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In linux, shell scripts are a way of programming, which is a collection of commands. Shell script will write some shell syntax and instructions inside, using pipeline commands and traffic redirection and other functions to achieve the desired processing purpose, is temporarily written Mini Program to complete the current work.

The operating environment of this tutorial: linux7.3 system, Dell G3 computer.

What is a shell script in linux

I. the concept and significance of shell script

Shell script is a programming method under linux, script (shell script) is a program written by the function of shell, this procedure is to use plain text files, write some shell syntax and instructions in it, and then use normal representation, pipeline commands and data flow redirection and other functions to achieve the processing purpose we want. To put it bluntly, it is to solve the practical problems of linux, temporarily write a Mini Program to complete the current work, if you need to use it many times, it can also be saved and recycled.

two。 The specification of the script

The script uses the shebang mechanism, that is, the format:! # / bin/bash 、! # / use/bin/python . You need to simply interpret the shell script. Here is an example:

#! / bin/bash#-# Filename: hello.sh# Revision: 1.The Date: 2017Unip 08 Universe Author: Mr.Dong# Email: Dong@gmail.com# Website: www.magedu.com# Description: This is the first script#-echo "hello world"

With this information, you can quickly know the usefulness of this script for your work.

You can use the following two commands to debug scripts at run time

Bash-n hello.sh to check the syntax errors of the script

Bash-x hello.sh can view the execution steps of the script

three。 Variable

1. Local variable

The local variable is valid only for the current shell and not for the child shell (how many levels of shell is the SHLVL view).

Variable assignment: name='value'

String assignment: name= "root"

Variable reference: name= "$USER"

Command reference: name=COMMAND name=$ (COMMAND)

Show all defined variables: set

Delete variable: unset name

two。 Environment variable

The environment variable is valid for the current SHELL and its child SHELL.

Declaration and assignment commands for variables:

Export name=VALUEdeclare-x name=VALUE

Reference to the variable: $name,$ {name}

Show all environment variable commands:

Env printenv export declare-x

Bash comes with environment variables:

PATH SHELL USER UID HOME PWD SHLVL LANG MAIL HOSTNAME HISTSIZE

3. Read-only variable

Declare a read-only variable:

Readonly namedeclare-r name

View the read-only variable: readonly-p

4. Location variable

$1, $2,... Corresponding to the first and second parameters, shift [n] changes position.

$0: command itself

$: all parameters passed to the script, all of which are combined into one string

$@: all parameters passed to the script, each of which is a separate string

$#: the number of parameters passed to the script

$@ $makes a difference only when it is enclosed in double quotation marks

Set-

four。 Arithmetic operation and logic operation

1. Arithmetic operation

Arithmetic operator: +-* /% (remainder) * * (multiplier)

Common operations:

(1) let var= arithmetic expression

(2) var=$ [arithmetic expression]

(3) var=$ ((arithmetic expression))

(4) var=$ (expr arg1 arg2 arg3 …)

(5) declare-I var = value

(6) echo 'arithmetic expression' | bc

Generate random number command: random number between echo $[$RANDOM%number] 0-number

Assignment operation: + =-= * /% =

Increase and decrease:

Let var+=1let var++let var-=1let var-

two。 Logical operation

True=0 false=1

It is the same as 1, 1, 1, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0.

Or 1 | | 1: 1 / 1 | | 0: 1 0 | | 1: 1 0 | | 0: 0

No! 1-0! 0-1

Five. Test command

The test command is a utility for testing conditional expressions in the shell environment.

For example:

Test "$A" = = "$B" & & echo "Strings are equal" ["$A" = = "$B"] & & echo "Strings are equal"

The two expressions express the same meaning.

[[]] indicates support for regular expressions

Numeric comparison: string comparison:

-whether the gt is greater than >

Whether ge is greater than or equal to > =

Is-eq equal to = =

Whether ne is not equal to! =

-whether the lt is less than

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