[learn shell programming easily]-4. The difference between single quotation marks and double quotation marks, the operation of integer values, the definition of arrays in the shell and the detailed us

 1、 The difference between single quotation mark and double quotation mark

What you see is what you get

2、 Operation of integer value ：

[[email protected] ~]# a=10
[[email protected] ~]# b=20
[[email protected] ~]# echo $(($a+$b))
30
[[email protected] ~]# echo $[$a+$b]
30

#  Use expr
[[email protected] ~]# expr $a+$b
10+20
[[email protected] ~]# expr $a + $b
30

Floating point , Just use bc

3、shell In the array

Definition ： Use parentheses 、 Use a space as a separator in the middle

[[email protected] case]# a=(xx yy zz ff)
[[email protected] case]# echo $a
xx
[[email protected] case]# echo ${a[*]}
xx yy zz ff
[[email protected] case]# echo ${a[@]}
xx yy zz ff
[[email protected] case]# echo ${a[@]:1:3}
yy zz ff
[[email protected] case]# echo ${#a[@]}
4
[[email protected] case]# a[3]='hh'
[[email protected] case]# unset a[2]
[[email protected] case]# echo ${!a[@]}
0 1 3

4、sed

sed Is a non interactive stream editor that supports regular expressions (stream editor).Linux In the system sed The command mainly uses script To process files .

• In memory about sed There are two spaces ：patterrn space（ Mode space ; Processing workshop ） and hold space（ Reserved space ; Temporary warehouse ）
• The two spaces can access each other .
• Generally reserved space is not used , Just use pattern space . The schema space has processed one line , It will be cleared and output to the screen . Then proceed to the next line . If you want to keep this line , You can put it in the reserved space .

The concept of pattern space

Mode space ：sed The tool reads a line of text from a file and stores it in a buffer , Then the command operates on the contents of the mode space , In the initial state, the schema space has no content , In the process of reading data in each cycle , The schema space is emptied and new content is written （ Add a little here ,sed Command operation is “ Mode space ” Instead of memory , That's why sed The reason why the contents of the original document will not be changed , If you want to change the contents of the original document, you need to add -i Options ）

sed Tool execution flow chart

command n Work flow chart of ：

Clear the current mode space , Then read in the next line （ If sed Statement encountered n The command will change the normal execution flow ）, The execution process is as follows ：
 

4.1、sed Syntax command format and common options

Command format ：

Generally, only the first two grammars are used . 

  Common options ：

sed How to find it ：

1. According to line number
2. According to the pattern --》 Regular expressions = character + Special symbols
3. According to the string  

4.1.1、sed Common editing commands for

4.1.2、sed Of p Order sample

[[email protected] rough_book]# sed -n '3,5p' passwd 
daemon:x:2:2:daemon:/sbin:/sbin/nologin
adm:x:3:4:adm:/var/adm:/sbin/nologin
lp:x:4:7:lp:/var/spool/lpd:/sbin/nologin

# -n, Suppress automatic printing of mode space , Only show script The result of the treatment . Without this  -n, Will output the entire "passwd" file , And 3 To 5 Guild output twice . because 3 To 5 Line just meets the conditions we set , So in the patter space Stay in , After processing, it will be output again , So it will cause output twice .
#  See clearly in this way 
[[email protected] rough_book]# cat passwd -n |sed -n '3,5p' 
     3	daemon:x:2:2:daemon:/sbin:/sbin/nologin
     4	adm:x:3:4:adm:/var/adm:/sbin/nologin
     5	lp:x:4:7:lp:/var/spool/lpd:/sbin/nologin

• sed -n '10p' passwd    According to the first 10 That's ok
• sed -n '$p' passwd      Show last line
• sed -n '2+2p' passwd    Show 2,3,4 That's ok
• sed -n '10,30!p' passwd    It shows 10 To 30 The line of
• sed -n '2 ~2p' passwd    Show even rows . from 2 The starting step is 2 Began to take .

4.1.3、sed -n '/ Pattern /p'  Input file

• The pattern here can be either a regular expression or an exact character
• Mode use / Include , To extend a regular rule, add -r Options

[[email protected] rough_book]# sed -n '/bash$/p' passwd 

#  Inside this ^ You can also use 

Using extended regularization

[[email protected] rough_book]# sed -nr '/^fan|^zhao/p' passwd 
fan1:x:7800:7803::/home/fan1:/bin/bash
fan2:x:7801:7804::/home/fan2:/bin/bash
zhaoliying1:x:7802:7805::/home/zhaoliying1:/bin/bash

Use escape characters

[[email protected] rough_book]# df -h|sed -n '/\/$/p'
/dev/mapper/centos-root   17G  3.2G   14G   19% /

4.2、sed The difference between single quotation marks and double quotation marks

• You can use... In double quotation marks shell Variables in

[[email protected] rough_book]# a=5
[[email protected] rough_book]# b=10
[[email protected] rough_book]# cat -n passwd |sed -n "${a},${b}p"
     5	lp:x:4:7:lp:/var/spool/lpd:/sbin/nologin
     6	sync:x:5:0:sync:/sbin:/bin/sync
     7	shutdown:x:6:0:shutdown:/sbin:/sbin/shutdown
     8	halt:x:7:0:halt:/sbin:/sbin/halt
     9	mail:x:8:12:mail:/var/spool/mail:/sbin/nologin
    10	operator:x:11:0:operator:/root:/sbin/nologin

• Single quotation marks don't work
• You can store patterns in both single and double quotation marks