Master Essential Linux Commands for System Management on CentOS 7.6

System Service Management

systemctl

List all service units:

<code>systemctl list-units --type=service</code>

Check a service status:

<code>systemctl status firewalld</code>

Stop a service:

<code>systemctl stop firewalld</code>

Start a service:

<code>systemctl start firewalld</code>

Restart a service (regardless of current state):

<code>systemctl restart firewalld</code>

Reload configuration without stopping the service:

<code>systemctl reload firewalld</code>

Disable a service from starting at boot:

<code>systemctl disable firewalld</code>

Enable a service to start at boot:

<code>systemctl enable firewalld</code>

File Management

ls

List all files in the root directory:

<code>ls -l /</code>

pwd

Show the absolute path of the current working directory.

cd

Change the current directory:

<code>cd /usr/local</code>

date

Display or modify system date and time:

<code>date '+%Y-%m-%d %H:%M:%S'</code>

passwd

Set a user password (example for root):

<code>passwd root</code>

su

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

<code>su -</code>

clear

Clear the terminal screen.

man

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

<code>man ls</code>

who

Show the current runlevel:

<code>who -r</code>

List currently logged‑in users:

<code>who -bUT</code>

free

Display memory usage in megabytes:

<code>free -m</code>

ps

Show running processes:

<code>ps -ef</code>

Filter for sshd processes:

<code>ps -ef | grep sshd</code>

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:

<code>more -c -10 /var/log/boot.log</code>

cat

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

<code>cat -Ab /var/log/boot.log</code>

touch

Create an empty file named text.txt:

<code>touch text.txt</code>

rm

Delete a file:

<code>rm text.txt</code>

Force delete a directory and its contents:

<code>rm -rf testdir/</code>

cp

Copy a directory recursively:

<code>cp -r /mydata/tes1 /mydata/test2</code>

mv

Move or rename a file:

<code>mv text.txt text2.txt</code>

Compression and Extraction

tar

Archive /etc without compression:

<code>tar -cvf /mydata/etc.tar /etc</code>

Compress /etc with gzip:

<code>tar -zcvf /mydata/etc.tar.gz /etc</code>

Compress /etc with bzip2:

<code>tar -jcvf /mydata/etc.tar.bz2 /etc</code>

List contents of a gzip archive (paged):

<code>tar -ztvf /mydata/etc.tar.gz | more -c -10</code>

Extract a gzip archive to the current directory:

<code>tar -zxvf /mydata/etc.tar.gz</code>

Disk and Network Management

df

Show disk space usage in human‑readable format:

<code>df -hT</code>

du

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

<code>du -h --max-depth=1 ./*</code>

ifconfig

Display network interface status.

netstat

Show routing table:

<code>netstat -rn</code>

Show all active TCP connections:

<code>netstat -an</code>

Show listening services with program names:

<code>netstat -tulnp</code>

Show all connections with process info:

<code>netstat -atunp</code>

wget

Download a file from the internet.

Software Installation and Management

rpm

Install a package:

<code>rpm -ivh nginx-1.12.2-2.el7.x86_64.rpm</code>

Search packages (fuzzy):

<code>rpm -qa | grep nginx</code>

Query exact package name:

<code>rpm -qa nginx</code>

List files installed by a package:

<code>rpm -ql nginx-1.12.2-2.el7.x86_64</code>

Show package information:

<code>rpm -qi nginx-1.12.2-2.el7.x86_64</code>

Verify package integrity:

<code>rpm -V nginx-1.12.2-2.el7.x86_64</code>

Upgrade a package:

<code>rpm -Uvh nginx-1.12.2-2.el7.x86_64</code>

Remove a package:

<code>rpm -e nginx-1.12.2-2.el7.x86_64</code>

yum

Install a package:

<code>yum install nginx</code>

Check for available updates:

<code>yum check-update</code>

Update a specific package:

<code>yum update nginx</code>

Search package information in repositories:

<code>yum info nginx*</code>

List all installed packages:

<code>yum info installed</code>

List package names matching a pattern:

<code>yum list nginx*</code>

Fuzzy search for packages:

<code>yum search nginx</code>

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