Master Essential Linux Commands and System Administration Basics

ls Command

Common usage examples of ls with options -l, -a, -F, -r, -R, -lS, -g, -lG, -li, -lt, and --version.

# ls
# ls -l
# ls -a
# ls -F
# ls -r
# ls -R
# ls -lS
# ls -g
# ls -lG
# ls -li
# ls -lt
# ls --version

cat Command

Examples of cat for displaying files, numbering lines, creating and appending files, and showing help.

# cat -n a
# cat > b <<EOF
# cat a > b
# cat a >> b
# cat --help

User and Group Management

Explanation of root and regular users, UID/GID ranges, and the formats of /etc/passwd, /etc/shadow, and /etc/group files with examples.

# cat /etc/passwd
# cat /etc/shadow
# cat /etc/group

Package Management Basics

Overview of RPM, yum, and related commands for installing and managing packages.

# rpm -ivh package.rpm
# yum install package

Process Management Basics

Descriptions of process types, attributes, and common tools: ps, top, kill, /proc, and free.

# ps aux
# top
# kill -9 1234
# free -m

File Search Utilities

Usage of which, whereis, locate, grep, and find with practical examples.

# which ls
# whereis ls
# locate filename
# grep pattern file
# find . -name "test*"

Scheduled Tasks

Creating one‑time tasks with at and recurring tasks with crontab, including syntax examples.

# at 21:17
# crontab -e
# 30 20 * * * /usr/local/nginx/sbin/nginx -s reload

Expect Scripting

Brief introduction to Expect for automating interactive sessions, with a sample script for password‑less SSH.

#!/usr/bin/expect -f
spawn ssh root@ip
expect {
    "yes/no" { send "yes\r"; exp_continue }
    "*password:" { send "password\r" }
}
interact

These sections provide a solid foundation for Linux system administration and command‑line proficiency.