Essential Linux Commands That Solve Over 95% of Common Problems

Operating System Overview

Linux is an operating system that runs directly on bare hardware and manages computer resources; all other software depends on it.

Filesystem Layout

/var

– variable files such as logs, spools, lock files, temporary files and page‑cached files. /home – each user’s personal files, configuration, documents, caches, etc. /proc – virtual files that expose kernel and process information; they occupy no disk space. /bin – essential binaries required for system boot, usable by all users. /etc – system configuration files (firewall rules, startup scripts, etc.). /root – home directory of the super‑user. /dev – device files that abstract hardware as files, enabling read/write operations.

Common Command Operations

Show current directory: pwd Change directory: cd [directory] – e.g., cd ~ (home), cd .. (parent), cd - (previous), cd / (root).

List files: ls, ls -l (long format), ls -a (include hidden), ls -la (both).

Create directories: mkdir folder, recursive mkdir -p a/b/c.

Remove empty directories: rmdir folder, recursive rmdir -p folder.

Delete files or directories: rm -rf path (forceful), rm -ri path (interactive).

Copy files/directories: cp source dest, recursive cp -r src dst, interactive cp -ri src dst.

Move/rename: mv old new (rename) or mv src dst (move).

Create empty file: touch filename.

Edit or view with vi: vi filename (opens existing file or creates a new one).

Display file content: cat filename, create and write cat > filename.

Show file head: head -n N filename.

Show file tail: tail -f filename (follow), tail -n N filename (last N lines).

Permission Management

Files have read ( r), write ( w) and execute ( x) bits for owner, group and others. Example listing:

drwxr-x-x. 2 root root 4096 Sep 23 2011 bin

d

– directory. rwx – owner has read, write, execute. r-x – group has read and execute. r-x – others have read and execute.

Change permissions with chmod using two modes:

Symbolic mode: specify u, g, o with + or -. Example: chmod u+rwx,g+rw,o+rw f01.

Numeric mode: three octal digits represent rwx bits. Example: chmod 753 f01 gives rwx to owner, r‑x to group, -wx to others; chmod 777 f01 gives full access to everyone.

User and Group Management

Each user has a unique UID; each group has a unique GID. IDs below 500 are typically reserved for system accounts, so new users should use IDs > 500.

Root is the super‑user (UID 0) and belongs to the root group.

Only root can add or delete users.

When a user is added without specifying a group, a private group with the same name is created.

User Operation Commands

Switch user: su username (temporary, retains current environment) or su - username (full login).

Current user: whoami.

Groups of current user: groups.

Show UID/GID: id.

Add user: useradd username or with UID useradd -u 1001 username.

Set password: passwd username.

Delete user: userdel username or remove home with userdel -r username.

Modify user: usermod [options] username (e.g., usermod -l newname oldname, usermod -g newgroup username).

Add group: groupadd groupname or with GID groupadd -g GID groupname.

Process Management

List processes: ps -ef | grep keyword. Output columns include user, PID, PPID, CPU usage, and command path.

Terminate process: kill -9 PID.

Other Common Commands

clear

– clear the terminal. man – view command manual. mnt – mount file systems.

SSH service control: service sshd start, service sshd restart, service sshd stop.

Run a Java JAR in background: nohup java -jar jar-0.0.1-SNAPSHOT.jar & (nohup prevents termination when the terminal closes).

Software Installation Methods

Three common methods on Linux:

Tar installation: extract source packages with tar -zxvf file.tar.gz, tar -jxvf file.tar.bz2 or tar -xvf file.tar and follow the software’s install steps.

RPM installation: Red Hat package manager; install with rpm -ivh package.rpm, query with rpm -qa | grep name, remove with rpm -e package.

YUM installation: front‑end to RPM that resolves dependencies automatically from online repositories.