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Is Shell Scripting really a big technology?

2022-06-12 14:00:00 【Oriental invincible is me】

In this paper, shell Script knowledge . It is best to have before study linux Command knowledge reserve .

After reading an article , The next time you look for a job, please write on your resume shell Script , bolt Q.

shell What is a script ？

shell A script is a script that contains shell Command script , Used to say linux command , You could say yes shell command .

function shell Script , It can realize such as installing software , Update software , execute , Restart the software, etc . such as , Install and deploy a package , It needs to be implemented shell Script . This shell Scripts are usually written by development engineers .

shell The script suffix is .sh, Such as a.sh, To execute shell Script , Just three steps

1、 create a file , Write the content of the document . Such as a.sh

2、 Make the script executable chmod +x a.sh

3、 Execute the script Execute... On the command line ./a.sh

Shell Variable

Variable naming

Such as your_name="runoob.com"

Only English letters can be used for naming , Numbers and underscores , The first character cannot begin with a number .
No spaces in between , You can use underscores _.
You can't use punctuation .
Out of commission bash Keywords in （ You can use help Command to view reserved keywords ）

Reference variables $yourname perhaps ${yourname} Curly braces are optional .

shell character string

Strings can be in single quotes , You can also use double quotes , You can also use no quotes .

str='this is a string'

Get string length ：

string="abcd"
echo ${#string}

Add... Before the string variable #

Extract substring

From the string No 2 Characters begin to intercept 4 Characters ：

string="runoob is a great site"
echo ${string:1:4} # Output unoo
Be careful ： The index value of the first character is 0.

shell Array

 Array name =( value 1  value 2 ...  value n)

The subscripts of array elements are 0 Numbered starting .

The general format for reading array element values is ：

${ Array name [ Subscript ]}

Use @ The symbol can get all the elements in the array , for example ：

echo ${array_name[@]}

shell Of echo command

Used to output strings to the command line

echo " Hello "

Note that the space is echo The space after must have

Show escape characters

echo "\"It is a test\""

You need to use a backslash before the character

Show variable

echo $ Variable name

Such as echo $javahome

Show wrap

echo -e "OK \n"

among -e To open the transfer

No, -e Output effect

Output the results to a file

echo " Hello " > file name

File does not exist automatically created , If exist , The original content will be cleared and new content will be written .

Show command execution results

echo `date`

Show date Results of command

shell Pass parameters

adopt $n towards shell Script pass through parameters $0 Indicates the path of the execution file

Example ： file a.sh The script is as follows ：

echo "Shell Pass parameter instance !";
echo " File name of execution :$0";
echo " The first parameter is zero :$1";
echo " The second parameter is :$2";
echo " The third parameter is zero :$3";

perform ：

When entering the execute file command , Write parameters , Pass it into the script and output .

$#	The number of parameters passed to the script
$*	Display all the parameters passed to the script in a single string . Such as "$*" use 「"」 In a nutshell 、 With "$1 $2 … $n" Output all parameters in the form of .
$$	The current process the script runs ID Number
$!	Of the last process running in the background ID Number
[email protected]	And $* identical , But use quotation marks , And return each parameter in quotation marks . Such as "[email protected]" use 「"」 In a nutshell 、 With "$1" "$2" … "$n" Output all parameters in the form of .
$-	Show Shell Current options used , And set command Function the same .
$?	Display the exit status of the last command .0 No mistakes , Any other value indicates an error .

Example ：

echo "Shell Pass parameter instance !";
echo " The number of parameters passed to the script :$#"
echo " File name of execution :$0";
echo " The first parameter is zero :$1";
echo " A single string shows all the parameters passed to the script :$*";
echo " The process in which the script runs ID:$$";
echo " Of the last process running in the background ID:$!"
echo " Output all parameters passed to the script :#@";
echo " Show shell Current options used :$-";
echo " Displays the last exit status of the command :$?";

perform ：

$* And [email protected] difference ：

The same thing ： All references to all parameters .
Difference ： Only in double quotes . Suppose you write three parameters when the script runs 1、2、3,, be " * " Equivalent to "1 2 3"（ Passed a parameter ）, and "@" Equivalent to "1" "2" "3"（ Three parameters are passed ）.

echo "-- \$* demonstration ---"
for i in "$*"; do
echo $i
done

echo "-- \[email protected] demonstration ---"
for i in "[email protected]"; do
echo $i
done

Shell Array

Only one-dimensional arrays are supported , Multidimensional arrays are not supported .

From the subscript 0 Start , Elements are separated by spaces

Define an array

Mode one ： Array name =(value0 value1 value2...)

Mode two ： Array name [0]=value0

Little surprise ：vi In command mode ：%d The contents of the file can be emptied

Read array

The general format for reading array element values is ：

${array_name[index]}

my_array=(A B "C" D)

echo " The first element is : ${my_array[0]}"
echo " The second element is : ${my_array[1]}"
echo " The third element is : ${my_array[2]}"
echo " The fourth element is : ${my_array[3]}"

Get all the elements in the array

Use @ or * You can get all the elements in the array

my_array[0]=A
my_array[1]=B
my_array[2]=C
my_array[3]=D

echo " The elements of the array are : ${my_array[*]}"
echo " The elements of the array are : ${my_array[@]}"

Gets the length of the array ：

echo " The number of array elements is : ${#my_array[*]}"
echo " The number of array elements is : ${#my_array[@]}"

Shell printf command

default printf Don't like echo Automatically add line breaks , We can add... Manually \n.

printf  format-string  [arguments...]

Parameter description ：

format-string: Control string for format
arguments: For parameter list .

echo "Hello, Shell"
printf "Hello, Shell\n"

in addition printf You can format strings

%s %c %d %f It's all format substitutions ,％s Output a string ,％d Integer output ,％c Output a character ,％f Output real number , Output in decimal form .

Shell Basic operators

Arithmetic operator

Other commands such as awk expr

#!/bin/bash

val=`expr 2 + 2`
echo " The sum of the two is : $val"

Note that this is a back quote, not a single quote There must be a space between numbers and operators

a=10
b=20

val=`expr $a + $b`
echo "a + b : $val"

val=`expr $a - $b`
echo "a - b : $val"

val=`expr $a \* $b`
echo "a * b : $val"

val=`expr $b / $a`
echo "b / a : $val"

val=`expr $b % $a`
echo "b % a : $val"

if [ $a == $b ]
then
echo "a be equal to b"
fi
if [ $a != $b ]
then
echo "a It's not equal to b"
fi

Relational operator

a=10
b=20

if [ $a -eq $b ]
then
echo "$a -eq $b : a be equal to b"
else
echo "$a -eq $b: a It's not equal to b"
fi
if [ $a -ne $b ]
then
echo "$a -ne $b: a It's not equal to b"
else
echo "$a -ne $b : a be equal to b"
fi
if [ $a -gt $b ]
then
echo "$a -gt $b: a Greater than b"
else
echo "$a -gt $b: a No more than b"
fi
if [ $a -lt $b ]
then
echo "$a -lt $b: a Less than b"
else
echo "$a -lt $b: a Not less than b"
fi
if [ $a -ge $b ]
then
echo "$a -ge $b: a Greater than or equal to b"
else
echo "$a -ge $b: a Less than b"
fi
if [ $a -le $b ]
then
echo "$a -le $b: a Less than or equal to b"
else
echo "$a -le $b: a Greater than b"
fi

remarks ：shell Scripts are space sensitive , Such as if There must be a space after

Boolean operator

Logical operators

String operators

File test operators

if [ -r $file ]
then
echo " Documents are readable "
else
echo " The file is unreadable "
fi
if [ -w $file ]
then
echo " Documents can be written "
else
echo " The document is not writable "
fi
if [ -x $file ]
then
echo " The document is executable "
else
echo " The file is not executable "
fi
if [ -f $file ]
then
echo " The file is a normal file "
else
echo " Documents are special documents "
fi
if [ -d $file ]
then
echo " A file is a directory "
else
echo " File is not a directory "
fi
if [ -s $file ]
then
echo " The file is not empty "
else
echo " The file is empty "
fi
if [ -e $file ]
then
echo " File exists "
else
echo " file does not exist "
fi

Shell Process control

if sentence

if condition
then
    command1 
    command2
    ...
    commandN 
fi

if-else

if condition
then
    command1 
    command2
    ...
    commandN
else
    command
fi

if else -if else

if condition1
then
    command1
elif condition2 
then 
    command2
else
    commandN
fi

Determine whether two variables are equal

a=10
b=20
if [ $a == $b ]
then
echo "a be equal to b"
elif [ $a -gt $b ]
then
echo "a Greater than b"
elif [ $a -lt $b ]
then
echo "a Less than b"
else
echo " There are no conditions that meet "
fi

for loop

for var in item1 item2 ... itemN
do
    command1
    command2
    ...
    commandN
done

in The list can contain substitutions 、 String and file name .

for loop in 1 2 3 4 5
do
echo "The value is: $loop"
done

while sentence

while condition
do
    command
done

int=1
while(( $int<=5 ))
do
echo $int
let "int++"
done

while Loops can be used to read keyboard information .

Infinite loop

Infinite loop syntax format ：

while :
do
    command
done

until loop

until Loop through a series of commands until the condition is true Stop when .

until condition
do
    command
done

Use until Command to output 0 ~ 9 The number of ：
#!/bin/bash

a=0

until [ ! $a -lt 10 ]
do
echo $a
a=`expr $a + 1`
done

case ... esac

amount to switch case sentence , Multi branch selection structure

Every case The branch begins with a right parenthesis , Use two semicolons ;; Express break, The end of execution , Jump out of the whole case ... esac sentence ,esac（ Namely case In turn, ） As the closing tag .

It can be used case Statement matches a value with a pattern , If the match is successful , Execute the matching command .

Value must be followed by word in, The first mock exam must be closed with right brackets. . Value can be variable or constant , Match found that after the first mock exam meets a certain pattern, , During this period, all commands are executed until ;;.

Value will detect every matching pattern . Once the patterns match , After the corresponding command of matching mode is executed, other modes will not be continued . If there is no matching pattern , Use the asterisk * Capture the value , Then execute the following command .

case  value  in
 Pattern 1)
    command1
    command2
    ...
    commandN
    ;;
 Pattern 2)
    command1
    command2
    ...
    commandN
    ;;
esac

The following script prompts for 1 To 4, Match with each pattern ：
echo ' Input 1 To 4 Number between :'
echo ' The number you enter is :'
read aNum
case $aNum in
  1)  echo ' You chose 1'
  ;;
  2)  echo ' You chose 2'
  ;;
  3)  echo ' You chose 3'
  ;;
  4)  echo ' You chose 4'
  ;;
  *)  echo ' You didn't enter 1 To 4 Number between '
  ;;
esac

Out of the loop

break command

break The command allows you to jump out of all loops （ Terminate all loops after execution ）.

The following script is a loop , When the input is not 1 To 5 Between the numbers when jumping out of the loop .

#!/bin/bash
while :
do
  echo -n " Input 1 To 5 Number between :"
  read aNum
  case $aNum in
  1|2|3|4|5) echo " The number you enter is $aNum!"
  ;;
  *) echo " The number you entered is not 1 To 5 Between ! Game over "
  break
  ;;
  esac
done

continue

continue Command and break Command similar , There's only one difference , It doesn't jump out of all loops , Just jump out of the current loop .

#!/bin/bash
while :
do
  echo -n " Input 1 To 5 Number between : "
  read aNum
  case $aNum in
  1|2|3|4|5) echo " The number you enter is $aNum!"
  ;;
  *) echo " The number you entered is not 1 To 5 Between !"
  continue
  echo " Game over "
  ;;
  esac
done

Run code discovery , When the input is greater than 5 Digital time , The loop in this case does not end , sentence echo " Game over " It will never be carried out .

Shell test command

Shell Medium test The command is used to check if a condition holds , It can do numerical 、 Character and file testing .

The numerical test

num1=100
num2=100
if test $[num1] -eq $[num2]
then
echo ' Two numbers are equal ！'
else
echo ' The two numbers are not equal ！'
fi

In code [] Perform basic arithmetic operations

String test

num1="goodmorning"
num2="goodevening"
if test $num1 = $num2
then
echo " Two strings are equal "
else
echo " Two strings are not equal "
fi
if test $num1 != $num2
then
echo " Two strings are not equal "
else
echo " Two strings are equal "
fi
if test -z $num1
then
echo " The string length is zero "
else
echo " String length is not zero "
fi
if test -n $num1
then
echo " String length is not zero "
else
echo " The string length is zero "
fi

File test

if test -e a.sh
then echo " File exists "
else echo " file does not exist "
fi


if test -r a.sh
then echo " Documents are readable "
fi

if test -w a.sh
then echo " Documents can be written "
fi

if test -x a.sh
then echo " The document is executable "
fi

if test -s a.sh
then echo " The file exists and has at least one character "
fi


if test -d a.sh
then echo " The directory file exists "
fi

if test -f a.sh
then echo " The file exists and is a normal file "
fi

if test -c a.sh
then echo " The file exists and is a special character file "
fi

if test -b a.sh
then echo " File exists and is a block special file "
fi

shell function

stay shell Function can be defined in , And call .

demoFun(){
    echo " This is my first  shell  function !"
}
echo "----- The function starts executing -----"
demoFun
echo "----- The function is finished -----"

with return Function of

funWithReturn(){
    echo " This function will add two input numbers ..."
    echo " Enter the first number : "
    read aNum
    echo " Enter the second number : "
    read anotherNum
    echo " The two figures are  $aNum  and  $anotherNum !"
    return $(($aNum+$anotherNum))
}
funWithReturn
echo " The sum of the two numbers entered is  $? !"

$? You can get the return value of the function

Function parameter

stay Shell in , You can pass parameters to a function when you call it . Inside the body of the function , adopt $n To get the value of the parameter , for example ,$1 Represents the first parameter ,$2 Represents the second parameter ...

funWithParam(){
    echo " The first parameter is zero  $1 !"
    echo " The second parameter is  $2 !"
    echo " The tenth parameter is  $10 !"
    echo " The tenth parameter is  ${10} !"
    echo " The eleventh parameter is  ${11} !"
    echo " The total number of parameters is  $#  individual !"
    echo " Output all parameters as a string  $* !"
}
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