Shell: basic learning

Write it at the front

shell Scripting language is similar to chatting with several people . You just need to think of all the orders as people who can help you , As long as you ask them to do it in the right way .

~ % who am I
jdchi  ttys000 Jul 29 10:33 
~ % date
2021 year  7 month 29 Japan   Thursday  10 when 40 branch 27 second  CST

first Shell Script

#!/bin/bash
echo "Hello World"

#! It's a convention mark , Tell the system what interpreter this script needs

function Shell Script

chmod +x ./test.sh # Give the script permission to execute
./test.sh # Execute the script

./ Tell the system to find in the current directory , Otherwise the system will go PATH Look in the catalog .

Getting input from the keyboard

#!/bin/bash

echo "What's your name"
read NAME
echo "My name is $NAME"

Assign keyboard input to variables NAME

shell % chmod +x ./test2.sh
shell % ./test2.sh 
What's your name
JD
My name is JD

Multiline comment

:<<EOF  notes   notes   notes  EOF

Shell Variable

There can be no spaces between variable names and equal signs ！

A read-only variable

#!/bin/bash
myName="hello"
readonly myName
myName="hi"

./shell_var.sh: line 5: myName: readonly variable

Delete variables

#!/bin/bash
hello="hello"
unset hello
echo $hello

There is no output

Variable type

• local variable
  • Define... In a script or command , At present only shell Valid in instance , Other shell Started program cannot access local variables
• environment variable
  • All programs can be accessed , When necessary shell Scripts can also define environment variables
• shell Variable
  • shell Special variables for program settings

Shell Array

Supports one dimensional array

Define an array

Use parentheses to represent arrays , Separate... With spaces .

#!/bin/bash
array=(1 2 3 4)
echo ${array[0]}

Read array

# Read an element 
value=${array[1]}
# Read all the elements in the array 
echo ${array[*]}
echo ${array[@]}

Get array length

echo ${
    #array[*]}

Shell Pass parameters

When executing the script , Pass parameters to script , The format of getting parameters in the script is ：$n,n On behalf of the n Parameters .

$0 For the filename of the execution

#!/bin/bash
echo " The file named  $0"
echo " The first parameter is zero  $1"
echo " The second parameter is  $2"

shell % ./shell_args.sh aa bb
 The file named  ./shell_args.sh
 The first parameter is zero  aa
 The second parameter is  bb

Special characters

echo " The number of parameters is  $#"
echo " Display all parameters in a single string  $*"
echo " The current process  ID  by  $$"
echo " The last process running in the background  ID  by  $!"
echo "Shell  Currently used options  $-"
# 0  No mistakes 
echo " The exit state of the last command  $?"

Shell Basic operators

Native bash Simple mathematical operations are not supported , But it can be achieved by other commands , for example awk and expr.

Arithmetic operator

expr Is an expression calculation tool , It can be used to evaluate expressions .

#!/bin/bash

a=10
b=20

echo a = $a 
echo b = $b
echo a + b = `expr $a + $b`
echo a - b = `expr $a - $b`

shell % ./cal.sh
a = 10
b = 20
a + b = 30
a - b = -10

Relational operator

#!/bin/bash
a=10
b=20

echo a = $a
echo b = $b

#  Whether two numbers are equal 
if [ $a -eq $b ]
then
	echo a  And  b  equal 
else
	echo a  And  b  It's not equal 
fi
#  Whether the two numbers are not equal 
if [ $a -ne $b ]
then
	echo a  And  b  It's not equal 
else
	echo a  And  b  equal 
fi

#  Whether the number on the left is greater than that on the right 
if [ $a -gt $b ]
then
	echo a  Than  b  Big 
else
	echo a  Than  b  Small 
fi

#  Whether the number on the left is less than that on the right 
if [ $a -lt $b ]
then
	echo a  Than  b  Small 
else
	echo a  Than  b  Big 
fi

#  Whether the number on the left is greater than or equal to that on the right 
if [ $a -ge $b ]
then
	echo a  Greater than or equal to  b
else
	echo a  Less than  b
fi

#  Whether the number on the left is less than or equal to that on the right 
if [ $a -le $b ]
then
	echo a  Less than or equal to  b
else
	echo a  Greater than  b
fi

shell % ./rel_operator.sh
a = 10
b = 20
a  And  b  It's not equal 
a  And  b  It's not equal 
a  Than  b  Small 
a  Than  b  Small 
a  Less than  b
a  Less than or equal to  b

Boolean operator

#!/bin/bash
a=10
b=100

#  Non operation 
if [ a != b ]
then
	echo a  It's not equal to  b
else
	echo a  be equal to  b
fi

#  And operation 
if [ $a -eq 10 -a $b -eq 100 ]
then
	echo a  be equal to  10 b  be equal to  100
else
	echo a  or  b  Not equal to the initial value 
fi

#  Or operations 
if [ $a -eq 10 -o $b -eq 100 ]
then
	echo a  be equal to  10  or  b  be equal to  100
else
	echo a  and  b  Not equal to the initial value 
fi

shell % ./bool_operator.sh
a  It's not equal to  b
a  be equal to  10 b  be equal to  100
a  be equal to  10  or  b  be equal to  100

String operators

#!/bin/bash
a='abc'
b='def'

if [ $a = $b ]
then
	echo a  And  b  equal 
else
	echo a  And  b  It's not equal 
fi

if [ $a != $b ]
then
	echo a  And  b  It's not equal 
else
	echo a  And  b  equal 
fi

#  Check if the string length is  0
if [ -z $a ] 
then
	echo a  The length of is  0
else
	echo a  The length is not  0
fi

#  Check whether the string length is not  0
if [ -n $b ] 
then
	echo b  The length is not  0
else
	echo b  The length of is  0
fi

#  Check if the string is empty 
if [ $a ] 
then
	echo a  Not empty 
else
	echo a  It's empty 
fi

shell % ./string_operator.sh
a  And  b  It's not equal 
a  And  b  It's not equal 
a  The length is not  0
b  The length is not  0
a  Not empty 

File test operators

#!/bin/bash

file="cal.sh"

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  The 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 % ./file_operator.sh
 Documents are readable 
 Documents can be written 
 The document is executable 
 The file is a normal file 
 File is not a directory 
 The file is not empty 
 File exists 

Shell print command

printf Command imitation C library , Transplantability ratio echo good .

grammar ：printf format-string [arguments…]

printf "%-10s %-8s %-4.2f\n"  Guo Jing   male  66.1234

Guo Jing male 66.12

%-10s A width of 10 Characters （- Indicates left alignment , If not, it means right alignment ）, Any character will be displayed in 10 Within characters wide , If it is not enough, it will be automatically filled with spaces , More than will also show all the content .

%-4.2f It means to format it as a decimal , among .2 Finger retention 2 Decimal place .

Shell test command

test The command is used to check if a condition holds .

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

 Two numbers are equal ！

Shell Process control

for loop

Format

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

Example

#!/bin/bash
array=(1 2 3 4 5)
for i in ${array[@]}
do
	echo $i
done

shell % ./process_control.sh
1
2
3
4
5

while sentence

Format

while condition
do
    command
done

Example

#!/bin/bash
a=0
while (($a<=5))
do
	echo $a
	let "a++"
done

0
1
2
3
4
5

case … esac

Be similar to switch case, Every case The branch begins with a right parenthesis , Use two semicolons ;; Express break, use esac End mark .

Format

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

Example

#!/bin/bash
echo  Input number 
read num
case $num in
	  1) echo  I chose  1
    ;;
    2) echo  I chose  2
    ;;
    *) echo  Other selected 
    ;;
esac

Shell function

Format

[ function ] funname [()]

{
    

    action;

    [return int;]

}

Example

#!/bin/bash

fun(){
    
	echo  Custom function 
}

fun

funWithParam(){
    
	echo  The first parameter is zero  $1
	echo  The second parameter is  $2
}

funWithParam 1 3

funWithReturn(){
    
	return 1
}

funWithReturn2(){
    
	return 2
}

funWithReturn

echo  The return value is  $?

funWithReturn2

echo  The return value is  $?

shell % ./fun.sh
 Custom function 
 The first parameter is zero  1
 The second parameter is  3
 The return value is  1
 The return value is  2

Shell Input / Output redirection

majority UNIX The system command receives the input from your terminal and sends the output back to your terminal . A command usually reads input from a place called standard input , By default , This happens to be your terminal . Again , A command usually writes its output to standard output , By default , This is also your terminal .

Output redirection

shell % touch myfile
shell % who > myfile
shell % echo 'hello' > myfile
shell % cat myfile
hello
shell % echo 'hello again' >> myfile
shell % cat myfile                  
hello
hello again

Input redirection

shell % wc -l myfile 
       2 myfile
shell % wc -l < myfile
       2

Reference resources

Shell course