Basics of shell programming-1

1. What is shell?

A scripting language: is a collection of Linux system commands, written to the script file, execution; support logical judgment, loops and other structured statements; can customize functions, etc.

two。 The beginning of the script file #! / bin/bash

#! / bin/bash specifies which command interpreter to run under

3. The way the script runs

1) add x permission to the script file and run it directly

2) run through the sh command

4. Sh command

-n only check for syntax errors in shell statements, not shell commands

-x displays each command line and its results during shell execution of the script

5. Date command

Format: date [OPTION] [+ FORMAT]

-d displays the date and time referred to in the string

1) string format

% Y year

% m month

% d days

% H hours, 24-hour system (00-23)

% M minutes (00-59)

% s seconds

% D display date format (mm/dd/yy)

% F display date format (% Y-%m-%d)

% T display time (HH:MM:SS)

2)-d

Date-d "+ 1 day" displays the date of the following day

Date-d "- 1 day" shows the date of the previous day

Date-d "- 1 month" +% Y%m%d displays the date of the previous month, specifying the format

Date-d "+ 1 month" +% Y%m%d displays the date of the next month, specifying the format

3)% s timestamp

% s seconds experienced since 00:00:00 on January 1, 1970

6. Shell variable

1) when a string is used frequently in a script and the length of the string is very long, you should use variables instead of

2) when referencing the result of the command, replace it with a variable, such as n = `wc-l 1.txt`

3) variables are essential when writing scripts that interact with users.

Example: if the read command does not specify a variable, the default variable is REPLY

4) shell built-in variables

5) Mathematical operations, conditional statements, etc.

Axia1 bliss 2

Centering $(($astatb))

7. Logical judgment in shell

1) format 1

If condition; then statement; fi

2) format 2

If condition; then statement; else statement; fi

3) format 3

If condition; then statement; elif condition; then statement; else statement; fi