Bash Builtins


Linux Luminarium

Many see shells as shallow: simply small wrappers around useful utilities. This is true to an extent, but ignoring the power of the shell can hamper you when you try to accomplish complex things in Linux. In this module, we will delve into using the shell itself, as opposed to the commandline tools that Linux offers.

Some questions that we will explore:

We will learn how to use the functionality built into the shell itself, often termed builtins. Builtins are invoked just like commands, but the shell handles them internally instead of launching other programs. You can get a list of shell builtins by running the builtin help, as so:

hacker@dojo:~$ help

That will list a bunch of builtins. You can get help on a specific one by passing it to the help builtin. Let's look at a builtin that we've already used earlier, export!

hacker@dojo:~$ help export
export: export [-fn] [name[=value] ...] or export -p
    Set export attribute for shell variables.
    
    Marks each NAME for automatic export to the environment of subsequently
    executed commands.  If VALUE is supplied, assign VALUE before exporting.
    
    Options:
      -f	refer to shell functions
      -n	remove the export property from each NAME
      -p	display a list of all exported variables and functions
    
    An argument of `--' disables further option processing.
    
    Exit Status:
    Returns success unless an invalid option is given or NAME is invalid.

Some good information! Don't forget to read the help on your builtins when you need info. No, go forth and learn!


Challenges

We'll start with reading input from the user (you). That's done using the aptly named read builtin, which reads input!

Here is an example using the -p argument, which lets you specify a prompt (otherwise, it would be hard for you, reading this now, to separate input from output in the example below):

hacker@dojo:~$ read -p "INPUT: " MY_VARIABLE
INPUT: Hello!
hacker@dojo:~$ echo "You entered: $MY_VARIABLE"
You entered: Hello!

Now that you're familiar with the read command, here it is without that prompt:

hacker@dojo:~$ read MY_VARIABLE
Hello!
hacker@dojo:~$ echo "You entered: $MY_VARIABLE"
You entered: Hello!

In this challenge, your job is to use read to set the PWN variable to the value COLLEGE. Good luck!

Often, when shell users want to read a file into an environment variable, they do something like:

hacker@dojo:~$ echo "test" > some_file
hacker@dojo:~$ VAR=$(cat some_file)
hacker@dojo:~$ echo $VAR
test

This works, but it represents what grouchy hackers call a "Useless Use of Cat". That is, running a whole other program just to read the file is a waste. It turns out that you can just use the powers of the shell!

Previously, you read user input into a variable. You've also previously redirected files into command input! Put them together, and you can read files with the shell.

hacker@dojo:~$ echo "test" > some_file
hacker@dojo:~$ read VAR < some_file
hacker@dojo:~$ echo $VAR
test

What happened there? The example redirects some_file into the standard input of read, and so when read reads into VAR, it reads from the file! Now, use that to read /challenge/read_me into the PWN environment variable, and we'll give you the flag! The /challenge/read_me will keep changing, so you'll need to read it right into the PWN variable with one command!


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