Linux Commands

Introduction to Linux commands

Most installations of new software on the Linux operating system take place on the terminal. In fact, operations to update and uninstall software must also be performed on the terminal. Even using certain software may require extensive use of the terminal, which makes being familiar with the command-line interface (CLI) very important. Then there’s creating files, editing them, moving them around, changing their permissions, and a plethora of operations that many users today take for granted with a simple mouse click. As such, we will be using terminals throughout the course, especially since some of the most powerful penetration testing tools are best used on the CLI.

List directory command

A good starting point would be to find out how to check the contents of folders on the terminal. Information on what kinds of files are available on the system, when they were made, who made them, etc., may all come in handy during penetration tests. For example, the shadow and passwd files in the etc folder hold user account and password details, respectively. When we gain unauthorized access to a system, attaining these files is usually an important step.

To list the contents of the current directory, we use the ls command. Relative paths can also be provided to check the contents of folders we are not currently in. ls also accepts a number of useful options:

• ls -l: The -l option lets us look at the long listing of directories. This outputs not just the files and directories but their owners, filegroups, sizes, dates, and times of modification.
• ls -lh: This option decreases the precision in the description of the file size. Instead of bytes, we get to see file sizes in megabytes, kilobytes, etc.
• ls -a: This options allows us to view hidden files. Some files are hidden because they may contain sensitive information or to prevent them from accidental deletion. In Linux and macOS, hidden files begin with a period; for example, .dockerenv and .bashrc are hidden files.
• ls -al: This option simply provides the long listing of hidden files as well.

Change directory command

It is pertinent to know how to change our current working directory. We can use the cd command like so: cd <name of directory>. For example, if we’re in home and would like to switch to usr (both home and usr are in the root directory), we would use cd /usr. If we’re in opt and would like to switch to a folder called google inside opt itself, we would use cd google. To quickly go to the root directory, cd / will work.

The cd command also comes with a couple of useful options:

• cd -: This option will change our current directory to the one we previously came from.
• cd ..: This option will switch us back to the parent directory.

Playground

Practice makes perfect. As such, try out all the commands discussed above in the terminal below. For the cd commands, note the change in the path as you run them.

Note: Sometimes, the terminal may not be as verbose or we may get a bit too engrossed in a task and may forget our current working directory. Of course, we could use a combination of ls and cd to figure it out, but there is also another simpler way; just use pwd.