c语言编写一个假的linux（2）

这次我们实现一个假的cat命令。

因为是假的，所以不管参数。只要输入cat README.md 能输出一些内容就行。并没有打开文件这种操作。

下面是代码：

#include <stdio.h>
#include <string.h>

int print_logo(){
    printf("%s\n","");
    printf("%s\n","  __       _        _ _");
    printf("%s\n"," / _| __ _| | _____| (_)_ __  _   ___  __");
    printf("%s\n","| |_ / _` | |/ / _ \\ | | '_ \\| | | \\ \\/ /");
    printf("%s\n","|  _| (_| |   <  __/ | | | | | |_| |>  <");
    printf("%s\n","|_|  \\__,_|_|\\_\\___|_|_|_| |_|\\__,_/_/\\_\\");
    printf("%s\n","");
}

int main(){
    print_logo();
    char cmd[100];
    while(1){
        printf("\n%s ","#");
        scanf("%s",cmd);
        if(strcmp(cmd,"ls")==0){
            printf("%s","README.md");
        }else if(strcmp(cmd,"cat README.md")==0){
            printf("%s\n","## FakeLinux");
            printf("%s\n","> a fake linux");
            printf("%s","http://github.com/fakelinux");
        }else{
            printf("Command '%s' not found",cmd);
        }
    }
    return 0;
}

运行效果：

怎么回事儿？一条带有参数的命令cat README.md 怎么被空格拆成2条命令了？

原来，scanf 函数遇到空格就读取一次，所以一个字符串里有空格就会被分成2次输入。现在把 scanf 改成 gets 就可以解决空格的问题：

#include <stdio.h>
#include <string.h>
#include <unistd.h>

int print_logo(){
    printf("%s\n","");
    printf("%s\n","  __       _        _ _");
    printf("%s\n"," / _| __ _| | _____| (_)_ __  _   ___  __");
    printf("%s\n","| |_ / _` | |/ / _ \\ | | '_ \\| | | \\ \\/ /");
    printf("%s\n","|  _| (_| |   <  __/ | | | | | |_| |>  <");
    printf("%s\n","|_|  \\__,_|_|\\_\\___|_|_|_| |_|\\__,_/_/\\_\\");
    printf("%s\n","");
}

int main(){
    print_logo();
    char cmd[100];
    while(1){
        printf("\r\n%s ","#");
        gets(cmd);
        if(strcmp(cmd,"")==0){
            printf("%s","#");
        }else if(strcmp(cmd,"ls")==0){
          printf("%s","README.md");
        }else if(strcmp(cmd,"cat README.md")==0){
            printf("%s\n","## FakeLinux");
            printf("%s\n","> a fake linux");
            printf("%s","http://github.com/fakelinux");
        }else{
            printf("Command '%s' not found",cmd);
        }
    }
    return 0;
}

缺点是编译时会产生一个可以忽略的警告：

gcc -o fakelinux main.c
main.c: In function ‘main’:
main.c:20:9: warning: implicit declaration of function ‘gets’; did you mean ‘fgets’? [-Wimplicit-function-declaration]
   20 |         gets(cmd);
      |         ^~~~
      |         fgets
/usr/bin/ld: /tmp/ccOAlP8X.o: in function `main':
main.c:(.text+0xc4): warning: the `gets' function is dangerous and should not be used.

运行效果：