Essential Linux Commands Every Java Backend Engineer Should Master

System Service Management

systemctl

systemctl is a combination of the service and chkconfig commands used to manage system services.

List the status of all services:

systemctl list-units --type=service

Check the running status of a service (example: firewalld):

systemctl status firewalld

Stop a service:

systemctl stop firewalld

Start a service:

systemctl start firewalld

Restart a service (regardless of its current state): systemctl restart firewalld Reload configuration without interrupting 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 a specified directory, e.g., the root directory:

ls -l /

pwd

Display the absolute path of the current working directory.

cd

Change the current working directory.

cd /usr/local

date

Show or modify the system date and time.

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

passwd

Set a user password.

passwd root

su

Switch user identity (e.g., become superuser).

su -

clear

Clear the terminal screen.

man

Display help information for a specific command.

man ls

who

Show the current runlevel:

who -r

Display users currently logged in:

who -buT

free

Show memory usage in megabytes.

free -m

ps

Display dynamic information about system processes: ps -ef Show the running status of a specific process (e.g., sshd):

ps -ef | grep sshd

top

View real‑time active processes, similar to Windows Task Manager.

mkdir

Create a directory.

more

Paginate file viewing, e.g., display 10 lines per page of boot.log:

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

cat

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

cat -Ab /var/log/boot.log

touch

Create a file, e.g., text.txt:

touch text.txt

rm

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

rm -rf testdir/

cp

Copy files or directories, e.g., copy test1 to test2:

cp -r /mydata/tes1 /mydata/test2

mv

Move or overwrite a file:

mv text.txt text2.txt

Compression and Extraction

tar

Archive files in /etc to etc.tar (no compression): tar -cvf /mydata/etc.tar /etc Compress /etc with gzip to etc.tar.gz: tar -zcvf /mydata/etc.tar.gz /etc Compress /etc with bzip2 to /etc.tar.bz2:

tar -jcvf /mydata/etc.tar.bz2 /etc

Page through a gzip archive's contents:

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

Extract a gzip archive to the current directory: tar -zxvf /mydata/etc.tar.gz Extract a gzip archive to a specific directory:

tar -zxvf /mydata/etc.tar.gz -C /mydata/etc

Disk and Network Management

df

Show disk space usage:

df -hT

du

Show size of files and directories in the current directory:

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

ifconfig

Display the status of network interfaces.

netstat

Show routing information:

netstat -rn

List all active TCP connections: netstat -an Show listening services:

netstat -tulnp

Display system resources in connection state:

netstat -atunp

wget

Download files from the internet.

File Upload and Download

Install the upload/download tool lrzsz: yum install -y lrzsz Upload a file using XShell (command rz triggers the upload dialog).

Download a file using XShell (command sz fileName triggers the save dialog).

Software Installation and Management

rpm

RPM (Red‑Hat Package Manager) is a generic package format for Linux, handling .rpm files.

Install a package: rpm -ivh nginx-1.12.2-2.el7.x86_64.rpm Fuzzy search for a package: rpm -qa | grep nginx Exact query for a package: rpm -qa nginx List files installed by a package: rpm -ql nginx-1.12.2-2.el7.x86_64 Show package summary information: rpm -qi nginx-1.12.2-2.el7.x86_64 Verify package contents against installed files: rpm -V nginx-1.12.2-2.el7.x86_64 Update 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

Yum (Yellow dog Updater, Modified) automatically downloads RPM packages, resolves dependencies, and installs them.

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