Question

I wrote this code for overriding the cat command in Ubuntu. The following three formats for cat instruction are working properly but rest are not working.

Working ones:

• ./catf > File.txt
• ./catf < File.txt

Not Working:

• ./catf File1.txt > File.txt
• ./catf File.txt

I am sending the terminal arguments to program as listed above. One other problem is every file also contains Enter your String: also which it shouldn’t.Here is code:

int main(int argc, char *argv[])
{
    int readbytes,fp;
    char buf[1024];
    if(argc==1)
    {
        printf("Enter your String :\n\n");
        readbytes=read(STDIN_FILENO,buf,1024);
        write(STDOUT_FILENO,buf,readbytes);
    }
    else if(argc==2)
    {
        fp=open(argv[1],O_RDONLY);
        dup2(0,fp);
        close(fp);
        readbytes=read(STDIN_FILENO,buf,1024);
        write(STDOUT_FILENO,buf,readbytes);
    }
    else if(argc==3)
    {
        if(argv[1][0]=='<')
        {
            fp=open(argv[2],O_WRONLY|O_CREAT,S_IRWXU);
            dup2(1,fp);
            close(fp);
            readbytes=read(STDIN_FILENO,buf,1024);
            write(STDOUT_FILENO,buf,readbytes);
        }
        else if(argv[1][0]=='>')
        {
            fp=open(argv[2],O_RDONLY);
            dup2(1,fp);
            close(fp);
            readbytes=read(STDIN_FILENO,buf,1024);
            write(STDOUT_FILENO,buf,readbytes);
        }
    }
    else if(argc==4)
    {
        printf("inside");
        fp=open(argv[1],O_RDONLY);
        dup2(0,fp);
        close(fp);
        fp=open(argv[3],O_WRONLY|O_CREAT,S_IRWXU);
        dup2(1,fp);
        close(fp);
        readbytes=read(STDIN_FILENO,buf,1024);
        printf("%c",buf);
        write(STDOUT_FILENO,buf,readbytes);
    }
    return 0;
}

UPDATE:

I made the changes suggested by William in the code and the rest of the code is working fine but this ./catf File.txt ">" File2.txt is still not working. Why is that?

if(argc==4)
    {
        printf("inside4");
        fp=open(argv[1],O_RDONLY);

        dup2(fp,0);

        close(fp);

        fp=open(argv[3],O_WRONLY|O_CREAT|O_TRUNC,S_IRWXU);

        dup2(fp,1);

        close(fp);

        readbytes=read(STDIN_FILENO,buf,1024);

        //printf("%c",buf);

        write(STDOUT_FILENO,buf,readbytes);
    }

What is the reason for above mentioned problems?

Answer 1

When you invoke cat > file1.txt, *argv[1] is not <. Rather, argc is 1 and stdout is already associated with the file by the time main is called. If you want to pass < as an argument via the shell, you will need to quote it:

$ cat '<' filename

Also, your invocation of dup2 has the arguments reversed; STDOUT_FILENO should be the 2nd argument, not the first. The way you have it in your argc==2 clause is closing fp and reopening it to the original stdout.