Data & Development Cheat Sheet

Linux Commands Cheat Sheet: Essential Commands & Examples

A practical Linux command reference for navigating the terminal, managing files and permissions, finding text, monitoring processes, troubleshooting networks, installing packages, and administering everyday systems.

Copyable commands
Practical examples
Distribution notes
Safety warnings

20 structured sections Beginner to intermediate Ubuntu, Debian, Fedora, RHEL, and Arch guidance

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Essential Linux commands with examples
Task	Command	What it does
Show current directory	`pwd`	Prints the absolute path of the current working directory.
List files	`ls -la`	Lists entries in long format, including hidden names.
Change directory	`cd /path/to/directory`	Changes the shell’s current working directory.
Create directories	`mkdir -p project/src`	Creates the directory and any missing parent directories.
Create an empty file	`touch notes.txt`	Creates the file if absent or updates its timestamps if it exists.
Copy a directory	`cp -r source/ backup/`	Copies the source directory and its contents recursively.
Move or rename	`mv old-name.txt new-name.txt`	Moves an entry or renames it when both paths share a filesystem.
Remove with confirmation	`rm -i file.txt`	Prompts before removing the named file.
Read a text file	`less file.txt`	Opens text in a scrollable pager without editing the file.
Search text recursively	`grep -Rni "error" .`	Searches below the current directory and prints line numbers.
Find files by name	`find . -type f -name "*.log"`	Finds regular files ending in `.log` below the current directory.
Inspect processes	`ps aux`	Displays a snapshot of processes with user and resource details.
Check free disk space	`df -h`	Reports filesystem space using human-readable units.
Show network addresses	`ip address show`	Displays network interfaces and their configured addresses.
Show kernel and system details	`uname -a`	Prints available kernel, machine, and operating-system information.
Open command documentation	`man command`	Opens the installed manual page for a command when available.

Linux terminal navigation commands with descriptions and examples
Command	What it does	Example
`pwd`	Prints the absolute path of the current working directory.	`pwd`
`ls`	Lists files and directories in the current location.	`ls`
`ls -lah`	Shows hidden entries, permissions, ownership, and human-readable sizes.	`ls -lah /var/log`
`cd /path/to/directory`	Moves to a directory using an absolute path.	`cd /var/log`
`cd directory`	Moves to a directory relative to the current location.	`cd projects`
`cd ..`	Moves up one level to the parent directory.	`cd ..`
`cd ../..`	Moves up two directory levels.	`cd ../..`
`cd ~`	Moves to the current user’s home directory.	`cd ~`
`cd -`	Returns to the previous working directory.	`cd -`
`realpath FILE`	Prints the resolved absolute path of a file or directory.	`realpath ./report.txt`
`pushd DIRECTORY`	Saves the current location on the directory stack and moves elsewhere.	`pushd /etc`
`popd`	Returns to the most recently saved directory on the stack.	`popd`

Absolute path

An absolute path begins at the filesystem root and starts with /. It identifies the same location regardless of your current directory.

cd /home/alex/projects

Relative path

A relative path starts from your current directory. Use . for the current directory and .. for its parent.

cd ../shared

Linux commands for managing files and directories
Command	What it does	Example
`touch FILE`	Creates an empty file or updates an existing file’s timestamps.	`touch notes.txt`
`mkdir DIRECTORY`	Creates a new directory.	`mkdir reports`
`mkdir -p PATH`	Creates a directory and any missing parent directories.	`mkdir -p projects/site/assets`
`cp SOURCE DESTINATION`	Copies a file to another path.	`cp report.txt backup.txt`
`cp -r SOURCE DIRECTORY`	Recursively copies a directory and its contents.	`cp -r website website-backup`
`cp -a SOURCE DESTINATION`	Copies recursively while preserving metadata and symbolic links.	`cp -a config config-backup`
`mv SOURCE DESTINATION`	Moves a file or directory, or renames it at the same location.	`mv draft.txt final.txt`
`rm -i FILE`	Prompts for confirmation before removing a file.	`rm -i old-report.txt`
`rmdir DIRECTORY`	Removes a directory only when it is empty.	`rmdir empty-folder`
`ln -s TARGET LINK`	Creates a symbolic link that points to another path.	`ln -s /var/log/app.log app.log`
`stat FILE`	Displays detailed size, permission, ownership, and timestamp information.	`stat report.txt`
`file FILE`	Examines a file and reports its detected type.	`file archive.tar.gz`
`basename PATH`	Returns the final filename or directory component of a path.	`basename /var/log/syslog`
`dirname PATH`	Returns the directory portion of a path.	`dirname /var/log/syslog`

Copy without accidental overwrites

Add -i to request confirmation when the destination already exists.

cp -i report.txt backups/report.txt

Rename several files carefully

Preview filenames before running a loop that modifies them. Quoting variables prevents spaces and wildcard characters from being split.

printf '%s\n' *.txt

Linux commands for viewing, editing, and processing text files
Command	What it does	Example
`cat FILE`	Writes the complete contents of one or more files to standard output.	`cat notes.txt`
`less FILE`	Opens a file in a scrollable viewer without loading it into an editor.	`less /var/log/syslog`
`head -n 20 FILE`	Displays the first 20 lines of a file.	`head -n 20 access.log`
`tail -n 20 FILE`	Displays the final 20 lines of a file.	`tail -n 20 access.log`
`tail -f FILE`	Continues displaying lines as they are appended to a file.	`tail -f app.log`
`nl FILE`	Displays text with line numbers added to non-empty lines.	`nl config.ini`
`wc -l FILE`	Counts newline characters, commonly used to report a file’s line count.	`wc -l users.csv`
`sort FILE`	Sorts text lines and writes the result to standard output.	`sort names.txt`
`sort FILE \| uniq`	Sorts lines and collapses adjacent duplicate lines.	`sort names.txt \| uniq`
`cut -d',' -f1 FILE`	Prints the first delimiter-separated field from every line.	`cut -d',' -f1 users.csv`
`tr 'a-z' 'A-Z'`	Translates lowercase ASCII letters to uppercase in standard input.	`tr 'a-z' 'A-Z' < names.txt`
`sed 's/OLD/NEW/g' FILE`	Replaces every occurrence of a pattern in the displayed output.	`sed 's/http:/https:/g' links.txt`
`nano FILE`	Opens a file in the Nano terminal text editor.	`nano notes.txt`
`vim FILE`	Opens a file in Vim when that editor is installed.	`vim notes.txt`

Search inside less

Press /, type a search term, and press Enter. Use n for the next match, N for the previous match, and q to quit.

less application.log

Count unique values

Sort the input before using uniq -c, because uniq only detects duplicate lines that are adjacent.

sort status.txt | uniq -c | sort -nr

Linux commands for locating files and searching text
Command	What it does	Example
`find PATH -name 'PATTERN'`	Searches below a path for names matching a case-sensitive pattern.	`find . -name '*.log'`
`find PATH -iname 'PATTERN'`	Searches for names without distinguishing uppercase and lowercase.	`find . -iname '*.jpg'`
`find PATH -type f`	Returns regular files while excluding directories and other file types.	`find ./reports -type f`
`find PATH -type d`	Returns directories below the selected path.	`find . -type d`
`find PATH -size +100M`	Finds entries larger than 100 mebibytes.	`find /var -type f -size +100M`
`find PATH -mtime -7`	Finds entries modified within the last seven 24-hour periods.	`find . -type f -mtime -7`
`grep 'PATTERN' FILE`	Prints lines in a file that match a basic regular expression.	`grep 'ERROR' application.log`
`grep -in 'PATTERN' FILE`	Searches without case sensitivity and includes matching line numbers.	`grep -in 'warning' application.log`
`grep -Rni 'PATTERN' PATH`	Recursively searches readable files below a directory.	`grep -Rni 'database_url' ./config`
`grep -v 'PATTERN' FILE`	Prints lines that do not match the pattern.	`grep -v '^#' settings.conf`
`grep -E 'ONE\|TWO' FILE`	Uses an extended regular expression to match either alternative.	`grep -E 'ERROR\|WARNING' application.log`
`locate NAME`	Queries a filename database when the locate utility and database are available.	`locate nginx.conf`
`command -v COMMAND`	Reports the command that the current shell would execute.	`command -v python3`
`type COMMAND`	Identifies whether a name is an alias, function, builtin, or executable.	`type cd`

Restrict the search depth

Use -maxdepth immediately after the starting path when you do not want find to descend through an entire tree.

find . -maxdepth 2 -type f -name '*.conf'

Use ripgrep when available

rg recursively searches text and normally respects ignore files such as .gitignore. It may need to be installed separately.

rg -n -i 'timeout' ./src

Linux commands for managing file permissions and ownership
Command	What it does	Example
`ls -l FILE`	Displays the file type, permission bits, owner, and group.	`ls -l deploy.sh`
`stat FILE`	Shows detailed metadata, including numeric and symbolic permissions.	`stat deploy.sh`
`chmod u+x FILE`	Adds execute permission for the file’s owner.	`chmod u+x deploy.sh`
`chmod g-w FILE`	Removes write permission from the file’s group.	`chmod g-w settings.conf`
`chmod 644 FILE`	Gives the owner read and write access and everyone else read-only access.	`chmod 644 settings.conf`
`chmod 755 FILE`	Gives the owner full access and everyone else read and execute access.	`chmod 755 deploy.sh`
`chmod -R MODE DIRECTORY`	Recursively changes permissions below a directory.	`chmod -R u+rwX project`
`chown USER FILE`	Changes the owner of a file when run with sufficient privileges.	`sudo chown alex report.txt`
`chown USER:GROUP FILE`	Changes both the owner and group of a file.	`sudo chown alex:developers app.conf`
`chgrp GROUP FILE`	Changes a file’s group ownership.	`chgrp developers report.txt`
`umask`	Displays the current mask used when permissions are assigned to new files.	`umask`
`umask 022`	Sets a common session mask that removes group and other write permission.	`umask 022`
`getfacl FILE`	Displays access control list entries when ACL tools are installed.	`getfacl shared.txt`
`setfacl -m u:USER:rw FILE`	Grants a named user read and write access through an ACL entry.	`setfacl -m u:alex:rw shared.txt`

Numeric permission values

Add 4 for read, 2 for write, and 1 for execute. The three digits represent the owner, group, and other users.

chmod 750 private-script.sh

Capital X for directories

In symbolic mode, X adds execute permission only to directories and files that already have an execute bit. This is useful for recursive directory changes.

chmod -R u+rwX,go+rX project

Linux commands for inspecting and managing users and groups
Command	What it does	Example
`whoami`	Prints the effective username of the current session.	`whoami`
`id`	Displays the current user ID, primary group, and supplementary groups.	`id`
`id USER`	Displays identity and group information for a specified account.	`id alex`
`groups USER`	Lists the groups associated with a user.	`groups alex`
`getent passwd USER`	Queries the configured account databases for a user entry.	`getent passwd alex`
`getent group GROUP`	Queries the configured account databases for a group entry.	`getent group developers`
`sudo useradd -m USER`	Creates a local account with a home directory on systems providing useradd.	`sudo useradd -m alex`
`sudo passwd USER`	Sets or changes the password for an account.	`sudo passwd alex`
`sudo usermod -aG GROUP USER`	Adds a user to a supplementary group without replacing existing memberships.	`sudo usermod -aG developers alex`
`sudo gpasswd -d USER GROUP`	Removes a user from a supplementary group.	`sudo gpasswd -d alex developers`
`sudo groupadd GROUP`	Creates a new local group.	`sudo groupadd developers`
`sudo usermod -l NEW OLD`	Changes an account’s login name but does not automatically rename its home directory.	`sudo usermod -l alexander alex`
`sudo userdel USER`	Removes a local account while normally leaving its home directory intact.	`sudo userdel olduser`
`last`	Displays recorded login sessions from the system login database.	`last`

Debian and Ubuntu account creation

Debian-based systems commonly provide adduser, an interactive front end that creates the account, home directory, and initial configuration.

sudo adduser alex

Apply new group membership

Existing processes do not automatically receive newly assigned group memberships. The user normally needs to start a new login session.

id alex

Linux commands for monitoring and controlling processes and shell jobs
Command	What it does	Example
`ps aux`	Displays a detailed snapshot of processes from all users.	`ps aux`
`ps -ef`	Displays processes using a full-format, parent-oriented listing.	`ps -ef`
`top`	Opens an interactive, continuously updated process monitor.	`top`
`htop`	Opens a user-friendly interactive monitor when htop is installed.	`htop`
`pgrep PATTERN`	Returns process IDs whose names match a pattern.	`pgrep nginx`
`pgrep -af PATTERN`	Displays matching process IDs with their complete command lines.	`pgrep -af python`
`kill PID`	Sends SIGTERM, requesting that a process shut down cleanly.	`kill 4242`
`kill -KILL PID`	Immediately stops a process without allowing cleanup.	`kill -KILL 4242`
`pkill PATTERN`	Sends SIGTERM to processes whose names match a pattern.	`pkill firefox`
`jobs`	Lists jobs started from the current shell session.	`jobs`
`COMMAND &`	Starts a command as a background job in the current shell.	`python3 server.py &`
`bg %JOB`	Continues a stopped shell job in the background.	`bg %1`
`fg %JOB`	Brings a background or stopped job into the foreground.	`fg %1`
`nohup COMMAND &`	Runs a command while ignoring hangup signals and places it in the background.	`nohup ./backup.sh > backup.log 2>&1 &`
`nice -n VALUE COMMAND`	Starts a process with an adjusted CPU scheduling priority.	`nice -n 10 ./report.sh`
`renice VALUE -p PID`	Changes the nice value of an existing process when permitted.	`renice 10 -p 4242`

Suspend a foreground job

Press Ctrl+Z to suspend the current foreground job. Then use bg to continue it in the background or fg to return it to the foreground.

jobs

Show a process tree

A process tree makes parent and child relationships easier to inspect. The standalone pstree utility may require a separate package on minimal installations.

ps -ef --forest

Linux commands for viewing system and hardware information
Command	What it does	Example
`uname -a`	Displays available kernel, hostname, release, and architecture information.	`uname -a`
`uname -r`	Prints the running kernel release.	`uname -r`
`uname -m`	Prints the machine hardware name, such as x86_64 or aarch64.	`uname -m`
`cat /etc/os-release`	Displays standardized operating-system identification data.	`cat /etc/os-release`
`hostname`	Prints the system’s current hostname.	`hostname`
`hostnamectl`	Displays hostname and system metadata on systems using systemd.	`hostnamectl`
`uptime`	Shows the current time, uptime, logged-in users, and load averages.	`uptime`
`date`	Displays the current system date, time, and timezone information.	`date`
`timedatectl`	Displays clock, timezone, and synchronization status on systemd systems.	`timedatectl`
`lscpu`	Displays CPU architecture, core, thread, and virtualization details.	`lscpu`
`free -h`	Displays memory and swap usage in human-readable units.	`free -h`
`lsblk`	Lists block devices and their mount relationships.	`lsblk`
`lspci`	Lists PCI devices when the pciutils package is installed.	`lspci`
`lsusb`	Lists USB devices when the usbutils package is installed.	`lsusb`
`who`	Displays users currently logged in and their terminal sessions.	`who`

Show selected OS fields

The values in /etc/os-release are designed for scripts as well as human inspection. This example prints the distribution name and version.

grep -E '^(NAME|VERSION)=' /etc/os-release

Read Linux load averages

The three values shown by uptime represent average runnable or uninterruptible tasks over approximately 1, 5, and 15 minutes.

cat /proc/loadavg

Linux commands for inspecting disks, storage, and filesystems
Command	What it does	Example
`df -h`	Displays used and available filesystem space in human-readable units.	`df -h`
`df -i`	Displays inode usage instead of storage blocks.	`df -i /var`
`du -sh PATH`	Reports the total apparent disk usage below a path.	`du -sh ~/Downloads`
`du -h --max-depth=1 PATH`	Summarizes usage for the immediate contents of a directory.	`du -h --max-depth=1 /var`
`lsblk -f`	Lists block devices with filesystem types, labels, UUIDs, and mount points.	`lsblk -f`
`findmnt`	Displays mounted filesystems in a tree-like structure.	`findmnt`
`findmnt PATH`	Identifies the mounted filesystem containing a selected path.	`findmnt /home`
`blkid`	Displays available block-device attributes such as UUID and filesystem type.	`sudo blkid`
`sudo fdisk -l`	Lists recognized disks and partition tables without changing them.	`sudo fdisk -l`
`sudo mount DEVICE PATH`	Mounts a filesystem at an existing directory.	`sudo mount /dev/sdb1 /mnt/backup`
`sudo umount PATH`	Detaches a mounted filesystem after active use has stopped.	`sudo umount /mnt/backup`
`sync`	Requests that buffered filesystem writes be committed to persistent storage.	`sync`
`sudo fsck DEVICE`	Checks and may repair an unmounted filesystem using the appropriate checker.	`sudo fsck /dev/sdb1`
`ncdu PATH`	Opens an interactive disk-usage browser when ncdu is installed.	`ncdu /var`

Find large top-level entries

Summarize the immediate contents of a directory and sort the human-readable results from largest to smallest.

du -h --max-depth=1 . | sort -hr

Check what keeps a mount busy

If unmounting reports that a target is busy, inspect processes using the mount before closing them normally. The fuser utility may be supplied by an additional package.

sudo fuser -vm /mnt/backup

Linux commands for creating, inspecting, and extracting compressed archives
Command	What it does	Example
`tar -cf ARCHIVE.tar PATH`	Creates an uncompressed tar archive from files or directories.	`tar -cf project.tar project/`
`tar -czf ARCHIVE.tar.gz PATH`	Creates a tar archive compressed with gzip.	`tar -czf project.tar.gz project/`
`tar -cJf ARCHIVE.tar.xz PATH`	Creates a tar archive compressed with xz.	`tar -cJf project.tar.xz project/`
`tar -tf ARCHIVE.tar`	Lists archive members without extracting them.	`tar -tf project.tar.gz`
`tar -xf ARCHIVE.tar`	Extracts an archive into the current directory.	`tar -xf project.tar.gz`
`tar -xf ARCHIVE.tar -C DIRECTORY`	Extracts an archive into an existing destination directory.	`tar -xf project.tar.gz -C /tmp/project`
`gzip FILE`	Compresses a file to FILE.gz and normally removes the original file.	`gzip access.log`
`gzip -k FILE`	Creates a gzip-compressed copy while keeping the original file.	`gzip -k access.log`
`gunzip FILE.gz`	Decompresses a gzip file and normally removes the compressed version.	`gunzip access.log.gz`
`xz -k FILE`	Creates an xz-compressed copy while preserving the original file.	`xz -k database.sql`
`unxz FILE.xz`	Decompresses an xz file and normally removes the compressed version.	`unxz database.sql.xz`
`zip -r ARCHIVE.zip PATH`	Recursively creates a ZIP archive when the zip utility is installed.	`zip -r project.zip project/`
`unzip -l ARCHIVE.zip`	Lists ZIP archive members without extracting them.	`unzip -l project.zip`
`unzip ARCHIVE.zip -d DIRECTORY`	Extracts a ZIP archive into a selected directory.	`unzip project.zip -d /tmp/project`

Extract one archive member

Supply the member’s path exactly as it appears in the archive listing to extract only that file.

tar -xf project.tar.gz project/README.md

Stream compressed text

Use zcat to send gzip-compressed text to standard output without creating an uncompressed file on disk.

zcat access.log.gz | less

Linux commands for inspecting and troubleshooting network connections
Command	What it does	Example
`ip address show`	Displays network interfaces and their assigned addresses.	`ip address show`
`ip link show`	Displays network interfaces and their link states.	`ip link show`
`ip route show`	Displays the current IPv4 routing table.	`ip route show`
`ping -c 4 HOST`	Sends four ICMP echo requests to test reachability and latency.	`ping -c 4 example.com`
`ss -tuln`	Lists listening TCP and UDP sockets using numeric addresses and ports.	`ss -tuln`
`sudo ss -tulpn`	Lists listening sockets and available process information.	`sudo ss -tulpn`
`curl -I URL`	Requests HTTP response headers without downloading the response body.	`curl -I https://example.com`
`curl -O URL`	Downloads a resource using the filename from its URL path.	`curl -O https://example.com/file.zip`
`wget URL`	Downloads a resource when the wget utility is installed.	`wget https://example.com/file.zip`
`dig DOMAIN`	Queries DNS and displays detailed response information.	`dig example.com`
`dig +short DOMAIN`	Prints a concise DNS answer, commonly an address or hostname.	`dig +short example.com`
`host DOMAIN`	Performs a concise DNS lookup when the host utility is installed.	`host example.com`
`traceroute HOST`	Displays network hops toward a destination when traceroute is installed.	`traceroute example.com`
`resolvectl status`	Displays DNS configuration on systems using systemd-resolved.	`resolvectl status`
`nmcli device status`	Displays interface status on systems managed by NetworkManager.	`nmcli device status`

Test a TCP port

Use Bash’s TCP redirection when supported to test whether a remote TCP connection can be opened. A successful command normally produces no output.

timeout 5 bash -c '</dev/tcp/example.com/443'

Discover the public IP address

This sends a request to an external service. Use only a service you trust, because it receives the requesting public IP address.

curl https://api.ipify.org

Package management commands for major Linux distributions
Command	What it does	Distribution
`sudo apt update`	Refreshes available package information from configured repositories.	Debian, Ubuntu
`sudo apt upgrade`	Upgrades installed packages when dependencies can be satisfied without removing packages.	Debian, Ubuntu
`sudo apt install PACKAGE`	Installs a package and required dependencies.	Debian, Ubuntu
`sudo apt remove PACKAGE`	Removes a package while normally retaining its system configuration files.	Debian, Ubuntu
`apt search TERM`	Searches available package names and descriptions.	Debian, Ubuntu
`dpkg -L PACKAGE`	Lists files installed by a local Debian package.	Debian, Ubuntu
`sudo dnf upgrade`	Refreshes metadata as needed and upgrades installed packages.	Fedora, RHEL family
`sudo dnf install PACKAGE`	Installs a package and resolves its dependencies.	Fedora, RHEL family
`sudo dnf remove PACKAGE`	Removes a package and dependencies that are no longer required.	Fedora, RHEL family
`dnf search TERM`	Searches enabled package repositories.	Fedora, RHEL family
`rpm -ql PACKAGE`	Lists files installed by an RPM package.	RPM-based systems
`sudo pacman -Syu`	Refreshes package databases and performs a full system upgrade.	Arch Linux
`sudo pacman -S PACKAGE`	Installs or upgrades a package from synchronized repositories.	Arch Linux
`sudo pacman -Rns PACKAGE`	Removes a package, unused dependencies, and associated backup configuration files.	Arch Linux
`pacman -Ss TERM`	Searches synchronized package databases.	Arch Linux
`sudo zypper refresh`	Refreshes repository metadata.	openSUSE, SUSE
`sudo zypper install PACKAGE`	Installs a package using configured repositories.	openSUSE, SUSE

Identify the package manager

Read the operating-system identification file before choosing distribution-specific instructions.

cat /etc/os-release

Find which package owns a file

Use the database command appropriate for the installed package format.

dpkg -S /usr/bin/curl

Linux systemctl commands for managing systemd services
Command	What it does	Example
`systemctl status UNIT`	Shows the unit’s current state and recent log messages.	`systemctl status ssh.service`
`sudo systemctl start UNIT`	Starts a unit for the current boot without enabling future startup.	`sudo systemctl start nginx.service`
`sudo systemctl stop UNIT`	Stops a running unit.	`sudo systemctl stop nginx.service`
`sudo systemctl restart UNIT`	Stops and starts a unit, interrupting the running service.	`sudo systemctl restart nginx.service`
`sudo systemctl reload UNIT`	Requests a supported service to reload configuration without a full restart.	`sudo systemctl reload nginx.service`
`sudo systemctl enable UNIT`	Configures a unit to start through its defined boot dependencies.	`sudo systemctl enable nginx.service`
`sudo systemctl enable --now UNIT`	Enables a unit for future boots and starts it immediately.	`sudo systemctl enable --now nginx.service`
`sudo systemctl disable UNIT`	Removes enablement links but does not necessarily stop a running unit.	`sudo systemctl disable nginx.service`
`systemctl is-active UNIT`	Reports whether a unit is currently active.	`systemctl is-active nginx.service`
`systemctl is-enabled UNIT`	Reports the unit’s enablement state.	`systemctl is-enabled nginx.service`
`systemctl --failed`	Lists units currently in a failed state.	`systemctl --failed`
`systemctl list-units --type=service`	Lists service units currently loaded by systemd.	`systemctl list-units --type=service`
`systemctl list-unit-files --type=service`	Lists installed service unit files and their enablement states.	`systemctl list-unit-files --type=service`
`sudo systemctl daemon-reload`	Makes systemd reread unit files after they have been changed.	`sudo systemctl daemon-reload`
`systemctl cat UNIT`	Displays the unit’s backing files and drop-in configuration.	`systemctl cat nginx.service`

Create a unit override

Use systemctl edit to create a drop-in override instead of modifying a vendor-supplied unit file directly.

sudo systemctl edit nginx.service

Manage a user service

Add --user to manage units belonging to the current user’s systemd manager rather than the system manager.

systemctl --user status my-app.service

Linux commands for viewing systemd journal and text log entries
Command	What it does	Example
`journalctl`	Displays entries available in the systemd journal.	`journalctl`
`journalctl -b`	Displays journal entries from the current boot.	`journalctl -b`
`journalctl -b -1`	Displays entries from the previous boot when retained journal data is available.	`journalctl -b -1`
`journalctl -u UNIT`	Filters entries associated with a systemd unit.	`journalctl -u nginx.service`
`journalctl -u UNIT -f`	Follows new journal entries for a selected unit.	`journalctl -u nginx.service -f`
`journalctl -n NUMBER`	Displays the requested number of most recent entries.	`journalctl -n 100`
`journalctl -p err`	Shows entries at error priority and more severe levels.	`journalctl -p err -b`
`journalctl --since TIME`	Displays entries on or after a specified date or relative time.	`journalctl --since "1 hour ago"`
`journalctl --since START --until END`	Restricts entries to a selected time range.	`journalctl --since "09:00" --until "10:00"`
`journalctl -k`	Displays kernel messages from the current boot by default.	`journalctl -k`
`journalctl --disk-usage`	Reports disk space currently used by journal files.	`journalctl --disk-usage`
`sudo journalctl --vacuum-time=14d`	Removes archived journal files older than the selected retention period.	`sudo journalctl --vacuum-time=14d`
`tail -f LOGFILE`	Follows appended lines in a traditional text log.	`sudo tail -f /var/log/syslog`
`grep -i 'PATTERN' LOGFILE`	Finds matching lines in a readable text log without case sensitivity.	`sudo grep -i 'failed' /var/log/auth.log`

Show concise service errors

Combine unit, boot, and priority filters to reduce unrelated output while troubleshooting a failed service.

journalctl -u nginx.service -b -p err --no-pager

Use precise timestamps

Include a full date when investigating older incidents so relative expressions and day boundaries do not select the wrong period.

journalctl --since "2026-08-16 09:00:00" --until "2026-08-16 10:00:00"

Linux SSH commands for remote access and secure file transfers
Command	What it does	Example
`ssh USER@HOST`	Opens an encrypted shell session on a remote SSH server.	`ssh alex@server.example.com`
`ssh -p PORT USER@HOST`	Connects to an SSH server listening on a nondefault port.	`ssh -p 2222 alex@server.example.com`
`ssh -i KEY USER@HOST`	Uses a selected private identity file for authentication.	`ssh -i ~/.ssh/work_ed25519 alex@server.example.com`
`ssh USER@HOST COMMAND`	Runs one command remotely and returns its output.	`ssh alex@server.example.com 'uptime'`
`ssh-keygen -t ed25519 -C 'LABEL'`	Creates an Ed25519 key pair and prompts for its location and passphrase.	`ssh-keygen -t ed25519 -C 'alex@example.com'`
`ssh-copy-id USER@HOST`	Installs a public key for remote authentication when the utility is available.	`ssh-copy-id alex@server.example.com`
`scp FILE USER@HOST:PATH`	Copies a local file to a remote path using SSH transport.	`scp report.pdf alex@server.example.com:/home/alex/`
`scp USER@HOST:FILE PATH`	Copies a remote file to a local destination.	`scp alex@server.example.com:/var/tmp/report.pdf ./`
`scp -r DIRECTORY USER@HOST:PATH`	Recursively copies a local directory to a remote path.	`scp -r website alex@server.example.com:/srv/backups/`
`sftp USER@HOST`	Starts an interactive SSH File Transfer Protocol session.	`sftp alex@server.example.com`
`rsync -av SOURCE USER@HOST:PATH`	Synchronizes files while preserving common metadata and showing progress details.	`rsync -av website/ alex@server.example.com:/srv/website/`
`rsync -av --dry-run SOURCE DESTINATION`	Previews which files rsync would transfer or update.	`rsync -av --dry-run website/ alex@server.example.com:/srv/website/`
`ssh -L LOCAL:HOST:REMOTE USER@SERVER`	Forwards a local TCP port through the SSH server to another host and port.	`ssh -L 8080:127.0.0.1:80 alex@server.example.com`
`ssh-keygen -F HOST`	Searches known-host entries for a hostname or address.	`ssh-keygen -F server.example.com`

Create a reusable SSH host alias

Add host settings to ~/.ssh/config so connections can reuse the same hostname, username, port, and identity file.

ssh production

Verify a private key’s permissions

OpenSSH rejects private keys that are accessible too broadly. Limit access to the owning user.

chmod 600 ~/.ssh/work_ed25519

Bash keyboard shortcuts and command history expressions
Shortcut or command	What it does	Typical use
`Ctrl+C`	Sends an interrupt signal to the current foreground process.	Stop a running command.
`Ctrl+Z`	Suspends the current foreground job.	Pause a command before using `bg` or `fg`.
`Ctrl+A`	Moves the cursor to the beginning of the command line.	Edit the command or add a prefix.
`Ctrl+E`	Moves the cursor to the end of the command line.	Continue typing after editing earlier text.
`Ctrl+U`	Cuts text from the cursor back to the beginning of the line.	Remove an incorrect command prefix.
`Ctrl+K`	Cuts text from the cursor to the end of the line.	Remove remaining command arguments.
`Ctrl+W`	Cuts the word before the cursor.	Remove the previous argument or path component.
`Ctrl+Y`	Pastes text most recently cut with a Readline shortcut.	Restore text removed with `Ctrl+U` or `Ctrl+K`.
`Ctrl+R`	Starts a reverse incremental search through command history.	Find a previously used command by typing part of it.
`Ctrl+L`	Clears the terminal display while preserving shell history.	Get a clean screen without deleting commands.
`Tab`	Completes commands, paths, or supported arguments.	Reduce typing and avoid path spelling errors.
`history`	Displays commands retained in the current Bash history list.	`history`
`history \| grep TERM`	Filters displayed history for matching text.	`history \| grep ssh`
`!!`	Expands to the complete previous command.	`sudo !!` reruns the previous command with `sudo`.
`!$`	Expands to the final argument of the previous command.	`less !$` reuses the previous final argument.
`!NUMBER`	Expands to the command with the selected history number.	`!125`
`Alt+.`	Inserts the final argument from a previous command.	Press repeatedly to move through earlier final arguments.

Reuse a command without executing it

Add :p to a history expansion to print the expanded command and place it in history instead of running it immediately.

!!:p

Save current session history

Bash normally writes history when the shell exits. This command appends new entries from the current session to the history file.

history -a

Bash operators for pipes, redirection, and command chaining
Operator	What it does	Example
`COMMAND1 \| COMMAND2`	Sends standard output from the first command to standard input of the second.	`ps aux \| grep nginx`
`COMMAND > FILE`	Writes standard output to a file, replacing its existing contents.	`date > timestamp.txt`
`COMMAND >> FILE`	Appends standard output to the end of a file.	`date >> timestamps.txt`
`COMMAND < FILE`	Provides a file as standard input to a command.	`sort < names.txt`
`COMMAND 2> FILE`	Writes standard error to a file, replacing its existing contents.	`find / -name '*.conf' 2> errors.log`
`COMMAND 2>> FILE`	Appends standard error to the end of a file.	`./backup.sh 2>> backup-errors.log`
`COMMAND > FILE 2>&1`	Writes standard output and standard error to the same file.	`./backup.sh > backup.log 2>&1`
`COMMAND \| tee FILE`	Displays standard output and also writes it to a file.	`df -h \| tee disk-report.txt`
`COMMAND \| tee -a FILE`	Displays standard output and appends it to a file.	`date \| tee -a activity.log`
`COMMAND1 && COMMAND2`	Runs the second command only when the first returns a successful status.	`mkdir reports && cd reports`
`COMMAND1 \|\| COMMAND2`	Runs the second command only when the first returns a nonzero status.	`ping -c 1 example.com \|\| echo 'Host unavailable'`
`COMMAND1 ; COMMAND2`	Runs commands sequentially regardless of the first command’s status.	`date ; uptime`
`COMMAND \| xargs COMMAND`	Builds command arguments from standard input.	`printf '%s\n' file1 file2 \| xargs wc -l`
`COMMAND &`	Starts a command as a background job in the current shell.	`./report.sh &`

Preserve pipeline failures

By default, a pipeline’s status usually comes from its final command. Enable pipefail when a script should also detect failure from an earlier pipeline stage.

set -o pipefail

Handle filenames safely with xargs

Use null-delimited output and input when filenames may contain spaces, quotes, or newline characters.

find . -type f -print0 | xargs -0 wc -l

Linux commands for environment variables and Bash configuration
Command	What it does	Example
`env`	Displays environment variables passed to the current process.	`env`
`printenv VARIABLE`	Prints the exported value of a selected environment variable.	`printenv HOME`
`echo "$VARIABLE"`	Expands and displays a shell variable while preserving its spacing.	`echo "$PATH"`
`VARIABLE='VALUE'`	Creates or updates a variable in the current shell.	`PROJECT_DIR="$HOME/projects/site"`
`export VARIABLE='VALUE'`	Sets a variable and exports it to subsequently started child processes.	`export EDITOR='nano'`
`unset VARIABLE`	Removes a variable or function from the current shell.	`unset PROJECT_DIR`
`VARIABLE='VALUE' COMMAND`	Supplies a variable to one command without permanently changing the shell.	`LANG=C sort names.txt`
`export PATH="$HOME/bin:$PATH"`	Adds a personal executable directory before the existing command search path.	`export PATH="$HOME/bin:$PATH"`
`source FILE`	Reads and executes a file in the current shell environment.	`source ~/.bashrc`
`. FILE`	Uses the portable dot syntax to read a file in the current shell.	`. ~/.profile`
`alias NAME='COMMAND'`	Creates a command alias for the current shell session.	`alias ll='ls -lah'`
`unalias NAME`	Removes an alias from the current shell session.	`unalias ll`
`type NAME`	Shows how the shell resolves an alias, function, builtin, or executable.	`type ll`
`bash --noprofile --norc`	Starts Bash without reading normal personal startup files.	`bash --noprofile --norc`

Common Bash startup files

Interactive non-login Bash shells commonly read ~/.bashrc. Login shells read the first available file among ~/.bash_profile, ~/.bash_login, and ~/.profile.

nano ~/.bashrc

Test a clean environment

Use env -i to start a command with an initially empty environment, then provide only the variables it requires.

env -i HOME="$HOME" PATH="/usr/bin:/bin" bash --noprofile --norc

Linux troubleshooting commands and diagnostic checks
Command	What it checks	Example
`echo $?`	Displays the exit status of the most recently completed foreground pipeline.	`echo $?`
`command -v COMMAND`	Checks whether the current shell can resolve a command name.	`command -v curl`
`type -a COMMAND`	Shows every shell alias, function, builtin, or executable matching a name.	`type -a python3`
`systemctl --failed`	Lists systemd units currently in a failed state.	`systemctl --failed`
`journalctl -p err -b`	Shows error-level and more severe journal entries from the current boot.	`journalctl -p err -b`
`dmesg --level=err,warn`	Displays available kernel ring-buffer errors and warnings on supporting systems.	`sudo dmesg --level=err,warn`
`df -h`	Checks whether a mounted filesystem is running out of storage space.	`df -h`
`df -i`	Checks whether a filesystem has exhausted its available inodes.	`df -i`
`free -h`	Displays current memory and swap usage.	`free -h`
`ip route show`	Checks current routes, including the default gateway when configured.	`ip route show`
`sudo -l`	Lists the current user’s permitted and prohibited sudo commands.	`sudo -l`
`man COMMAND`	Opens the installed manual page for a command.	`man rsync`
`COMMAND --help`	Requests concise usage information from commands that support this option.	`find --help`
`help BUILTIN`	Displays Bash documentation for a shell builtin.	`help cd`

Why does Linux say “command not found”?

The program may not be installed, its directory may be missing from PATH, or the command name may be incorrect. Check it with command -v, then use the correct distribution package manager if installation is required.

What does “permission denied” mean?

The current user may lack permission to read, write, execute, or traverse the target. Inspect the complete path with namei -l PATH when available, and correct ownership or permissions instead of automatically using sudo.

Why can a disk appear full when files are small?

The filesystem may have exhausted its inodes, deleted files may still be held open by processes, or data may exist beneath a mount point. Compare df -h, df -i, and du before deleting anything.

Should every administrative command use sudo?

No. Use normal user privileges whenever possible and elevate only the specific command that requires administrative access. Review the command, resolved paths, wildcards, and expected result first.

Are Linux commands identical on every distribution?

Core command behavior is often similar, but package managers, filesystem layouts, service names, defaults, installed utilities, and tool versions can differ. Check the local manual and your distribution’s official documentation.

How should a failing command be investigated?

Preserve the exact error, check the exit status, confirm inputs and permissions, reproduce the problem with the smallest safe command, and inspect relevant logs before changing the system.