Examples of If Case statement usage in Linux

If and case two conditional statements in the shell

1. If Syntax 1:
If
Conditions
Then
Commands
Else
Commands
Fi

Syntax 2 for if:
Syntax: If condition
Then
Commands
Elif conditions
Then
Commands
Elif conditions
Then
Commands
....
Else
Commands
Fi

Description: Read is a row for reading user input

Some descriptions of IF statements

First: If condition 1
Then
Commands
Elif Condition 2
Then
Commands
Elif Condition 3
Then
Commands
....
Else
Commands
Fi

The conditions in the 1,2,3 etc, are equal status, that is, the equivalent of the condition of 1 is not satisfied, then see conditions 2, Conditions 1, 2 are not satisfied, see conditions 3 to push down (-a for and, but can not be replaced with &&, otherwise will be an error,-O for or meaning).

Case Condition Statement:
Syntax: Case conditions in
xxx
commands;;
xxx
commands;;
xxx
commands;;
Esac

Description: This ESAC is the end of the case, want to if...fi the same,
Pay attention to commands;; the ";;" cannot be lost.

Example
1, write a script/root/bin/createuser.sh, to achieve the following functions: Using a username as a parameter, if the user of the specified parameter exists, it is added; Displays the ID number of the added user

[Root@localhost bin]# Cat createuser.sh
#!/bin/bash
# date:2016.8.12
# AUTHOR:CYH
# Description:determine whether a user exists

User= ' cut-d:-f1/etc/passwd |grep-o "$"
[$#-ne 1] && exit 1
if [[-N $user]]; Then
echo "User already exists!"
echo "The User information: ' grep ' ^$1\>"/etc/passwd |cut-d:-F 1,3 ' "
Else
Useradd $ >/dev/null
echo "The user has been created!"
echo "The User information: ' grep $1/etc/passwd |cut-d:-F 1,3 '"
Fi

Script Test

[Root@localhost bin]# getent passwd Tom
[Root@localhost bin]# createuser.sh Tom
The user has been created!
The user information:tom:2028
[Root@localhost bin]# getent passwd Tom
Tom:x:2028:2033::/home/tom:/bin/bash
[Root@localhost bin]#
[Root@localhost bin]# getent passwd Zhangsan
Zhangsan:x:2022:2024::/home/zhangsan:/bin/bash
[Root@localhost bin]# createuser.sh Zhangsan
User already exists!
The user information:zhangsan:2022

2, write a script/root/bin/yesorno.sh, prompts the user to enter Yes or no, and to determine whether the user entered Yes or no, or other information

[Root@localhost bin]# Cat yesorno.sh
#!/bin/bash
# description:determine the user input characters (yes or no).

Read-t5-p "Please input yes or no:" ans
[Z $ans] && echo "You have no input, script has quit" && exit 10
Case $ans in
[YY] [EE] [ss]|y| Y
echo "Input is yes!"
;;
[NN] [oo]|n| N
echo "Input is no!"
;;
*)
echo "Input error, please enter again."
;;
Esac

Script Test

[Root@localhost bin]# yesorno.sh
Please input yes or no:y
Input is yes!
[Root@localhost bin]# yesorno.sh
Please input yes or no:yes
Input is yes!
[Root@localhost bin]# yesorno.sh
Please input yes or no:no
Input is no!
[Root@localhost bin]# yesorno.sh
Please input yes or no:n
Input is no!
[Root@localhost bin]# yesorno.sh
Please input yes or NO:TT
Input error, please enter again.

3, write a script/root/bin/filetype.sh, judge the user input file path, display its file type (normal, directory, link, other file type)

[Root@localhost bin]# Cat filetype.sh
#!/bin/bash
# AUTHOR:CYH
# date:2016.8.15
# Description:input a file, and determine the type of the file

If [$#-ge 1]; Then
If [-Z $]; Then
echo "Your input is empty"
elif [D $]; Then
echo "This is a directory."
elif [-H $]; Then
echo "This is a link file."
Elif [F $]; Then
echo "This is a common file."
Elif [C $]; Then
echo "This is a character device file."
Elif [B $]; Then
echo "This are block device file."
Elif [S $]; Then
echo "This is a socket file."
elif [-P $]; Then
echo "This is a pipe file."
Else
echo "The input character is empty or file does not exist,please specify again."
Fi
Else
Read-t 10-p "Please input file name:" File
[[!-e $file | |-Z $file]] && echo "The input is empty or file does not exist,please specify again." && exit
Type= ' ls-ld $file 2>/dev/null | Cut-c1 '
Case $type in
D
echo "This is a directory."
;;
-)
echo "This is a common file."
;;
L
echo "This is a link file."
;;
S
echo "This is a socket file."
;;
C
echo "This is a character device file."
;;
b
echo "This are block device file."
;;
P
echo "This is a pipe file."
;;
*)
echo "This isn't is file."
Esac
Fi

Script Test

[Root@localhost bin]# filetype.sh DFDFD
The input character is empty or file does not exist,please specify again.
[Root@localhost bin]# Filetype.sh/var
This is directory.
[Root@localhost bin]# filetype.sh/var/log/
This is directory.
[Root@localhost bin]# Filetype.sh/var/log/messages
This is a common file.
[Root@localhost bin]# Filetype.sh/tmp/cat
This is a link file.
[Root@localhost bin]# Filetype.sh/tmp/hogsuspend
This is a pipe file.
[Root@localhost bin]# Filetype.sh/var/log/messages21
The input character is empty or file does not exist,please specify again.

[Root@localhost bin]# filetype.sh
Please input file name:dfsdfsfq2
The input character is empty or file does not exist,please specify again.
[Root@localhost bin]# filetype.sh
Please input file name:/etc/fstab
This is a common file.
[Root@localhost bin]# filetype.sh
Please input file name:/tmp/cat
This is a link file.
[Root@localhost bin]# filetype.sh
Please input file name:/tmp/hogsuspend
This is a pipe file.

4, write a script/root/bin/checkint.sh to determine whether the user input parameter is a positive integer

[Root@localhost bin]# Cat checkint.sh
#!/bin/bash
#

Read-p "Please input an integer:" Integer
#[$integer-lt 0] && echo "Please enter at least one value" && exit 10
if [[$integer =~ ^0*[0-9][0-9]*$]]; Then
echo "This is an integer"
Else
echo "This isn't an integer"
Fi

Script Test

[Root@localhost bin]# checkint.sh
Please input an integer:12
This is an integer
[Root@localhost bin]# checkint.sh
Please input an integer:3.14
This isn't an integer
[Root@localhost bin]# checkint.sh
Please input an INTEGER:DF
This isn't an integer
[Root@localhost bin]# checkint.sh
Please input an integer:09
This is an integer
[Root@localhost bin]# checkint.sh
Please input an integer:9f
This isn't an integer