Lesson 23 · Input redirection

Input redirection

Redirection works the other way too - `<` feeds a file into a command's standard input. Today you point file descriptor 0 at a file so programs can read from it as if you had typed it.

The goal

Implement `< file` so the command reads its standard input from that file.

Start here - the target

TO DO

Scenario: Redirecting standard input from a file

Giventhe file in.txt contains "hello\n"

Whenthe line: cat < in.txt runs

Thenthe shell output is "hello\n" (cat read its input from the file)

Andopening a missing input file reports an error and the command does not run

Background

Input redirection is the mirror image of output. Standard input is file descriptor 0; < opens a file read-only and dup2s it onto descriptor 0, so when the program reads “standard input” it reads the file’s contents instead of your keyboard. cat < in.txt and cat in.txt produce the same output, but by completely different routes - in the first, cat reads stdin and never knows a file was involved.

There is one asymmetry worth handling: an output file is created if missing, but an input file that does not exist is an error. Opening it fails, and the shell should report that and skip running the command, setting a non-zero status - there is no sensible input to give the program. That failure path is the one new wrinkle on top of the dup2 pattern you already know.

Make it work

// input redirection points fd 0 at the file, read-only
int fd = open(target, O_RDONLY);
if (fd < 0) { perror(target); /* fail: skip exec, status 1 */ }
dup2(fd, 0);           // stdin now reads from the file
close(fd);

CheckpointDONE

`cat < in.txt` reads from a file. Commit and stop here.