Section 1.1: The sum Command

This command is about as straightforward as it gets. With only four recognized parameters (including the --help flag), the sum utility generates a checksum and block count for the specified input file.

For instance, if you create a simple text file containing:

line 1

line 2

line 3

and then process it with the sum command:

> sum file.txt

22592 1

The output displays the checksum on the left and the number of blocks occupied by the file on the right—remarkably simple.

This utility is valuable for quickly comparing files or verifying if a file has changed since the last checksum. Alternatively, the cksum utility can be employed for a checksum that counts bytes instead of blocks.

Section 1.2: The mktemp Utility

This tool is perfect for creating temporary files. Sometimes, you need to hold onto information in a file until a certain process is complete. With mktemp, you can do this swiftly and securely. Instead of manually creating files in the /tmp directory, mktemp automates the process with a single command, eliminating the hassle of checking for pre-existing files or generating unique filenames.

Simply executing mktemp yields a name and path for your new temporary file:

/tmp/tmp.xWrpgjoJgk

Now, you can seamlessly store data in a temporary file without the need for redundant effort.

Section 1.3: The namei Command

Navigating paths in Linux can often be intricate, especially when dealing with multiple symlinks. The namei utility provides clarity by showing you the full hierarchy of a single path and the steps it takes to reach the end.

For instance, if you have a directory structured like this:

/tmp/real_dir/test_file

and create a symlink called symlink_dir, you can use namei to trace the path to the test file through the symlink:

f: /tmp/symlink_dir/test_file

d /

d tmp

l symlink_dir -> /tmp/real_dir

d /

d tmp

d real_dir

• test_file

This output reveals the complete path to test_file, including traversing through symlink_dir and back to real_dir. Instead of laboriously checking each directory or symlink, namei presents a clear overview instantly.

Section 1.4: The sg Command

Whether you are already familiar with this command or not, the sg utility is incredibly useful. It allows you to switch groups and execute commands as that group. For example, if you want to run a command as an admin group without being a member, you can use:

sg admin 'ls -lah'

This command runs ls -lah as the admin group and returns you to your original group afterward. It's particularly useful for testing group permissions or executing scripts with different privileges.

Section 1.5: The last Utility

Maintaining security is crucial on any Linux system, especially those hosted in the cloud. If you suspect unauthorized access or malicious activity, the last utility can provide insights.

Executing last displays the login history of all users, including their remote IP addresses, login times, and current statuses. Additionally, it reveals the duration of each session, helping you identify any unusual patterns, such as numerous short-duration sessions that may indicate a script or automated process logging in and out.

Section 1.6: The tac Command

Most users are acquainted with the cat command, but are you aware of its lesser-known counterpart, tac? The tac command (which is "cat" spelled backward) performs almost the same function but displays the contents of a file in reverse order.

For instance, if your file contains:

line 1

line 2

line 3

line 4

line 5

Using cat would yield the standard output, while tac produces:

line 5

line 4

line 3

line 2

line 1

While this may seem trivial at first glance, it can be particularly beneficial for large log files where you want to view the most recent entries first.

Thanks for exploring these hidden gems of Linux utilities! If you have your own favorite commands, don't hesitate to share them with us. For even more Linux command insights, check out the links below:

• 6 More Terminal Commands You Should Know
• 5 Linux Utilities That Will Brighten Your Day