2020-09_ Shell Programming Notes

Chapter one Shell Script programming application guide

1. Study Shell The foundation of programming .

• vi/vim To use skillfully
• Linux Skilled use of common commands
• common Linux Network service deployment troubleshooting

2. Shell Introduction to the concept and principle of script .

shell Is a command interpreter . It's on the outer layer of the operating system , Be responsible for direct dialogue with users , Explain user input to the operating system , And handle the output of various operating systems , Output to screen return to user .

3. Simple and complex shell Script .

Example 1. eliminate /var/log Under the message Simple command script for log file .

#  Put all the commands in one file and pile them up to form a script .
#  To use  root  Identity to run the script 
#  Clear log script 
cd /var/log
cat /dev/null > messages
echo "Logs cleaned up."
#  Tips ：/var/log/messages  Is the log file of the system , Very important 

problem ：
(1) No root Can't clean up the log .
(2) There are no process control statements , In short, sequential operation , There is no success in judgment and logicality .

Example 2. Include command 、 Clearing variables and process control statements /var/log Under the messages Of the log file shell Script .

#!/bin/bash
#  Clear log script 
LOG_DIR=/var/log
ROOT_UID=0 # $UID by 0 When , Only users have root User's rights 
#  To use root User to run , Brackets are separated by spaces 
if [ "$UID" -ne "$ROOT_UID" ]
then
    echo "Must be root ro tun this script."
    exit 1
fi
cd $LOG_DIR || {
    echo "Cannot change to necessary directory." >&2
    exit 1
}
cat /dev/null > messages && echo "Logs cleaned up."
exit 0
#  Return before exiting 0 Indicates successful return 1 It means failure 

Expand ： Three ways to empty the log and file contents ：
[[email protected] day1]# echo >test.log # There is a blank line
[[email protected] day1]# > test.log # Redirect
[[email protected] day1]# cat /dev/null > test.log

4. Linux The default script interpreter of the system ？

Centos linux By default shell yes bash.
Check the method ：echo $SHELL or grep root /etc/passwd

5. shell Script creation and execution

• At the beginning of the script ( first line )
  A standard shell The first line of the script will indicate which program ( Interpreter ) To execute the contents of the script , stay linux bash Programming is generally ：

#!/bin/bash
 or 
#!bin/sh < ==255  Within characters 

“#！” Also known as magic number , When executing a script , The kernel will be based on "#!" The latter interpreter determines which program should be used to interpret the contents of the script . Be careful ： This line must be the first line at the top of each script .
sh by bash The soft links .

If python, Then the first behavior , The execution is as follows ：python test.py perform

#!/usr/bin/env python

6. shell Basic norms and habits of script development

(1) Start by specifying the script interpreter

#!bin/sh  or  #!/bin/bash

(2) Add copyright and other information at the beginning

#Date: 16:29 2020-3-30
#Author: Created by oldboy
#Mail: [email protected]
#Function: This scripts function is....
#Version: 1.1

Tips ： Configurable vim The above information is automatically added when editing the file , The way is to modify ~/.vimrc The configuration file .

(3) The script does not need Chinese comments
Try to use English notes to prevent the trouble of Chinese garbled code after this machine or switching the system environment .

(4) Script to .sh Extension name

(5) Excellent code writing habits
I. The paired symbol contents are written at one time , Prevent omission , Such as ：{},[],’’,""
II. [] There should be spaces at both ends of brackets , When writing, you can leave a space [ ]
III. Process control statements are written at one time , Adding content , Such as ：
if The statement format is completed at one time ：
if Condition content
then
Content
fi
for Cycle format completed at one time ：
for
do
Content
done

(6) Make the code readable by indentation

Chapter two Shell Variable basis and depth

1. environment variable

Environment variables are used to define Shell Operating environment , Guarantee Shell Correct execution of commands . Home directory .bash_profile File or global configuration /etc/bashrc,/etc/profile,/etc/profile.d In the definition of .
Login shows that the loaded content can be placed in /etc/profile.d Under the table of contents .

Traditionally , All environment variables are capitalized . Before the environment variable is applied to the user process , Must use export Command export .

use evn or set Show default environment variables .unset Eliminate local variables and environment variables .

2. local variable

I. Ordinary string variable definition ：
Variable name =value; Variable name =‘value’; Variable name =’'value"

II. Command variables ：
Variable name =``; Variable name =$()

habit ： Numbers are not quoted , Other default double quotes .

Single quotation marks ： What you see will be output .
Double quotes ： Output everything in double quotation marks , If there is an order in the content , Variable , Special escape , The command result will be parsed first , And then output the final content .
No quotes ： Generally continuous string , Numbers i, The path, etc. can be without quotation marks . However, it is best to replace it with double quotation marks .

3. Special variables

I. Positional variable
$0 Gets the currently executed shell The filename of the script , Include script path , Such as
dirname $0 # File path
basename $0 # File name

$natakeWhenfrontOf\; boardThat\text{'}s\; okOfshellfootBenOfThe\; firstnindividualginsengCountvalue,n=1..9,Whennby0whensurfaceinfootBenOfwritingPieces\; ofname,Such\; asfruitnBigOn9,be${10}

$# Get current shell The total number of parameters in the script command line

Enterprise case ： Control the number of parameters transmitted by the user

[ $# -ne 2 ] && {
    echo "must two"
    exit 1
}
echo oldgirl

$$ Get current shell The process number of

$? Gets the return value of the last instruction executed (0 For success , Not 0 For failure )

(1) The application case ： When there is only one script and only one process running in the system

# pid.sh  Script files 
[[email protected] scripts]# cat pid.sh
#!/bin/sh
pidPath=/tmp/a.pid
if [ -f "$pidPath" ]
   then
      kill -USR2 `cat $pidPath`
      rm -f $pidPath
fi
# script pid write to $pidPath
echo $$ >$pidPath
sleep 300

#  Check the process 
[[email protected] scripts]# ps -ef|grep pid.sh|grep -v grep
root      1758  1457  0 10:03 pts/0    00:00:00 sh pid.sh

4. Numerical calculation of variables

Common calculation commands ：(())、let、expr、bc（ decimal ）、$[]
I. (())) usage

[[email protected] scripts]# ((a=1+2**3-4%3))
[[email protected] scripts]# echo $a
8
[[email protected] scripts]# b=$((1+2**3-4%3))
[[email protected] scripts]# echo $b
8
[[email protected] scripts]# echo $((3>2))
1
[[email protected] scripts]# echo $((3<2))
0

Example ： One at a time （ Unmixed operation ）, That is, pass two numbers through parameters , Pass a calculation symbol , Produce results .

[[email protected] scripts]# cat d.sh
echo $(($1$2$3))
[[email protected] scripts]# sh d.sh 3*2
6

II. let usage

[[email protected] scripts]# i=2
[[email protected] scripts]# let i=i+8
[[email protected] scripts]# echo $i
10
[[email protected] scripts]# i=2
[[email protected] scripts]# i=i+8
[[email protected] scripts]# echo $i
i+8

III. expr usage

[[email protected] scripts]# expr 2 + 2
4
[[email protected] scripts]# expr 2+2
2+2
[[email protected] scripts]# expr $[2*3]
6
[[email protected] scripts]# expr $[2/3]
0

IIII. read usage

# -t timeout  Set the input waiting time , The default unit is seconds .
# -p prompt   Set the prompt message 
[[email protected] scripts]# read -t 5 -p "pls input:" a
pls input:1
[[email protected] scripts]# echo $a
1

The third chapter Branch and loop structure

1. if Conditional sentence

#  Single branch 
if [ Conditions ]
  then
     Instructions 
fi

#  Double branch 
if  Conditions 
  then 
     Instruction set 
else
     Instruction set 
fi

#  Multiple branches 
if  Conditions 
  then 
     Instructions 
elif   Conditions 
  then 
     Instructions 
    ...
else
     Instructions 
fi   

Special way of writing ：if [-f “$file1”]; then echo 1;fi amount to : [-f “file1”] && echo 1

Example 1： Single branch if Compare the size of two integers in conditional sentences .

[[email protected] scripts]# cat if-single.sh
#!/bin/sh
if [ 10 -lt 12 ]
   then
       echo "yes,10 is less than 12"
fi

Example 2： Use read And how to realize the comparison of the above integers by script parameter transfer ？

#!/bin/sh
read -p "pls input two num:" a b

if [ $a -lt $b ];then
   echo "yes,$a less than $b"
fi
if [ $a -eq $b ];then
   echo "yes,$a equal $b"
fi
if [ $a -gt $b ];then
   echo "yes,$a greater than $b"
fi

Chapter four Example demonstration

1. Use for The cycle is /oldboy Create in batch under directory 10 File , The names are in the following order ：

oldboy-1.html,oldboy-2.html,…,oldboy-10.html

[[email protected] oldboy]# cat /home/scripts/01.sh
[ ! -d /oldboy ] && mkdir -p /oldboy
for i in `seq 10`
do
  touch /oldboy/oldboy-${i}.html
done

2. use for The loop implementation will... In the above file name oldboy All changed linux, And the extension is changed to uppercase . requirement ：for The circular body of a cycle cannot appear oldboy character string .

[[email protected] scripts]# cat 02.sh
#!/bin/sh
cd /oldboy
for f in `ls *.html`
do
  mv $f `echo $f|sed 's#oldboy#linux#g'|sed 's#html#HTML#g'`
done

3. Batch creation 10 A system account oldboy01-oldboy10 And set the password ( Passwords cannot be the same ).

#!/bin/sh
for n in `seq -w 10`
do
  useradd oldboy$n&&\
  echo "root$n"|passwd --stdin oldboy$n
done

4. Array definition, addition, deletion, modification and query

[[email protected] scripts]# array=(1 2 3)
[[email protected] scripts]# echo ${#array[@]}
3
[[email protected] scripts]# echo ${#array[*]}
3
[[email protected] scripts]# echo ${array[0]}
1