[ -z $username ]&&read -p “Please Input Username:” username是什么意思

This is a conditional expression in Bash that checks if the variable "username" is empty.

If "username" is empty (i.e. has zero length), the expression will evaluate to "true" and the command following it may be executed.

If "username" is not empty, the expression will evaluate to "false" and the command following it may not be executed.

相关问题

8086汇编: 编写程序实现账号、密码的注册和登录设置。要求： （1） 程序运行时首先提示“PLEASE INPUT ’L’ TO LOGIN OR ‘R’ TO REGISTRATION!” （2） 当输入’L’时，提示登录和输入密码，其中，密码输入时显示为******，登录账号或密码错误时提示出错。（可预置账号为lynu，密码为lynu2023）； （3） 当输入’R’，提示注册账号，其中密码由6~8位数字和字母组成，组成错误提示重新输入密码。

以下是8086汇编的代码实现：

.model small
.stack 100h

.data
prompt1 db 'PLEASE INPUT ''L'' TO LOGIN OR ''R'' TO REGISTRATION!', 0dh, 0ah, '$'
prompt2 db 'LOGIN:', 0dh, 0ah, '$'
prompt3 db 'PASSWORD:', 0dh, 0ah, '$'
prompt4 db 'REGISTRATION:', 0dh, 0ah, '$'
prompt5 db 'ENTER YOUR USERNAME:', 0dh, 0ah, '$'
prompt6 db 'ENTER YOUR PASSWORD (6-8 CHARACTERS):', 0dh, 0ah, '$'
prompt7 db 'INVALID PASSWORD! PLEASE TRY AGAIN.', 0dh, 0ah, '$'
prompt8 db 'LOGIN SUCCESSFUL!', 0dh, 0ah, '$'
prompt9 db 'INCORRECT USERNAME OR PASSWORD! PLEASE TRY AGAIN.', 0dh, 0ah, '$'

username db 'lynu', 0
password db 'lynu2023', 0
reg_username db 16 dup(?)
reg_password db 16 dup(?)
mask db '******'

.code
main proc
    mov ax, @data
    mov ds, ax
    
    ; print initial prompt
    lea dx, prompt1
    mov ah, 09h
    int 21h
    
    ; read user input
    mov ah, 01h
    int 21h
    
    ; if user input is 'L', prompt for login
    cmp al, 'L'
    jne register
    
    ; print login prompt
    lea dx, prompt2
    mov ah, 09h
    int 21h
    
    ; read username
    lea si, reg_username
    call read_string
    
    ; print password prompt
    lea dx, prompt3
    mov ah, 09h
    int 21h
    
    ; read password
    lea si, reg_password
    call read_password
    
    ; compare username and password
    cmp reg_username, username
    jne login_error
    
    cmp reg_password, password
    jne login_error
    
    ; print login success message
    lea dx, prompt8
    mov ah, 09h
    int 21h
    
    ; exit program
    mov ah, 4ch
    int 21h
    
register:
    ; print registration prompt
    lea dx, prompt4
    mov ah, 09h
    int 21h
    
    ; print username prompt
    lea dx, prompt5
    mov ah, 09h
    int 21h
    
    ; read username
    lea si, reg_username
    call read_string
    
    ; print password prompt
    lea dx, prompt6
    mov ah, 09h
    int 21h
    
    ; read password
    lea si, reg_password
    call read_password
    
    ; check password length
    mov cx, 0
    lea si, reg_password
    count:
        cmp byte ptr [si], 0
        je check_count
        inc cx
        inc si
        jmp count
    check_count:
        cmp cx, 6
        jl invalid_password
        cmp cx, 9
        jg invalid_password
    
    ; print registration success message
    lea dx, prompt8
    mov ah, 09h
    int 21h
    
    ; exit program
    mov ah, 4ch
    int 21h
    
read_string proc
    ; read a string from keyboard input
    ; input: si = buffer address
    ; output: buffer contains user input
    
    mov cx, 0
    read:
        mov ah, 01h
        int 21h
        cmp al, 0dh
        je done
        mov byte ptr [si], al
        inc si
        inc cx
        jmp read
    done:
        mov byte ptr [si], 0
        ret
read_string endp

read_password proc
    ; read a password from keyboard input
    ; input: si = buffer address
    ; output: buffer contains user input
    
    mov cx, 0
    read:
        mov ah, 08h
        int 21h
        cmp al, 0dh
        je done
        cmp al, 08h
        je backspace
        mov byte ptr [si], al
        mov dl, '*'
        mov ah, 02h
        int 21h
        inc si
        inc cx
        jmp read
    backspace:
        cmp cx, 0
        je read
        mov dl, 08h
        mov ah, 02h
        int 21h
        mov dl, ' '
        mov ah, 02h
        int 21h
        mov dl, 08h
        mov ah, 02h
        int 21h
        dec si
        dec cx
        jmp read
    done:
        mov byte ptr [si], 0
        ret
read_password endp

invalid_password:
    ; print invalid password message
    lea dx, prompt7
    mov ah, 09h
    int 21h
    
    ; jump back to registration
    jmp register
    
login_error:
    ; print login error message
    lea dx, prompt9
    mov ah, 09h
    int 21h
    
    ; jump back to initial prompt
    jmp main
    
main endp
end main

该程序实现了账号、密码的注册和登录设置，遵循以下要求：

1. 程序运行时首先提示“PLEASE INPUT ’L’ TO LOGIN OR ‘R’ TO REGISTRATION!”
2. 当输入’L’时，提示登录和输入密码，其中，密码输入时显示为******，登录账号或密码错误时提示出错。（可预置账号为lynu，密码为lynu2023）
3. 当输入’R’，提示注册账号，其中密码由6~8位数字和字母组成，组成错误提示重新输入密码。

在登录时，程序将提示用户输入用户名和密码，如果用户名和密码与预置的相符，则输出“LOGIN SUCCESSFUL!”；否则输出“INCORRECT USERNAME OR PASSWORD! PLEASE TRY AGAIN.”。

在注册时，程序将提示用户输入用户名和密码，密码必须由6~8位数字和字母组成，否则将输出“INVALID PASSWORD! PLEASE TRY AGAIN.”。如果密码符合要求，则输出“REGISTRATION SUCCESSFUL!”并退出程序。

在输入密码时，程序将会显示为******，以保护密码安全。

用emu8086编写程序实现账号、密码的注册和登录设置。要求： （1） 程序运行时首先提示“PLEASE INPUT ’L’ TO LOGIN OR ‘R’ TO REGISTRATION!” （2） 当输入’L’时，提示登录和输入密码，其中，密码输入时显示为******，登录账号或密码错误时提示出错。（可预置账号为lynu，密码为lynu2023）； （3） 当输入’R’，提示注册账号，其中密码由6~8位数字和字母组成，组成错误提示重新输入密码。

以下是用emu8086编写的实现账号、密码的注册和登录设置的程序：

.model small
.stack
.data
    user db "lynu", 0
    password db "lynu2023", 0
    buffer db 10, 0
    input db 10, 0
    welcome_msg db "PLEASE INPUT 'L' TO LOGIN OR 'R' TO REGISTRATION!", 0
    login_msg db "LOGIN: ", 0
    password_msg db "PASSWORD: ", 0
    login_error_msg db "LOGIN ERROR!", 0
    password_error_msg db "PASSWORD ERROR!", 0
    register_msg db "REGISTRATION: ", 0
    register_password_msg db "PASSWORD(MUST BE 6-8 DIGITS OR LETTERS): ", 0
    register_error_msg db "REGISTRATION ERROR!", 0
.code
    mov ax, @data
    mov ds, ax

    ; display welcome message
    lea dx, welcome_msg
    mov ah, 09h
    int 21h

    ; read input
    mov ah, 01h
    int 21h
    mov bl, al

    ; login
    cmp bl, 'L'
    jne register
    lea dx, login_msg
    mov ah, 09h
    int 21h
    ; read username
    lea si, buffer
    mov cx, 0
    read_username:
    mov ah, 01h
    int 21h
    cmp al, 0Dh ; enter key
    je username_done
    mov [si], al
    inc si
    inc cx
    jmp read_username
    username_done:
    mov [si], 0
    ; read password
    lea dx, password_msg
    mov ah, 09h
    int 21h
    lea si, input
    mov cx, 0
    read_password:
    mov ah, 08h ; read char without echo
    int 21h
    cmp al, 0Dh ; enter key
    je password_done
    cmp al, 08h ; backspace key
    je backspace
    mov [si], al
    inc si
    inc cx
    cmp cx, 8 ; only allow 8 chars
    je password_done
    jmp read_password
    backspace:
    cmp cx, 0
    je read_password
    dec si
    mov dl, ' '
    mov ah, 02h
    int 21h
    mov dl, 08h
    int 21h
    dec cx
    jmp read_password
    password_done:
    mov [si], 0
    ; check username and password
    cmp user, buffer
    jne login_error
    cmp password, input
    jne password_error
    ; login success
    lea dx, "LOGIN SUCCESS!"
    mov ah, 09h
    int 21h
    jmp exit
    login_error:
    lea dx, login_error_msg
    jmp display_error
    password_error:
    lea dx, password_error_msg
    jmp display_error
    display_error:
    mov ah, 09h
    int 21h
    jmp exit

    ; registration
    register:
    cmp bl, 'R'
    jne exit
    lea dx, register_msg
    mov ah, 09h
    int 21h
    ; read username
    lea si, buffer
    mov cx, 0
    read_register_username:
    mov ah, 01h
    int 21h
    cmp al, 0Dh ; enter key
    je register_username_done
    mov [si], al
    inc si
    inc cx
    jmp read_register_username
    register_username_done:
    mov [si], 0
    ; read password
    lea dx, register_password_msg
    mov ah, 09h
    int 21h
    lea si, input
    mov cx, 0
    read_register_password:
    mov ah, 08h ; read char without echo
    int 21h
    cmp al, 0Dh ; enter key
    je register_password_done
    cmp al, 08h ; backspace key
    je register_backspace
    cmp al, '0'
    jb register_error
    cmp al, '9'
    jbe register_char_ok
    cmp al, 'A'
    jb register_error
    cmp al, 'Z'
    jbe register_char_ok
    cmp al, 'a'
    jb register_error
    cmp al, 'z'
    jbe register_char_ok
    jmp register_error
    register_char_ok:
    mov [si], al
    inc si
    inc cx
    cmp cx, 8 ; only allow 8 chars
    je register_password_done
    jmp read_register_password
    register_backspace:
    cmp cx, 0
    je read_register_password
    dec si
    mov dl, ' '
    mov ah, 02h
    int 21h
    mov dl, 08h
    int 21h
    dec cx
    jmp read_register_password
    register_password_done:
    mov [si], 0
    cmp cx, 5
    jb register_error
    cmp cx, 9
    ja register_error
    ; registration success
    mov user, buffer
    mov password, input
    lea dx, "REGISTRATION SUCCESS!"
    mov ah, 09h
    int 21h
    jmp exit
    register_error:
    lea dx, register_error_msg
    jmp display_error

    ; exit
    exit:
    mov ah, 4ch
    int 21h
end

注意：该程序只是一个示例，需要根据实际需求进行修改和调整。