Shell Scripting

Shell Programming

Once in a while Linux Write something simple in the environment Shell Script , This article can be used as a reference , Write some simple scripts .

Grammatical norms

Define the execution environment

#!/usr/bin/bash
 perhaps 
#!/usr/bin/env bash | python |perl

perform

 Modify the permissions 
chmod +x ./demo.sh
 perhaps 
chmod 700 ./demo.sh

./demo.sh
 perhaps 
bash ./demo.sh
sh ./demo.sh
#  adopt bash sh  There is no need to modify permissions , Specify the execution environment 

Special symbols

Symbol	explain
~	Home directory
-	Upper level directory
!!	Execute the last order
$	Get environment variables ${USER}
±*/%	Operator
*	wildcard Multiple
？	One
|	Pipe,
\	Escape character
>	Redirect （echo abc > ./demo.txt）
>>	Redirect （echo abc >> ./demo.txt） Additional
<	wc < abc.txt
<<

notes

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

Variable

#  Defining variables 
name="zhangsan"
age=18
#  read-only 
readonly name
#  Delete variables 
unset age
#  visit 
echo ${name}
#  Get string length 
echo ${
    #name}
#  Extract substring 
echo ${string:0:2} #  Output  unoo

Array

arr=("a" "b" "c" "d")
echo arr[0]
#  Get all 
echo arr[@]
echo arr[*]
#  Gets the length of the array 
length=${
    #arr[@]}

operation

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 .

$(( Arithmetic expression )) or $[ Arithmetic expression ], More efficient .

#!/bin/bash

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

# 1.(())
echo $((1+1))
# 2.let
let a+=6 c=a+b
#  Be careful , For similar let x+y This way of writing ,Shell  Although the calculation  x+y  Value , But discard the result ; If you don't want to , have access to let sum=x+y take  x+y  The result of is saved in the variable  sum  in .

read Read console input

Two points attention ：

• Space between expression and operator , for example 2+2 It is not right , Must be written as 2 + 2, This is different from most programming languages we are familiar with .

• The complete expression is to be `` contain , Note that this character is not a common single quotation mark , stay Esc Key bottom .

• 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 ;

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.

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

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 .

Process control

if

if condition1
then
    command1
elif condition2 
then 
    command2
else
    commandN
fi

for

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

for((i=1;i<=5;i++))
do
    echo " This is the first  $i  Secondary call ";
done;

while

while condition
do
    command
done

#  Example 
#!/bin/bash
int=1
while(( $int<=5 ))
do
    echo $int
    let "int++"
done

File contains

adopt . Import the files that need to be included