Shell Scripting Lesson II ·

The shell file contains

The shell can also contain external scripts in the following syntax format:

. FileName

Or

SOURCE filename

For example, create a two shell script.

Script 1:test1.sh

url = "Www.baidu.com"

Script 2:test2.sh

. ./test1.sh

echo "$url"

You can see the results by executing the test2.sh.

Shell input and output redirection

Command	Description
Command>file	REDIRECT output to file
Command<file	redirect input to File
Command>>file	Redirects the output to file in Append mode
N>file	redirect files with file descriptor N to file
N>>file	Redirects files with file descriptor n appended to file
N>&m	Merges the output file m and n
N<&m	Merges the input file M and n
<<tag	Enter the content between tag tag and end tag as input

Note: File descriptor 0 is typically standard input (STDIN), 1 is standard output (STDOUT), and 2 is standard error output (STDERR)

Redirect in-depth explanation

In general, each Unix/linux command will open three files when it is run:

• Standard input file (stdin): stdin file descriptor for 0,unix program reads data from stdin by default.
• Standard output file (stdout): StdOut file descriptor for 1,unix program defaults to stdout output data
• Standard error file (stderr): The stderr file descriptor for the 2,unix program writes an error message to the stderr stream.

By default, Command>file redirects stdout to File,command<file to redirect stdin to file.

If you want stderr to be redirected to file, you can write:

Command 2>file #2表示标准错误文件

Command 2>>file #追加 2 indicates a standard error file

If you want to redirect stdout and stderr to file after merging, you can write

Command > File 2>&1

Command >> file 2>&1

Shell functions

Format

[Function] funname [()]

{

Action

[Return int;] #return后跟数值n (0-255)

}

Example

Demofun () {

echo "This is my first Shell function"

}

echo "--start--"

Demofun

echo "--end--"

Example: With return

Funwithretun () {

echo "Enter first number"

Return Anum

echo "Enter a second number"

Return Bnum

Return $ (($aNum + $bNum))

}

Funwithreturn

echo "The sum of the two numbers entered is $?" #函数返回值在调用该函数后通过 $? To get

Example: function parameters

Funwithparam () {

echo "The first parameter is $" #$1 represents the first argument, and the value of $ is the second argument, obtaining a value greater than or equal to 10 parameters requires ${n}

echo "Fifth parameter is $ $"

echo "Tenth parameter is ${10}"

echo "14th parameter is ${14}"

echo "All Parameters $*"

}

Funwithparam 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

Parameter handling	Description
$#	The number of arguments passed to the script
$*	Displays all parameters passed to the script in a single string
$$	The current process ID number for the script to run
$!	ID number of the last process running in the background
[Email protected]	Same as $*, but quoted when used, and returns each parameter in quotation marks
$-	Displays the current options used by the shell, same as the SET command function
$?	Displays the exit status of the last command. 0 means no error, and any other value indicates an error

Shell Process Control

If Else

Format:

If condition

Then

Command1

Command2

Fi

If else-if Else

Format

If Condition1

Then

Command1

Elif Condition2

Then

Command2

Else

CommandN

Fi

For loop

Format

For Var in item1 item2 ... itemn

Do

Command1

...

CommandN

Done

Example

For loop in 1 2 3 4 5

Do

echo "The value is: $loop"

Done

While

Format

While condition

Do

Command

Done

Example

Int=1

while (($int <=5))

Do

Echo $int

Let "int++"

Done

Infinite loops

While:

Do

Command

Done

While True

Do

Command

Done

Until cycle

Until condition

Do

Command

Done

Case

Case value in

Mode 1)

Command1

Command2

...

CommandN

;;

Mode 2)

Command1

Command2

...

CommandN

;;

Esac

Jump out of the loop

Break

Continue

Shell Scripting Lesson II ·