Shell script Base numeric operation judgment and if statement

Numeric operations

Integer operation "Three kinds, one can be mastered"
Expr numeric operator number
[[email protected] ~]# Expr 1 + 1 (OP symbols are all +-*/NOTE: * Requires \*.) % is the remainder, only 0 1 2)
2
[[email protected] ~]# Expr 45 \* 1
45

echo$[numeric operator numbers]
[[email protected] ~]# echo $[89*89+454848*874851]
397924235569

Let
i++ = = I=i+1
i+=2 = = I=i+2
i*=2 = = I=i*2
i\=2 = = I=i\2
I-=3 = = I=i-2
[Email protected] ~]# a=13
[[email protected] ~]# let a++
[Email protected] ~]# echo $a
14
[[email protected] ~]# let a+=6
[Email protected] ~]# echo $a
20

decimal operation "BC" (Can do logical operation, pipeline operation is convenient)
[Email protected] ~]# echo "84515*4511.894654+4856.844*154.158-15451" |BC
382056047.039810
[Email protected] ~]# echo "899<1" |BC
0
[Email protected] ~]# echo "899>1" |BC
1

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Combination of commands:

Syntax format
Judge:
[]: Judgment statement to be executed in parentheses

Logical-delimited operations
&&: A given condition must be established, the entire test result is true
|| : The entire test result is true as long as one of the conditions is set
;: Execute first, then execute

A&&b: Only a succeeds, only executes B
A B: Execute a, then execute B
a| | B: Only a failed to execute B

Common Test actions: (directory, file)
-E: Determines whether the object exists or is true
-D: Determines whether the object is a directory or True
-F: Determines whether the object is a generic file, or True
-R: Determines whether the object has a readable permission, is true
-W: Determines whether the object has writable permissions, is True
-X: Determines whether an object has executable permissions or is True
-S: Determine if the file is not empty,

Integer value comparison: (quantity)
-eq: Equals
-ne: Not equal to
-ge: greater than or equal to
-le: Less than or equal to
-GT: Greater Than
-LT: Less than

string comparison:
= =: two strings same
! =: Two strings are not the same
-Z: null value of String
-N: The value of the string is not empty, (equivalent! -Z)

[[email protected] ~]# Vim a.sh (delete script)
#!/bin/bash
Read-p "Crouching groove, you really want to delete ah y/n" sure (read-p display content on the terminal)
[-Z $sure]&&exit (determine if $sure is empty (-Z) &&: matches, stops for)
[$sure = y]&& rm-rf $ (if you enter Y && match, continue to the next step)
[Email protected] ~]#/a.sh ftp.sh
You're really going to remove it, y/ny.

[[email protected] ~]# vim b.sh (monitor login user volume)
#!/bin/bash
Num= ' Who |wc-l ' (' anti-apostrophe ' write command, this command counts how many users, put the number in num)
[$num-ge 4]&& mail-s ' Errro ' root </etc/passwd (when the number of users is greater than (GE) 4 &&: compliant, execute send mail)
[[email protected] ~]# crontab-e (added to scheduled task monitoring)
* * * * * */root/b.sh

[[email protected] ~]# vim a.sh (guessing word game)
#!/bin/bash
xx=$ ((random%10)) (xx variable, the random number generated by the Linux system,%10 represents 10 of the number)
Read-p "hydra[0-10]:" Cai
[$cai-eq $xx]&& echo OK &&exit| | Echo Kill (when the number entered equals (eq) shows OK and exits, not kill)

[Email protected] ~]# vim a.sh
#!/bin/bash
[!-d/mnt/iso]&& Mkdir/mnt/iso (! -D Inverse, if true &&: compliant, create Mnt/iso)
[-e/iso/rhel-server-6.7-x86_64-dvd.iso]&& Mount-o loop/iso/rhel-server-6.7-x86_64-dvd.iso/mnt
/iso (-E: Determines whether the object exists, &&: Conforms, exists on Mount to Mnt/iso)

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If statement
Single-branch format: (executes the following command as long as the judgment is met)
If [Judge];then
Command
.. ..
Fi
[Email protected] ~]# vim a.sh
#!/bin/bash
if [$USER = root];then
Yum Clean All
Yum Repolist
Fi

Dual-branch format: (Judgment not satisfied, re-judge to perform the next condition)
If [Judge];then
Command
elif [Judging];then
Command
Else
Command
Fi
Test:
[Email protected] ~]# vim a.sh
#!/bin/bash
Read-p code? : Hydra
If [$hydra-gt 9999];then
echo "Hail Hydra"
elif [$hydra-gt 8888];then
echo "Hail"
elif [$hydra-gt 7777];then
echo "Hydra"
Else
echo "Mdzz"
Fi
Test: Guess the number game
[Email protected] ~]# vim b.sh
#!/bin/bash
SUIJI=$[RANDOM%100]
For i in {1..9} (i is a variable representing all integers from 1 to 9, representing a maximum of 9 guesses)
Do
Read-p "hydra[0-99]" Shuo
If [$shuo-eq $suiji];then
echo "Hail Hydra"
Exit
elif [$shuo-gt $suiji];then
echo "Mdzz"
Else
echo "Mdzz"
Fi
Done

Multi-branch format: (Judgment not satisfied, re-judge to execute next condition, and so on)
If [Judge]&&[judgment];then
Command
elif[Judgment]&&[judgment];then
Command
Else
Command
Fi

[Email protected] ~]# vim c.sh
#!/bin/bash
Read-p "Level (1-100)": "DJ"
If [$dj-ge]&&[$dj-le];then
echo "$DJ level! Dalao "
elif [$dj-ge]&&[$dj-le];then
echo "$DJ level! Emmm "
Else
echo "$DJ level! Mdzz "
Fi

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Shell script Base numeric operation judgment and if statement