Master Essential Linux Commands for System Management on CentOS 7.6

System Service Management

systemctl

List all service units: systemctl list-units --type=service Check a service status: systemctl status firewalld Stop a service: systemctl stop firewalld Start a service: systemctl start firewalld Restart a service (regardless of current state): systemctl restart firewalld Reload configuration without stopping the service: systemctl reload firewalld Disable a service from starting at boot: systemctl disable firewalld Enable a service to start at boot:

systemctl enable firewalld

File Management

ls

List all files in the root directory:

ls -l /

pwd

Show the absolute path of the current working directory.

cd

Change the current directory:

cd /usr/local

date

Display or modify system date and time:

date '+%Y-%m-%d %H:%M:%S'

passwd

Set a user password (example for root):

passwd root

su

Switch to another user, e.g., become superuser:

su -

clear

Clear the terminal screen.

man

Show manual page for a command, e.g., ls:

man ls

who

Show the current runlevel: who -r List currently logged‑in users:

who -bUT

free

Display memory usage in megabytes:

free -m

ps

Show running processes: ps -ef Filter for sshd processes:

ps -ef | grep sshd

top

Interactive view of active processes, similar to Windows Task Manager.

mkdir

Create a new directory.

more

Paginated view of a long file, e.g., show 10 lines per page of /var/log/boot.log:

more -c -10 /var/log/boot.log

cat

Display a file with line numbers, e.g., boot log:

cat -Ab /var/log/boot.log

touch

Create an empty file named text.txt:

touch text.txt

rm

Delete a file: rm text.txt Force delete a directory and its contents:

rm -rf testdir/

cp

Copy a directory recursively:

cp -r /mydata/tes1 /mydata/test2

mv

Move or rename a file:

mv text.txt text2.txt

Compression and Extraction

tar

Archive /etc without compression: tar -cvf /mydata/etc.tar /etc Compress /etc with gzip: tar -zcvf /mydata/etc.tar.gz /etc Compress /etc with bzip2: tar -jcvf /mydata/etc.tar.bz2 /etc List contents of a gzip archive (paged): tar -ztvf /mydata/etc.tar.gz | more -c -10 Extract a gzip archive to the current directory:

tar -zxvf /mydata/etc.tar.gz

Disk and Network Management

df

Show disk space usage in human‑readable format:

df -hT

du

Show size of files and directories in the current folder (depth 1):

du -h --max-depth=1 ./*

ifconfig

Display network interface status.

netstat

Show routing table: netstat -rn Show all active TCP connections: netstat -an Show listening services with program names: netstat -tulnp Show all connections with process info:

netstat -atunp

wget

Download a file from the internet.

Software Installation and Management

rpm

Install a package: rpm -ivh nginx-1.12.2-2.el7.x86_64.rpm Search packages (fuzzy): rpm -qa | grep nginx Query exact package name: rpm -qa nginx List files installed by a package: rpm -ql nginx-1.12.2-2.el7.x86_64 Show package information: rpm -qi nginx-1.12.2-2.el7.x86_64 Verify package integrity: rpm -V nginx-1.12.2-2.el7.x86_64 Upgrade a package: rpm -Uvh nginx-1.12.2-2.el7.x86_64 Remove a package:

rpm -e nginx-1.12.2-2.el7.x86_64

yum

Install a package: yum install nginx Check for available updates: yum check-update Update a specific package: yum update nginx Search package information in repositories: yum info nginx* List all installed packages: yum info installed List package names matching a pattern: yum list nginx* Fuzzy search for packages:

yum search nginx

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