Shell Script Basics

Shell Script

Standardize the general composition of shell scripts

#! Environmental Statement (Sha-bang)

# comment Text

Executable code

Considerations for setting variables

If the specified variable name already exists, it is equivalent to re-assigning a value to this variable

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, cannot use keywords and special characters

Basic format

Reference variable Value: $ variable Name

View variable values: Echo $ variable name, echo ${variable name}

Environment variables

Typically uppercase to set up a user/system environment

PWD PATH USER LOGNAME

SHELL HOME

Custom variables

User-owned settings use

Example

#!/bin/bash

Echo

echo $

Echo

echo $

Echo ${10}

Echo $#

Echo $*

echo [email protected]

Perform

# sh./1.sh a b c d E F

./1.sh# $ $ for Script name

A# $ is the first parameter of a script

b# $ $ for script second argument

C# $ $ is the third parameter of a script

# $ $ This is indicated as a no value status if "${10}" will give no value to null value

6# $ #给脚本参数的的个数

A b c d E F# $* List all parameters

A b c d E F# [email protected] List all parameters

#!/bin/bash

Cat-n $ | Head-$2 # View the first 2 lines of a file

Positional variables

Command-line arguments (non-interactive pass-through values) that are provided when the script is executed

$ ${10}

#无值 is not equal to a null value

#空值对于变量是属于有值的状态 no value for a variable is a non-value state

Pre-defined variables

The execution information used to save the script

Use these variables directly

You cannot assign values directly to these variables

$# $* [email protected]

$? : The status value after the exit of the execution program, 0 indicates normal, the other value is abnormal

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Operation

# expr 10/3#除法

# 3

# expr \* 3#乘法 * default to wildcard \ Restore

# 30

# expr 1 + 2#加法

# 3

# expr 3-1#减法

# 2

# expr 3#取余

# 1

\#反斜杠将后一个变量变为纯字符

" #相当于 $ ()#将命令执行后的结果输出

# mkdir $ (date +%f) date

Or

# mkdir ' Date +%f ' date

"#单引号内安纯字符输出

""#双引号保留其中变量输出

Useradd $user &>/dev/null # $>/dev/null output discarded (not shown)

&& #逻辑 and

|| #逻辑 or

! #逻辑 No

Common Test Options

Check file status

-e: Document exists as true

-D: Document exists and is directory-True

-F: Document exists and is file-true

-r: Document exists with Read permission true

-W: Document exists with Write permission true

-X: Document exists with Execute permission true

Compare integer size (e=equal g=greater l=less n=not T=than)

-GT: Greater Than

-ge: greater than or equal to

-eq: Equals

-ne: Not equal to

-LT: Less than

-le: Less than or equal to

string alignment

= = #相等

!= #不相等

REDIRECT Output

Covered:

: Correct output

2>: Error Output

&>: Error and correct output

Additional:

>>: Correct output

2>>: Error Output

&>>: Error and correct output

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If [condition test 1]; Then

Command sequence XX

elif [condition Test 2]; Then

Command Sequence yy

Else

Command Sequence ZZ

Fi

Cases

Create a/root/foo.sh script on Server0

1) When running/root/foo.sh Redhat, the output is Fedora

2) When running/root/foo.sh fedora, output is Redhat

3) when no parameters or parameters are not redhat or

When fedora, its error output produces the following information:

/root/foo.sh Redhat|fedora

Initial wording:

#!/bin/bash

If [$#-eq 0];then

Echo '/root/foo.sh Redhat|fedora '

Exit

elif [= = Redhat];then

echo Fedora

Exit

elif [= = = Fedora];then

Echo Redhat

Exit

Else

Echo '/root/foo.sh Redhat|fedora '

Fi

After improvement:

#!/bin/bash

If ["$" = = Redhat];then # $ No value is not assigned when no comparison can be made to error

echo Fedora# "$" will be given a null value when it is not assigned a value state can be compared

Exit# changed from $ to "$" to reduce a null value

elif ["$" = = Fedora];then

Echo Redhat

Exit

Else

Echo '/root/foo.sh redhat|fedora ' >&2# >&2 defines the output of echo as an error output

Exit 1#退出并将返回值改为1----echo $? is 1

Fi

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For loop structure

Loop structure: The repeated execution of the statement, loop to execute

For variable name in value list

Do

Command sequence

Done

Cases

Write a bulk Add user script

Create a/root/batchusers script on Server0

1 This script requires that the user Name list file be supplied as a parameter

2 If no arguments are provided, this script should give a hint

Usage:/root/batchusers, exit and return the corresponding value

3 If you provide a file that does not exist, this script should give a hint

Input file not found, exit and return the corresponding value

4 new User Login Shell is/bin/false, no need to set password

#!/bin/bash

If [$#-eq 0];then

Echo ' Usage:/root/batchusers ' >&2

Exit 1

Fi

If [-F $];then

For A in $ (cat $)

Do

Useradd-s/bin/false $a

Echo $A created successfully

Done

Else

Echo ' Input file not found ' >&2

Exit 1.

Fi

Shell Script Basics