In this reading, you’ll continue exploring Linux commands, which can help you filter for the information you need. You’ll learn a new Linux command, find, which can help you search files and directories for specific information.

Filtering for information

You previously explored how filtering for information is an important skill for security analysts. Filtering is selecting data that match a certain condition. For example, if you had a virus in your system that only affected the .txt files, you could use filtering to find these files quickly. Filtering allows you to search based on specific criteria, such as file extension or a string of text.

grep

The grep command searches a specified file and returns all lines in the file containing a specified string or text. The grep command commonly takes two arguments: a specific string to search for and a specific file to search through.

For example, entering grep OS updates.txt returns all lines containing OS in the updates.txt file. In this example, OS is the specific string to search for, and updates.txt is the specific file to search through.

Let’s look at another example: grep error time_logs.txt. Here grep is used to search for the text pattern. error is the term you are looking for in the time_logs.txt file. When you run this command, grep will scan the time_logs.txt file and print only the lines containing the word error.

Piping

The pipe command is accessed using the pipe character (|). Piping sends the standard output of one command as standard input to another command for further processing. As a reminder, standard output is information returned by the OS through the shell, and standard input is information received by the OS via the command line. 

The pipe character (|) is located in various places on a keyboard. On many keyboards, it’s located on the same key as the backslash character (\). On some keyboards, the | can look different and have a small space through the middle of the line. If you can’t find the |, search online for its location on your particular keyboard.

When used with grep, the pipe can help you find directories and files containing a specific word in their names. For example, ls /home/analyst/reports | grep users returns the file and directory names in the reports directory that contain users. Before the pipe, ls indicates to list the names of the files and directories in reports. Then, it sends this output to the command after the pipe. In this case, grep users returns all of the file or directory names containing users from the input it received.

Note: Piping is a general form of redirection in Linux and can be used for multiple tasks other than filtering. You can think of piping as a general tool that you can use whenever you want the output of one command to become the input of another command.

find

The find command searches for directories and files that meet specified criteria. There’s a wide range of criteria that can be specified with find. For example, you can search for files and directories that

• Contain a specific string in the name,

• Are a certain file size, or

• Were last modified within a certain time frame.

When using find, the first argument after find indicates where to start searching. For example, entering find /home/analyst/projects searches for everything starting at the projects directory.

After this first argument, you need to indicate your criteria for the search. If you don’t include a specific search criteria with your second argument, your search will likely return a lot of directories and files. 

Specifying criteria involves options. Options modify the behavior of a command and commonly begin with a hyphen (-). 

-name and -iname

One key criteria analysts might use with find is to find file or directory names that contain a specific string. The specific string you’re searching for must be entered in quotes after the -name or -iname options. The difference between these two options is that -name is case-sensitive, and -iname is not. 

For example, you might want to find all files in the projects directory that contain the word “log” in the file name. To do this, you’d enter find /home/analyst/projects -name "*log*". You could also enter find /home/analyst/projects -iname "*log*".

In these examples, the output would be all files in the projects directory that contain log surrounded by zero or more characters. The "*log*" portion of the command is the search criteria that indicates to search for the string “log”. When -name is the option, files with names that include Log or LOG, for example, wouldn’t be returned because this option is case-sensitive. However, they would be returned when -iname is the option.

Note: An asterisk (*) is used as a wildcard to represent zero or more unknown characters.

-mtime

Security analysts might also use find to find files or directories last modified within a certain time frame. The -mtime option can be used for this search. For example, entering find /home/analyst/projects -mtime -3 returns all files and directories in the projects directory that have been modified within the past three days. 

The -mtime option search is based on days, so entering -mtime +1 indicates all files or directories last modified more than one day ago, and entering -mtime -1 indicates all files or directories last modified less than one day ago. 

Note: The option -mmin can be used instead of -mtime if you want to base the search on minutes rather than days.

Key takeaways

Filtering for information using Linux commands is an important skill for security analysts so that they can customize data to fit their needs. Three key Linux commands for this are grep, piping (|), and find. These commands can be used to navigate and filter for information in the file system.

• Consider the privacy and security implications of using AI. Consider how using AI tools may affect the security of other people or organizations.

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Permission commands

Previously, you explored file permissions and the commands that you can use to display and change them.  In this reading, you’ll review these concepts and also focus on an example of how these commands work together when putting the principle of least privilege into practice.

Reading permissions

In Linux, permissions are represented with a 10-character string. Permissions include:

• read: for files, this is the ability to read the file contents; for directories, this is the ability to read all contents in the directory including both files and subdirectories

• write: for files, this is the ability to make modifications on the file contents; for directories, this is the ability to create new files in the directory

• execute: for files, this is the ability to execute the file if it’s a program; for directories, this is the ability to enter the directory and access its files

These permissions are given to these types of owners:

• user: the owner of the file

• group: a larger group that the owner is a part of

• other: all other users on the system

Each character in the 10-character string conveys different information about these permissions. The following table describes the purpose of each character:

Exploring existing permissions

You can use the ls command to investigate who has permissions on files and directories. Previously, you learned that ls displays the names of files in directories in the current working directory.

There are additional options you can add to the ls command to make your command more specific. Some of these options provide details about permissions. Here are a few important ls options for security analysts:

• ls -a: Displays hidden files. Hidden files start with a period (.) at the beginning.

• ls -l: Displays permissions to files and directories. Also displays other additional information, including owner name, group, file size, and the time of last modification.

• ls -la: Displays permissions to files and directories, including hidden files. This is a combination of the other two options.

Changing permissions

The principle of least privilege is the concept of granting only the minimal access and authorization required to complete a task or function. In other words, users should not have privileges that are beyond what is necessary. Not following the principle of least privilege can create security risks.

The chmod  command can help you manage this authorization. The chmod command changes permissions on files and directories.

Using chmod

The chmod command requires two arguments. The first argument indicates how to change permissions, and the second argument indicates the file or directory that you want to change permissions for.  For example, the following command would add all permissions to login_sessions.txt:

chmod u+rwx,g+rwx,o+rwx login_sessions.txt

If you wanted to take all the permissions away, you could use

chmod u-rwx,g-rwx,o-rwx login_sessions.txt

Another way to assign these permissions is to use the equals sign (=) in this first argument. Using = with chmod sets, or assigns, the permissions exactly as specified. For example, the following command would set read permissions for login_sessions.txt for user, group, and other:

chmod u=r,g=r,o=r login_sessions.txt

This command overwrites existing permissions. For instance, if the user previously had write permissions, these write permissions are removed after you specify only read permissions with =.

The following table reviews how each character is used within the first argument of chmod:

Note: When there are permission changes to more than one owner type, commas are needed to separate changes for each owner type. You should not add spaces after those commas.

The principle of least privilege in action

As a security analyst, you may encounter a situation like this one: There’s a file called bonuses.txt within a compensation directory. The owner of this file is a member of the Human Resources department with a username of hrrep1. It has been decided that hrrep1 needs access to this file. But, since this file contains confidential information, no one else in the hr group needs access.

You run ls -l to check the permissions of files in the compensation directory and discover that the permissions for bonuses.txt are -rw-rw----. The group owner type has read and write permissions that do not align with the principle of least privilege.  

To remedy the situation, you input chmod g-rw bonuses.txt. Now, only the user who needs to access this file to carry out their job responsibilities can access this file.

File Permissions in Linux — Google

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Responsible use of sudo

Previously, you explored authorization, authentication, and Linux commands with sudo, useradd, and userdel. The sudo command is important for security analysts because it allows users to have elevated permissions without risking the system by running commands as the root user. You’ll continue exploring authorization, authentication, and Linux commands in this reading and learn two more commands that can be used with sudo: usermod and chown. 

Responsible use of sudo

To manage authorization and authentication, you need to be a root user, or a user with elevated privileges to modify the system. The root user can also be called the “super user.” You become a root user by logging in as the root user. However, running commands as the root user is not recommended in Linux because it can create security risks if malicious actors compromise that account. It’s also easy to make irreversible mistakes, and the system can’t track who ran a command. For these reasons, rather than logging in as the root user, it’s recommended you use sudo in Linux when you need elevated privileges.

The sudo command temporarily grants elevated permissions to specific users. The name of this command comes from “super user do.” Users must be given access in a configuration file to use sudo. This file is called the “sudoers file.” Although using sudo is preferable to logging in as the root user, it's important to be aware that users with the elevated permissions to use sudo might be more at risk in the event of an attack.

You can compare this to a hotel with a master key. The master key can be used to access any room in the hotel. There are some workers at the hotel who need this key to perform their work. For example, to clean all the rooms, the janitor would scan their ID badge and then use this master key. However, if someone outside the hotel’s network gained access to the janitor’s ID badge and master key, they could access any room in the hotel. In this example, the janitor with the master key represents a user using sudo for elevated privileges. Because of the dangers of sudo, only users who really need to use it should have these permissions.

Additionally, even if you need access to sudo, you should be careful about using it with only the commands you need and nothing more. Running commands with sudo allows users to bypass the typical security controls that are in place to prevent elevated access to an attacker.

Note: Be aware of sudo if copying commands from an online source. It’s important you don’t use sudo accidentally. 

Authentication and authorization with sudo

You can use sudo with many authentication and authorization management tasks. As a reminder, authentication is the process of verifying who someone is, and authorization is the concept of granting access to specific resources in a system. Some of the key commands used for these tasks include the following:

useradd

The useradd command adds a user to the system. To add a user with the username of fgarcia with sudo, enter sudo useradd fgarcia. There are additional options you can use with useradd:

• -g: Sets the user’s default group, also called their primary group

• -G: Adds the user to additional groups, also called supplemental or secondary groups

To use the -g option, the primary group must be specified after -g. For example, entering sudo useradd -g security fgarcia adds fgarcia as a new user and assigns their primary group to be security.

To use the -G option, the supplemental group must be passed into the command after -G. You can add more than one supplemental group at a time with the -G option. Entering sudo useradd -G finance,admin fgarcia adds fgarcia as a new user and adds them to the existing finance and admin groups.

usermod

The usermod command modifies existing user accounts. The same -g and -G options from the useradd command can be used with usermod if a user already exists. 

To change the primary group of an existing user, you need the -g option. For example, entering sudo usermod -g executive fgarcia would change fgarcia’s primary group to the executive group.

To add a supplemental group for an existing user, you need the -G option. You also need a -a option, which appends the user to an existing group and is only used with the -G option. For example, entering sudo usermod -a -G marketing fgarcia would add the existing fgarcia user to the supplemental marketing group.

Note: When changing the supplemental group of an existing user, if you don't include the -a option, -G will replace any existing supplemental groups with the groups specified after usermod.  Using -a with -G ensures that the new groups are added but existing groups are not replaced.

There are other options you can use with usermod to specify how you want to modify the user, including:

• -d: Changes the user’s home directory.

• -l: Changes the user’s login name.

• -L: Locks the account so the user can’t log in.

The option always goes after the usermod command. For example, to change fgarcia’s home directory to /home/garcia_f, enter sudo usermod -d /home/garcia_f fgarcia. The option -d directly follows the command usermod before the other two needed arguments.

userdel

The userdel command deletes a user from the system. For example, entering sudo userdel fgarcia deletes fgarcia as a user. Be careful before you delete a user using this command.

The userdel command doesn’t delete the files in the user’s home directory unless you use the -r option. Entering sudo userdel -r fgarcia would delete fgarcia as a user and delete all files in their home directory. Before deleting any user files, you should ensure you have backups in case you need them later.

Note: Instead of deleting the user, you could consider deactivating their account with usermod -L. This prevents the user from logging in while still giving you access to their account and associated permissions. For example, if a user left an organization, this option would allow you to identify which files they have ownership over, so you could move this ownership to other users.

chown

The chown command changes ownership of a file or directory. You can use chown to change user or group ownership. To change the user owner of the access.txt file to fgarcia, enter sudo chown fgarcia access.txt. To change the group owner of access.txt to security, enter sudo chown :security access.txt. You must enter a colon (:) before security to designate it as a group name.

Similar to useradd, usermod, and userdel, there are additional options that can be used with chown.