fileRedirection.md

File redirection in MiniShell

Before diving into the concept of file redirection, let us understand a few concepts:

• A file is a resource on the computer stored on disk, whereas file descriptor (FD) is a integer that the kernel gives our process to handle the file ( or any I/O resource ).

Now how does a terminal knows whom to redirect output?

Each process has some predefined FDs, which points to a particular open resource for IO:

FD	Formal name	Pointed Resource
0	STDIN_FILENO	Standard Input
1	STDOUT_FILENO	Terminal
2	STDERR_FILENO	Terminal

now when we do: int fd = open("output.txt", O_WRONLY | O_CREAT | O_TRUNC, 0644);

• The kernel finds output.txt on disk, it opens it for writing and assigns the process a new file descriptor that points to the open file. for eg here fd = 3

now write(1 , "hello" , 5) -> redirects o/p to fd=1 which is the STDout terminal. and write(3, "hello" , 5) -> redirects o/p to fd=3 which is the output.txt file

Use of dup2() sys call to duplicate FDs

dup2() is a system call which duplicates FDs or make one FD point to the same resource as another. Syntax: dup2(oldfd, newfd) : duplicates the file descriptor oldfd onto newfd.

Next any function or system call that originally prints or writes to the resource pointed by newfd will now do it to the resource pointed by oldfd, as now newfd points to the same resource as oldfd.

Closing FDs:

doing close(fd) will tell the kernel that this descriptor pointing to a open resource is to be closed.

Terminal Redirection Examples:

1. Truncated Write to a File In this operation, terminal writes to a file, if anything was written previously it removes and writes from beginning. e.g ls > output.txt : this will print the output of ls to the file output.txt

2. Read from a file In this operation, the terminal reads from a file. It requires duplicating the file descriptor of an open file to STDIN_FILENO, so that the standrard input comes from the open file. e.g cat < logs.yml: reads the file and prints it to terminal.

3. Append to a file In this operation, the terminal appends to a file. If a file is not created it crates , and writes to it, else appends content to it, without truncating the file. e,g : cat >> file.txt