Linux Permissions and sudo in Practical DevSecOps Labs

Understanding file permissions and privilege escalation is essential for working in the Practical DevSecOps labs. This article covers how to check permissions, make files executable, and run commands as different users.

Table of Contents

1. Understanding File Permissions
2. Running Commands with sudo
3. Common Questions

Understanding File Permissions

Checking permissions

ls -l myfile      # long listing shows permissions
stat myfile        # detailed file status

Making files executable

To run a file as a script or program, it must have executable permissions:

chmod +x myfile

After adding executable permissions, verify with ls -l:

-rwxr-xr-x 1 root root 3 Aug 13 20:36 myfile

The x in the permission string indicates the file is now executable.

Running the file

./myfile

Note: The ./ prefix tells Linux to look for the executable in the current directory. Without it, Linux only searches directories in $PATH.

Running Commands with sudo

The sudo command lets you run commands with elevated privileges (as root by default).

Basic usage

sudo ls

Creating and switching to a new user

adduser --gecos "" john          # create user john
usermod -aG sudo john           # add john to sudo group
sudo su - john                   # switch to john

Once logged in as john, try reading a sensitive file:

cat /etc/shadow

cat: /etc/shadow: Permission denied

Even as a regular user in the sudo group, you need to explicitly use sudo to access privileged resources:

echo pdevsecops | sudo -S cat /etc/shadow

The -S flag tells sudo to read the password from standard input, enabling automation.

Returning to root

exit

Note: The sudoers file controls which users and groups can use sudo. Administrators use it to delegate permissions without sharing the root password.

Read more about Linux file permissions at Understanding Linux File Permissions.

Common Questions

Wrap-Up

File permissions control who can read, write, and execute files. Use chmod +x to make scripts executable, sudo to escalate privileges when needed, and ls -l to inspect permissions. These are fundamental skills used throughout every DevSecOps lab.