Piping

• A treatise on I/O redirection in Linux shells, which has applications in this assignment.
• A great visual guide to I/O redirection in Linux.

First, let's look at redirect stdout to files. You can accomplish this with the > character, as so:

hacker@dojo:~$ echo hi > asdf

This will redirect the output of echo hi (which will be hi) to the file asdf. You can then use a program such as cat to output this file:

hacker@dojo:~$ cat asdf
hi

In this challenge, you must use this input redirection to write the word PWN (all uppercase) to the filename COLLEGE (all uppercase).

Aside from redirecting the output of echo, you can, of course, redirect the output of any command. In this level, /challenge/run will once more give you a flag, but only if you redirect its output to the file myflag. Your flag will, of course, end up in the myflag file!

You'll notice that /challenge/run will still happily print to your terminal, despite you redirecting stdout. That's because it communicates its instructions and feedback over standard error, and only prints the flag over standard out!

Just like standard output, you can also redirect the error channel of commands. Here, we'll learn about File Descriptor numbers. A File Descriptor (FD) is a number the describes a communication channel in Linux. You've already been using them, even though you didn't realize it. We're already familiar with three:

• FD 0: Standard Input
• FD 1: Standard Output
• FD 2: Standard Error

When you redirect process communication, you do it by FD number, though some FD numbers are implicit. For example, a > without a number implies 1>, which redirects FD 1 (Standard Output). Thus, the following two commands are equivalent:

hacker@dojo:~$ echo hi > asdf
hacker@dojo:~$ echo hi 1> asdf

Redirecting errors is pretty easy from this point. If you have a command that might produce data via standard error (such as /challenge/run), you can do:

hacker@dojo:~$ /challenge/run 2> errors.log

That will redirect standard error (FD 2) to the errors.log file. Furthermore, you can redirect multiple file descriptors at the same time! For example:

hacker@dojo:~$ some_command > output.log 2> errors.log

That command will redirect output to output.log and errors to errors.log.

Let's put this into practice! In this challenge, you will need to redirect the output of /challenge/run, like before, to myflag, and the "errors" (in our case, the instructions) to instructions. You'll notice that nothing will be printed to the terminal, because you have redirected everything! You can find the instructions/feedback in instructions and the flag in myflag when you successfully pull this off!

Just like you can redirect output from programs, you can redirect input to programs! This is done using <, as so:

hacker@dojo:~$ echo yo > message
hacker@dojo:~$ cat message
yo
hacker@dojo:~$ rev < message
oy

You can do interesting things with a lot of different programs using input redirection! In this level, we will practice using /challenge/run, which will require you to redirect the PWN file to it and have the PWN file contain the value COLLEGE! To write that value to the PWN file, recall the prior challenge on output redirection from echo!

You know how to run commands, how to redirect their output (e.g., >), and how to search through the resulting file (e.g., grep). Let's put this together!

In preparation for more complex levels, we want you to:

1. Redirect the output of /challenge/run to /tmp/data.txt.
2. This will result in a hundred thousand lines of text, with one of them being the flag, in /tmp/data.txt.
3. Grep that for the flag!

It turns out that you can "cut out the middleman" and avoid the need to store results to a file, like you did in the last level. You can use this using the | (pipe) operator. Standard output from the command to the left of the pipe will be connected to (piped into) the standard input of the command to the right of the pipe. For example:

hacker@dojo:~$ echo no-no | grep yes
hacker@dojo:~$ echo yes-yes | grep yes
yes-yes
hacker@dojo:~$ echo yes-yes | grep no
hacker@dojo:~$ echo no-no | grep no
no-no

Now try it for yourself! /challenge/run will output a hundred thousand lines of text, including the flag. Grep for the flag!