Shell Script Basics 1 Overview and variables

Shell Overview:
The interpreter program between the Linux kernel and the user
Usually refers to/bin/bash
Responsible for directing to the kernel to translate and communicate user/program instructions
Equivalent to the "shell" of the operating system

How the shell is used:
Interactive--Command line: manual intervention, high degree of intelligence, the implementation of an article by article, low efficiency
Non-interactive-script: need to design in advance, intelligent difficult, batch execution, high efficiency, easy to run quietly in the background

To switch the shell environment:
Chsh Change the login shell via Usermod
Manually execute the target shell program
[Email protected] ~]# Cat/etc/shells
/bin/sh (most Unix default shell)
/bin/bash (most Linux uses the default shell)
/sbin/nologin (non-landing shell)
/bin/dash
/bin/tcsh
/bin/csh
/bin/ksh

Bash Basic Features:
command-line Environment review
Shortcut key, TAB key
Command history
Command aliases
Standard input and output
redirect
Pipeline operation

/etc/profile (Environment variables and startup programs)
/ETC/BASHRC (functions and aliases)
/root/.bash_profile
/root/.bashrc

Writing and executing scripts:
#!/bin/bash
For i in {1..10}
Do
Useradd hydra$i 2>>uerror.log >/dev/null
echo "123" |passwd--stdin user$i 2>>perror.log >/dev/null
Done

To add Execute permissions:
[Email protected] ~]# chmod +x x.sh
[Email protected] ~]#./x.sh
HAIL HYDRA!!!!!

Execute when not authorized: (Because interpreter rights have permission)
[Email protected] ~]# chmod-x x.sh
[Email protected] ~]# bash x.sh
[Email protected] ~]# sh x.sh
(Above two open sub-processes)
[Email protected] ~]# source x.sh
[Email protected] ~]#. x.sh
(above two non-open sub-processes)

Scripting Spec Ideas:
#! Script declaration (which interpreter to use)
#注释信息 (steps, ideas, uses, variable meanings, etc.)
The executable statement

Script creation:
Create a new text file
Add an executable script statement (command line)
Add x permission
[[email protected] ~]# vim x.sh (create file)
#!/bin/bash (Declaration interpreter)
#步骤, ideas, uses, variable meanings
echo "Hail HYDRA!!!!!" (write script statement)
[[email protected] ~]# chmod +x x.sh (add execute permission)
[Email protected] ~]#./x.sh
HAIL HYDRA!!!!!

——————————————————————————————

The shell variable is a weakly typed variable

Variable:
Variable name = value
Variable name: letter, Number, _, but cannot start with a number

Define/Assign Variables
Variable name = variable Value
[[email protected] ~]# x=12 define variable x, assign value 12
[[email protected] ~]# var1=redhat define variable var1, assign value Redhat
Related precautions:
If the specified variable name already exists, it is equivalent to re-assigning it to this variable name.
Don't have spaces on either side of the equals sign
Variable names are made up of letters/numbers/underscores and are case-sensitive
Variable names cannot start with a number, do not use keywords and special characters

Prevent conflicts can be in curly braces (case-sensitive)
$ variable Name
${variable Name}
[Email protected] ~]# a=7

To view the value of a variable:
Name of the echo $ variable
[Email protected] ~]# echo $a
7

To cancel a variable:
Name of the unset variable
[Email protected] ~]# unset a
[Email protected] ~]# vim b.sh
#!/bin/bash

Useradd $ (Variable $ variable)
echo "$" |passwd--stdin $

[Email protected] ~]#/b.sh Hydra Anonymous
Change the password for the user Hydra.
PASSWD: All the authentication tokens have been successfully updated.

Categories of variables:
Environment variables: variable names are usually case-sensitive, oil system maintenance, used to set the working environment
Location variables: Bash built-in, storing parameters provided when executing scripts
Predefined variables: Bash built-in, a class of special-purpose variables that can be called directly, but cannot be directly assigned or modified
Custom variables: Set, modify and use by the user

Variable configuration file
/etc/profile ~/.bash_profile
Related actions
ENV: List all the environment variables
Set: List all variables
Common environment variables:
PWD PATH USER LOGNAME UID SHELL HOME PS1 PS2 ....

Pre-defined variables:
The execution information used to save the script
Directly using these variables, you cannot assign values directly to these variables
Variable name:
$: Current process or pin name
$$: PID number of the current running process
$ : Return status after command execution, 0 Normal, non 0 indicates failure
$#: The number of position variables that have been loaded
$*: Values for all positional variables

Positional variables:
To provide command-line arguments when executing a script
expressed as $n,n as ordinal
$1,$2, ... ${10},${11}, ...
[Email protected] ~]# vim c.sh
#!/bin/bash
echo $ ${10} (view 1th, 10 positional parameters)
[Email protected] ~]#./c.sh 1 2 3 4 5 6 7 8 9 10 11
1 10

Instance:
Quickly add users and set a login password.
Provide the user name as a parameter when executing the script
Set the login password to 123456
[Email protected] ~]# vim x.sh
!/bin/bash
Useradd $2>/tmp/err.log
echo 123456 | passwd--stdin $ &>/dev/null

——————————————————————————————————————

Extended assignment operation
Distinguish three kinds of delimiters
Double quotation mark "": Allow extension to $ reference other variable
Single quote ': Disable extension, even if $ is also treated as a normal character
Anti-apostrophe ': Put the command, execute the command, take out the result of the command
(double quotes, single quotes are whole, but single quotes mask special symbols)

[Email protected] run]# echo "$USER"
Root
[Email protected] run]# echo ' $USER '
$USER
[[email protected] run]# echo ' date '. txt
July 25, 2017 Tuesday 07:04:09 CST.txt

Back up/var/log in tar, back up to/opt
[Email protected] opt]# tar-czf log_ ' Date +%y%m%d '. tar.gz/var/log/

————————————————————————————————————

Shell Script Basics 1 Overview and variables