Linux Learning Diary--Basic commands (7) Variables in--bash, configuration files

Definition and use of variables

Variable type

Strongly typed: You must specify a type when defining a variable, the participating operation must conform to the type requirement, and calling an undeclared variable produces an error;

Weak type: No need to specify type, default is character type, participate in operation will automatically do implicit type conversion, the variable can be called directly without prior definition;

Variable naming laws:
1. The reserved words in the program cannot be made: for example if, for;
2, can only use numbers, letters and underscores, and can not start with a number;
3. See the meaning of the name,

Types of variables in bash:
Based on criteria such as the scope of the variable's entry:
Local variable: The active scope is the current shell process and is not valid for Shell processes other than the current shell, including the current Shell's child shell process;
Environment variables: The effective scope is the current shell process and its child processes;
Local variables: The effective range is the current shell process in a code fragment (usually refers to a function);
Positional variables: $, $, ... To indicate that the script is used to invoke parameters passed to it through the command line in the script code;

Special variables: $?, $, $*, [email protected], $#

Local Variables:
Variable assignment: name= ' value '
You can use references:
Value
(1) can be a direct string; Name= "username"
(2) Variable reference: name= "$username"
(3) Order reference: Name= ' Command ', name=$ (command)
Variable reference: ${name}, $name
"": a weak reference in which the variable reference is replaced with the value of the variable;
": Strong reference, where the variable reference is not replaced with the value of the variable, and the original string is persisted;
Show all variables that have been defined
Set
Destroying variables:

unset name

Environment variables:
Variable declaration, assignment:
Export Name=value
Declare-x Name=value
Variable reference: $name, ${name}
Show all environment variables:
Export
Env
Printenv
Destroyed:
unset name

Bash has many built-in environment variables: PATH, SHELL, UID, Histsize, HOME, PWD, old, Histfile, PS1

Read-only variables:
ReadOnly Name
Declare-r Name

Positional variables:
Calling parameters passed to the script through the command line in the script code;
$, $, ... : Corresponding call 1th, 2nd and other parameters;

Shift [N]

$: the order itself;

$*: All parameters passed to the script;
[Email protected]: All parameters passed to the script;
$#: The number of arguments passed to the script;

Example: Determining the number of rows for a given file
#!/bin/bash
Linecount= "$ (wc-l $1| cut-d"-f1) "
echo "has $linecount lines."

Bash configuration file:

In terms of effective scope, there are two categories:
Global configuration:
/etc/profile
/etc/profile.d/*.sh
/etc/bashrc
Personal configuration:
~/.bash_profile
~/.bashrc

by function, there are two categories:
Profile class: Provides configuration for the interactive logon shell
Global:/etc/profile,/etc/profile.d/*.sh
Personal: ~/.bash_profile
Function:
(1) for defining environment variables;
(2) run the command or script;

BASHRC class: Provides configuration for a non-interactive logon shell
Global:/ETC/BASHRC
Personal: ~/.BASHRC

Function:
(1) Define command aliases;
(2) define local variables;

Shell Login:
Interactive login:
Enter the account password directly via the terminal;
Users switching with "Su-username" or "Su-l UserName"

Loading process
/etc/profile---/etc/profile.d/*.sh------~/.BASHRC----and/ETC/BASHRC//late loading will overwrite the same variables that were loaded first

Non-interactive logon:
Su UserName
The terminal opened under the graphical interface
Execute script

Loading process

~/.BASHRC--/ETC/BASHRC-/etc/profile.d/*.sh

Edit the new configuration defined by the profile to take effect:
(1) Restart the shell process;
(2) using the source or. Command process

Linux Learning Diary--Basic commands (7) Variables in--bash, configuration files