Linux Fundamentals
Linux Fundamentals tests how well you navigate and manipulate a Linux system: the filesystem hierarchy, permissions and ownership, the shell, processes, and basic package management. Questions are practical; you must read a command or permission string and predict the result.
Filesystem and paths
Linux presents one rooted tree. /etc holds host configuration, /var holds variable data such as logs, /home holds user files, /tmp is world-writable and periodically cleaned, and /usr holds most installed programs and libraries. Absolute paths begin at /; relative paths begin at the current directory. . is the current directory and .. is its parent.
Permissions
Each file has an owner, a group, and a permission triad for user, group, and other. Read, write, and execute are worth 4, 2, and 1. chmod 640 report.txt sets rw- for the owner, r-- for the group, and nothing for others. chown changes ownership; on most systems only root can give a file to another user. Execute on a directory means you may enter or traverse it, while read means you may list its names.
Processes and packages
ps shows processes, top shows live resource use, and signals such as SIGTERM (15) ask a process to stop while SIGKILL (9) cannot be caught. Package managers (apt, dnf, zypper) install from repositories and track dependencies.
Worked example: a script must be readable and executable by its owner, readable by its group, and inaccessible to others. Owner needs 4+2+1 = 7; group needs 4; other needs 0. The mode is 750, applied with chmod 750 script.sh. Choosing 770 would wrongly expose the file to other users if the group is broad.
Traps: treating rwx on a directory as file read/write; forgetting that root bypasses permission checks; confusing owner and group when a user belongs to several groups; and assuming chmod +x sets execute only for the owner, when it applies to all classes by default.