Redirection and files

Redirects and files

Programs can use files in two ways :

The first way is , Show opening a file with a specific function , Close file , Read the file , write file .

The second way is , Design programs that interact with keyboards and items , Redirect input value files and output from files through different channels . In other words , hold stdin Stream reassigned to file .

A major problem with redirection is that it is related to the operating system , And C irrelevant .

1. Redirect input

Redirect the input such as the operation instruction is "<"

Because it uses VSCode When running the program, you can input ./ecoh_eof Instructions

You can execute the file, but you need to compile the link generation before that .exe file .

When reading the book, the redirected input says words Text file for , At first I thought it was .txt The format of , But I'm trying VScode Its own terminal 、git bash And the system comes with Powershell The effect cannot be displayed after .

 

As shown in the figure above .

Later, it was found that the reason for the problem was .txt The reason for the document . So I deleted .txt After running the program, it will have an effect .

stay words Enter... In the file

aaaaaaaa
bbbbbbbb
cccccccc

C The procedure is ：

#include <stdio.h>
int main(void)
{
    int ch;
​
    while((ch = getchar()) != EOF)
        putchar(ch);
​
    return 0;
}

But in VSCode The terminal running result on will still report an error, so it is replaced by git bash Up operation .

The operation effect is as follows :

 

The successful running , This well proves that the above input problems are the cause of the operating system , because git bash Is similar to Linux Of .

2. Redirect output

The instructions used in the redirect output operation are ">"

Also in git bash Input in ./ecoh_eof >savewords

You can save the input characters to savewords On the file . But it's not clear git bash How did you quit editing text in , The book says Ctrl+z(DOS) or Ctrl+D(UNIX), I tried two methods, but I couldn't return the train , Then I switched back VSCode Interestingly VSCode Your terminal can use ">"

Then the display results are as follows :

 

3. Combined redirection

One thing to note about combined redirection is that the input file name and output file name cannot be the same in a command . The reason is that the length of the original file has been truncated before the output file is input. Similarly, the following commands :

./echo_eof < mywords > mywords

The redirected command is independent of the order of the redirected operators. For example, the execution effect of the following two commands is the same .

./echo_eof < mywords > savewords

./echo_eof > savewords < mywords

 

The effect of the two implementations is the same .

Points for attention when using redirection

1. Redirection operator connects an executable ( Including standard operating system commands ) And a data file , Cannot be used to connect one data file to another , Nor can it be used to connect one program to another .

2. You cannot read the input of multiple files using the redirection operator , You can't direct output to multiple files .

3. Usually , Spaces between file names and operators are not required , Except occasionally in UNIX shell、Linux shell or Windows.