Basic usage of shell script

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This article is adapted from https://www.runoob.com/linux/linux-shell.html
Although the tutorial has been well written , But if you don't copy it yourself, it's like you didn't learn . Or are you used to sorting things into your own order , This is convenient for later search and use .
in addition , On this basis, I will add my own examples and details .

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Basics

Interpreter

$ which bash
/bin/bash
$ which sh
/bin/sh

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Extension

It can be .sh, It can also be .shell

The extension does not affect script execution , See the name and know the meaning

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function shell

The way 1： As an executable

chmod +x test.sh
./test.sh

The first line must be correct , Such as #!/bin/bash, So that the system can find the correct interpreter .

add ./ Represents finding... In the current directory test.sh This script ;
If directly executed /test.sh, Will go to PATH In looking for （ There is no the ）.

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The way 2： As interpreter parameters

/bin/sh test.sh
/bin/php test.php

Scripts that run this way , You don't need to specify interpreter information on the first line , It's no use writing .

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notes

• With # The first line is the comment , Will be ignored by the interpreter .

#--------------------------------------------
#  This is a comment 
# author： Novice tutorial 
# site：www.runoob.com
# slogan： It's not just technology that you learn , Is the dream ！
#--------------------------------------------

#####  User configuration area   Start  #####
#
#
#  Here you can add script description information 
# 
#
#####  User configuration area   end  #####

:<<EOF  The comment ...  The comment ...  The comment ... EOF

# EOF  You can also use other symbols :
:<<'  The comment ...  The comment ...  The comment ... '

:<<!
 The comment ...
 The comment ...
 The comment ...
!

---

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characteristic

sh and bash Some features compared with other programming languages ：

• Function can only return strings , Cannot return array ;
• Object oriented... Is not supported ;
• interpreted ;

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Variable

Definition

a="abc"

• Definition time , Variable name without dollar sign （$,PHP Variable needs in language ）
• There can be no spaces between variable names and equal signs
• The second assignment , No need to add $, Only when calling

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assignment

The above definition uses Direct copy , You can also assign values to variables with statements

#  take  /etc  The file names in the following directory are looped out .
for file in `ls /etc`
    

for file in $(ls /etc)

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Call variables

Use a defined variable , To prefix the variable name with a dollar sign

a="abc"
echo $a
echo ${a} 

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{}

Curly brackets are optional , Add it or not , Plus for Help interpreter Identify the boundaries of variables
Curly brackets are recommended for all variables

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A read-only variable readonly

readonly Commands can define variables as read-only variables , The value of a read-only variable cannot be changed

#!/bin/bash
myUrl="https://www.google.com"
readonly myUrl
myUrl="https://www.runoob.com" #  Will report a mistake  

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Delete variables

unset variable_name

• Variable cannot be used again after being deleted .
• unset Command cannot delete read-only variables .

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Variable type

function shell when , There will be three variables at the same time ：

• local variable
• environment variable
• shell Variable

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character string

name='Alice'
 
#  String concatenation 
str1='hello, '$name' !'
str2='hello, ${name} !'
str3="Hello, \"$name\"!"
str4="hello, ${name} !" 

echo $str1  $str2  $str3  $str4 

About Quotes

• Strings can be in single quotes , You can also use double quotes , You can also use no quotes .
• In single quotation marks , Any character will be output as it is , The variables are invalid ;
• A single quotation mark cannot appear in a single quotation mark string , You can't escape a single quotation mark ; But in pairs , Use as string concatenation .
• You can have variables in double quotes
• Escape characters can appear in double quotes

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String manipulation

#  Get string length 
string="abcd"
echo ${
    #string} # Output  4

#  Extract substring ; The index value of the first character is  0. 
echo ${string:1:3} #  Output  bcd

#  Find substrings ; Use backquotes instead of single quotes  
echo `expr index "$string" b` #  Output  1
echo `expr index "$string" cd` #  Output  2

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Array

• bash Supports one dimensional array （ Multidimensional arrays are not supported ）, And there's no limit to the size of the array .
• Use parentheses to indicate , Element use " Space " Symbol split
• The subscript of the element is 0 Start

Definition

stay Shell in , Use parentheses to represent arrays , For array elements " Space " Symbol split . The general form is ：

#  Array name =( value 1  value 2 ...  value n)
#  Such as 

array_name=(value0 value1 value2 value3)

#  or 
array_name=(
value0
value1
value2
value3
)

#  Define each component of the array separately ：
array_name[0]=value0
array_name[1]=value1
array_name[n]=valuen

---

Concrete example

my_array=(A B "C" D)
echo " The first element is : ${my_array[0]}"

#  assignment  
my_array[2]=C

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

---

Read array

Format

${ Array name [ Subscript ]}

#  Such as ：
valuen=${array_name[n]}

#  Use  @  The symbol can get all the elements in the array 
echo ${array_name[@]}

---

Gets the length of the array

#  Get the number of array elements 
length=${
    #array_name[@]}

#  perhaps 
length=${
    #array_name[*]}

#  Get the length of a single element of an array 
lengthn=${
    #array_name[n]}

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Pass parameters

The format of obtaining parameters in the script is ：$n.
n Represents a number ,1 For executing the first parameter of the script ,2 For executing the second parameter of the script , And so on

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

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Special characters for processing parameters ：

Processing parameters	explain
$#	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 $\ast phaseSame\; as,butyessendusewhenAddleadNumber,andstayleadNumberinreturnreturnEvery\; timeindividualginsengCount.Such\; as"$@“ use 「”」 In a nutshell 、 With "$1" “$2"\dots "$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 .

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$* 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 123 （ Three parameters are passed ）.

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echo "Shell  Pass parameter instance ！";
echo " The first parameter is zero ：$1";

echo " The number of parameters is ：$#";
echo " The passed parameter is displayed as a string ：$*";

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

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

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Operator

Like any programming language , Support for multiple operators

• Arithmetic operator
• Relational operator
• Boolean operator
• String operators
• File test operators

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Arithmetic operations

Native bash Simple mathematical operations are not supported , But you can do it with other commands , for example awk and expr,expr The most commonly used .
expr Is an expression calculation tool , It can be used to evaluate expressions .
Add two numbers , Note the use of back quotes , Not single quotes '：

val=`expr 2 + 2`
echo " The sum of the two is  : $val"
[ $a != $b ] #  return  true.


a=10
b=20

val=`expr $a + $b`  # 30

val=`expr $a - $b`  # -10

val=`expr $a \* $b` # 200

val=`expr $b / $a`  # 0

val=`expr $b % $a`  # 2

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

• Space between expression and operator , for example 2+2 It is not right , Must be written as 2 + 2
• The complete expression is to be Two back quotes contain
• Conditional expressions should be placed between square brackets , And there should be spaces , for example : [$a==$b] It's wrong. , Must be written as [ $a == $b ].
• Multiplication sign * There must be a backslash in the front \ To achieve multiplication ;
• stay MAC in shell Of expr Grammar is ：$(( expression )), In this expression * You don't need escape symbols \ .

---

Relationship between operation

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

explain	give an example
-eq	Check whether two numbers are equal , Equal return true.	[ $a -eq $b ] return false.
-ne	Check if two numbers are not equal , Unequal return true.	[ $a -ne $b ] return true.
-gt	Check whether the number on the left is greater than that on the right , If it is , Then return to true.	[ $a -gt $b ] return false.
-lt	Check if the number on the left is less than the number on the right , If it is , Then return to true.	[ $a -lt $b ] return true.
-ge	Check whether the number on the left is equal to or greater than the number on the right , If it is , Then return to true.	[ $a -ge $b ] return false.
-le	Check whether the number on the left is less than or equal to the number on the right , If it is , Then return to true.	[ $a -le $b ] return true.

---

Boolean operation

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Operator	explain	give an example
!	Non operation , Expression for true Then return to false, Otherwise return to true.	[ ! false ] return true.
-o	Or operations , There is an expression for true Then return to true.	[ $a -lt 20 -o $b -gt 100 ] return true.
-a	And operation , Both expressions are true To return to true.	[ $a -lt 20 -a $b -gt 100 ] return false.

---

Logical operations

---

Operator	explain	give an example
&&	Logical AND	·[[ $a -lt 100 && $b -gt 100 ]]` return false
||	Logical OR	[[ $a -lt 100 || $b -gt 100 ]] return true

---

String operations

Assumed variable a by “abc”, Variable b by “efg”：

Operator	explain	give an example
=	Checks if two strings are equal , Equal return true.	[ $a = $b ] return false.
!=	Detect whether two strings are not equal , Unequal return true.	[ $a != $b ] return true.
-z	Check if the string length is 0, by 0 return true.	[ -z $a ] return false.
-n	Check whether the string length is not 0, Not for 0 return true.	[ -n "$a" ] return true.
$	Check if the string is empty , Not empty return true.	[ $a ] return true.

---

File test operation

file="/var/www/runoob/test.sh"
if [ -r $file ]
then
   echo " Documents are readable "
else
   echo " The file is unreadable "
fi

---

The file test operator is used to detect Unix Various attributes of the file .

Attribute detection is described as follows ：

The operator	explain	give an example
-b file	Check if the file is a block device file , If it is , Then return to true.	[ -b $file ] return false.
-c file	Check whether the file is a character device file , If it is , Then return to true.	[ -c $file ] return false.
-d file	Check if the file is a directory , If it is , Then return to true.	[ -d $file ] return false.
-f file	Check if the file is a normal file （ It's not a catalog , It's not a device file ）, If it is , Then return to true.	[ -f $file ] return true.
-g file	Check if the file is set SGID position , If it is , Then return to true.	[ -g $file ] return false.
-k file	Check whether the file is set with the adhesive bit (Sticky Bit), If it is , Then return to true.	[ -k $file ] return false.
-p file	Check if the file is a named pipeline , If it is , Then return to true.	[ -p $file ] return false.
-u file	Check if the file is set SUID position , If it is , Then return to true.	[ -u $file ] return false.
-r file	Check whether the file is readable , If it is , Then return to true.	[ -r $file ] return true.
-w file	Check whether the file is writable , If it is , Then return to true.	[ -w $file ] return true.
-x file	Check if the file is executable , If it is , Then return to true.	[ -x $file ] return true.
-s file	Check if the file is empty （ Is the file size greater than 0）, Not empty return true.	[ -s $file ] return true.
-e file	Test files （ Including directory ） Whether there is , If it is , Then return to true.	[ -e $file ] return true.

---

Other checkers ：

• -S: Determine whether a file is socket.
• -L: Check whether the file exists and is a symbolic link .

---

echo command

# 1. Show normal string :
echo "It is a test" # 
echo It is a test #  The effect same as above 

# 2. Show escape characters 
echo "\"It is a test\""   # "It is a test"

# 3. Show variable 
read name #  Read a line from standard input , And assign the value of each field of the input line to  shell  Variable 
echo "$name It is a test"

# 4. Show wrap 
echo -e "OK! \n" # -e  Open escape 
echo "It is a test" 

# 5. Show no line breaks 
echo -e "OK! \c" # -e  Open escape  \c  Don't wrap 
echo "It is a test"  # OK! It is a test

# 6. Show results directed to file 
echo "It is a test" > myfile

# 7. Output string as is , Do not escape or take variables ( single quote )
echo '$name\"' # $name\"

# 8. Show command execution results ; Use backquotes 
echo `date` 

---

printf command

• from POSIX Standard definition , So it's better than using echo Good portability .
• Use Reference text or Space Separated parameters
• It can be outside printf Using format string , You can also specify the width of the string 、 Left and right alignment, etc .
• Don't like echo Automatically add line breaks , We can add... Manually \n.

Format ：

printf  format-string  [arguments...]

---

printf "%-10s %-8s %-4s\n"  full name   Gender   weight kg  
printf "%-10s %-8s %-4.2f\n"  Guo Jing   male  66.1234 
printf "%-10s %-8s %-4.2f\n"  Yang2 guo4   male  48.6543 
printf "%-10s %-8s %-4.2f\n"  Guo Fu   Woman  47.9876

#  Output results 
 full name       Gender     weight kg
 Guo Jing       male       66.12
 Yang2 guo4       male       48.65
 Guo Fu       Woman       47.99

• %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 .
• %-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 .

---

# format-string Is double quotation mark 
printf "%d %s\n" 1 "abc"

#  Single quotation marks have the same effect as double quotation marks  
printf '%d %s\n' 1 "abc" 

#  You can output without quotation marks 
printf %s abcdef

#  The format specifies only one parameter , But the extra parameters will still be output in this format ,format-string  Be reused 
printf %s abc def

printf "%s\n" abc def

printf "%s %s %s\n" a b c d e f g h i j

#  without  arguments, that  %s  use NULL Instead of ,%d  use  0  Instead of 
printf "%s and %d \n" 

---

Process control

sh The process control of cannot be empty

<?php
if (isset($_GET["q"])) {
    
    search(q);
}
else {
    
    //  Not doing anything 
}

stay sh/bash You can't write that in , If else Branch has no statement execution , Don't write this else.

---

if

Format

if condition
then
    command1 
    command2
    ...
    commandN 
fi

---

Write in a line （ For terminal command prompt ）：

if [ $(ps -ef | grep -c "ssh") -gt 1 ]; then echo "true"; fi

---

if else

if condition
then
    command1  
    ...
    commandN
else
    command
fi

---

if else-if else

if condition1
then
    command1
elif condition2 
then 
    command2
else
    commandN
fi

---

Example ：

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

---

Write in a line ：

for var in item1 item2 ... itemN; do command1; command2… done;

---

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

for str in This is a string
do
    echo $str
done
'''
This
is
a
string
'''

---

while sentence

while condition
do
    command
done

---

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

---

Infinite loop

while :
do
    command
done

---

until loop

until Loop through a series of commands until the condition is true Stop when .
until Circulation and while The cycle is the opposite of the way it's handled .
commonly while Circulation is better than until loop , But sometimes — Only in a few cases ,until Circulation is more useful .
until Grammar format :

until condition
do
    command
done

---

Output 0 ~ 9 The number of ：

a=0

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

---

Out of the loop

• break: Jump out of all loops
• continue: Jump out of current loop

---

function

Format

[ function ] funname [()]
{
    
    action;
    [return int;]
}

• You can take function fun() Definition , It can also be direct fun() Definition , Without any parameters .
• Parameter return , Can display plus ：return return , If not , Results will be run with the last command , As return value .return Heel value n(0-255）

---

Example ：

demoFun(){
    
    echo " This is my first  shell  function !"
}

#  call  
demoFun

#  With parameters 
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} !" #  When n>=10 when , Need to use ${n} To get the parameters .
    echo " The eleventh parameter is  ${11} !"
    echo " The total number of parameters is  $#  individual !"
    echo " Output all parameters as a string  $* !"
}
funWithParam 1 2 3 4 5 6 7 8 9 34 73

---

Special characters for processing parameters ：

Processing parameters	explain
$#	Number of arguments passed to script or function
$*	Display all the parameters passed to the script in a single string
$$	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 .
$-	Show Shell Current options used , And set Same command function .
$?	Display the exit status of the last command .0 No mistakes , Any other value indicates an error .

---

---

Input / Output redirection

An order is usually called The standard input Where to read input , Output is written to standard Quasi output , By default , This happens to be your terminal .

Redirect command list ：

command	explain
command > file	Redirect output to file.
command < file	Redirect input to file.
command >> file	Redirect the output as an append to file.
n > file	Set the file descriptor to n To file.
n >> file	Set the file descriptor to n The file is redirected to by appending file.
n >& m	The output file m and n Merge .
n <& m	The input file m and n Merge .
<< tag	Will start marking tag And closing marks tag Between the content as input .

---

We need to pay attention to , File descriptor ：

• 0 It's usually standard input （STDIN）
• 1 It's standard output （STDOUT）
• 2 Standard error output （STDERR）.

---

#  Output redirection 
command1 > file1

#  Input redirection 
command1 < file1

• Output redirection will overwrite the contents of the file

---

Example

$ echo " Novice tutorial ：www.runoob.com" > users

---

In depth understanding of Redirect

In general , Every Unix/Linux The command runs with three files open ：

• Standard input file (stdin)：stdin The file descriptor of is 0,Unix The program defaults from stdin Reading data .
• Standard output file (stdout)：stdout The file descriptor of is 1,Unix The program defaults to stdout Output data .
• Standard error file (stderr)：stderr The file descriptor of is 2,Unix The program will go to stderr Write error messages in the stream .

By default ,command > file take stdout Redirect to file,command < file take stdin Redirect to file.

---

stderr

# stderr  Redirect to  file
$ command 2>file

# stderr  Append to  file  end of file 
$ command 2>>file

#  take  stdout  and  stderr  Redirect to after merge  file
$ command > file 2>&1
#  perhaps 
$ command >> file 2>&1

#  Yes  stdin  and  stdout  All redirects （ take  stdin  Redirect to  file1, take  stdout  Redirect to  file2.）
$ command < file1 >file2

---

Here Document

Here Document yes Shell A special way of redirection , Used to redirect input to an interactive Shell Script or program .

---

Format ：

command << delimiter document delimiter

• The function is to combine two delimiter Content between (document) Pass as input to command.
• At the end of the delimiter Be sure to write at the top , There can't be any characters in front of it , There can't be any characters after it , Including spaces and tab Indent .
• At the beginning delimiter Before and after the space will be ignored .

---

Call in terminal

$  wc -l << EOF
>  Hello 
>  Welcome to 
>  Mohe dance hall 
> EOF
       3

---

Use in script

cat << EOF  Welcome to   Novice tutorial  www.runoob.com EOF

---

/dev/null file

• /dev/null It's a special document , Everything written to it will be discarded ;
• If you try to read from the file , So you can't read anything .
• however /dev/null Documents are very useful , Redirect the output of the command to it , It's going to work " No output " The effect of .
• If you want to execute a command , But I don't want to display the output on the screen , So you can redirect the output to /dev/null .

---

If you want to shield stdout and stderr, It can be written like this ：

$ command > /dev/null 2>&1

• there 2 and > There can be no spaces between ,2> It means error output only when it is integrated .

---

References

Shell You can also include external scripts

#  Grammar format 
. filename   #  Note the number (.) There is a space between the file name and the file name 

#  or 

source filename

---

example
Create two shell Script files , test1.sh and test2.sh

---

test1.sh The variable is declared in

name='hello'

---

test2.sh Call in test1.sh

#  Use  .  Number test1.sh  file 
. ./test1.sh

#  Or use the following include file code 
# source ./test1.sh

echo "my name is ：$name"

---

Linux The command of
https://www.runoob.com/linux/linux-command-manual.html

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2022-02-15( Two )