Shell Scripting App (shell scripting Basics vs. Shell variables)

1. Shell: Command interpreter
1) supported shells are saved in/etc/shells

2) Default shell is/bin/bash

2. Shell script writing:

1) using the VI Editor

2) One command per line, followed by

3) Give executable permissions (x)

3, the way of implementation:

1) script file path: There are two, relative and absolute paths that must have X permissions

2) SH script file path: No x permission, relative and absolute path can be, on the child shell

3) Source (or.) script file path: Do not need x permission to execute at current sell

4. Script Composition:

1) Script declaration: Specifies the shell that the script executes to #! Beginning

2) Comment Information: explanatory text, beginning with #, does not execute

3) executable statement:

5. Input and output redirection and pipe characters

Input device: File name/dev/stdin number 0 for keyboard

Output device: File name/dev/stdout number 1 corresponds to the display

Error Output: File name/dev/stderr number 2 corresponds to display

: Output redirected to file, create or overwrite original file

<: Input Redirect to file

Append output Redirect to file

2>: Error message output redirected to file

&>: Mixed output redirected to the same file

|: Pipe character, give left command result to right command execution

awk command: Outputs the specified range of information with the specified character as a delimiter, and the default delimiter is a space

Awk-f: ' {print $1,$7} '

6. Variable classification:

1) Custom variables: User-defined

2) Environment variables: Automatic system creation

3) Position variable: Command parameter location: $1~$9

4) Pre-defined variables: variables with fixed meanings

7. Custom variables:

1) Define a variable: variable name = variable value proposed all uppercase (case-sensitive)

2) View variable value: Echo $ variable Name example: Echo $A [plus $ display value, no display name (A)]

3) The effect of quotation marks on variables:

(1) Double quotation marks: The value of the reference variable

(2) Single quote: Show variable name

(3) Anti-apostrophe: EXECUTE AS command, get command result, do not allow nesting, Can $ () instead

(4) Assigning values to variables from keyboard input:

READ-P "hint Information" variable name

(5) Publish variable: Export variable name "= Variable Value" Child shell also valid

(6) Integer operation: Expr variable 1 operator variable 2 (example: EXPR3 * 4)

Operator: Plus (+) minus (-) multiply (*) divide (/) remainder (%)

8. Environment variables:

Configuration file:

1) Global: Effective/etc/profile for all users

2) User: Valid only for users ~/.bash_profile

Common environment variables:

1) PWD: Current working directory

2) Path: Paths for command search

3) User: Current user

4) Shell: Shell of the current user

5) HOME: The current user's host directory

6) Histsize: Record the number of history commands, default to 1000

7) UID: Current User ID

9. Position variable: The position of the command parameter, represented by $1~$9, first to Nineth

10. Pre-defined variables:

1) $#: The number of positional variables in the command line

2) $*: Contents of all positional variables

3) $?: Result of previous command execution, normal 0, non 0 value indicates exception or error

4) $ A: Represents the script itself

Other commands:

Date +%y Year

Date +%y-%m Month

Date +%y-%m-%d Month Day

Shell script Application (shell script base and shell variable)