
⑤. + PIN Name

Tip: When you are finished viewing a path, use ESC +. , you can call Lu Jinglai to paste directly.

2. Variables
Common shell variables
Custom variables: Variables defined by the user according to their environment, for example: a=b
Environment variables, positional variables, pre-defined variables.

Call variable: Echo $+ variable name
(1) When 2 sets of variables need to be called: Echo $Linux $linux
(2) Variable name is easy to confuse with {} to enclose the variable name: Echo ${linux}system
(3) Variable values have some other special compliance: linux= "Rhel 6.5"
(4) Reference variable: system= "RHEL $linux"
(5) Single quotation mark: When assigning a special symbol case: a=1,b= ' $a ', echo $b
(6) Anti-apostrophe " “ ,esc下边那个按键。将命令输出结果赋给变量，反撇号括起来范围必须为可执行命令。例：rpm –qf which pwd"
3. Read command
Example: Read Dell HP
1 2
Reference variable: Echo $dell, outputs 1 echo $hp and outputs 2.
For interactive image display, add –p to display the prompt message.
Example: read–p "Input your password:" Password
4.
View all current variables by set
ENV View global Variables
Export setting global variables
Example: #export a=1, #bash, #echo $a, a value of 1 is returned.
5. Operation of numerical variables
There must be an empty space between the operator and the variable. The operation of integers is mainly done by the internal command expr command.
Example: Expr $A + $B (addition) expr $A * $B (multiplication) expr $A% $B (division)

Assigning the result of an operation to another variable

6. Position variable

Example: Vim weizhi.sh


7. Pre-defined variables
$#: Number of positional variables in the command line (the program executes several positional parameters)
$*: The contents of all positional variables (specific content such as/boot is a specific content)
$?: The state returned after the previous command was executed, when the return status value is 0 indicates normal execution, and a value other than 0 indicates an exception or error
Determine if an error occurs correctly for 0 exception error for non 0 value between 1-127
$: The currently executing process/program name (which is the name of the currently executing command or program)

Tip: Add Environment variables

Vim/etc/profile

At the end of the document, add:
Export path= "/opt/stm/stlinux-2.3/devkit/sh4/bin: $PATH"
(/opt/stm/stlinux-2.3/devkit/sh4/bin) is the path to add

Getting Started with shell scripting--variables