Linux (specifically Ubuntu) commands and techniques

date-and-time.sh

# Display current date and time (including timezone)
date

# Change timezone
sudo dpkg-reconfig tzdata
# or
sudo dpkg-reconfigure tzdata

# restart cron as otherwise the time will keep running on UTC
/etc/init.d/cron stop
/etc/init.d/cron start

general.sh

## General commands

# Truncate a file - note: if superuser privilege is needed for truncating, enclose the below commands in single quotes and run as rooted bash, i.e. sudo bash -c '<command>'
> <file>
# or
cat > <file>
# or
cat /dev/null > <file>

# Append a file to another file
cat <content> >> <file>

# Convert Mac line endings (Ctrl-r) into Unix ones (Ctrl-n)
cat <input-file> | tr "\r" "\n" > <output-file> 

# Print first/last 10 lines of a file
# first 10 lines
head <file>
# last 10 lines
tail <file>

# Print first/last n lines of a file
# first n lines
head -n <num> <file>
# last n lines
tail -n <num> <file>

# Describe an application (provided it has a man page) - e.g. nginx, apache2, mysql
whatis <service>

# Download a file and pass the output to a command
wget -O - <file-url> | <command>

# Download a file, following redirects if present (e.g. Sourceforge)
curl -L '<file-url>'

# Set a temporary environment variable
export <VARIABLE>=<value>

# Print all environment variables
printenv
#or
env

# Print the value of a particular environment variable
printevn <VARIABLE>
# or
echo $<VARIABLE>

# Nullify an environment variable
export <VARIABLE>=

# Unset an environment variable
unset <VARIABLE>

# Un-export an environment variable, thereby demoting it to become a shell variable while still retaining its value
export -n <VARIABLE>

# Copy a file's contents into clipboard (assuming xclip is installed)
xclip -sel clip < <file>

# Modify text in-place in a file
sed -i "s/<original>/<new>/g" <file>

# Copy a local file to a remote host via SSH, compressing the file during transfer and showing transfer progress
rsync -vz --progress -e ssh <source-file> <dest-host>:<dest-path>

# Copy a file from one remote host to another
rsync -vz --progress -e ssh <source-host>:<source-file> <dest-host>:<dest-path>

# Move the cursor to start of the current line of text in terminal
ctrl + a
# or to the end of the line
ctrl + e

# Clear the current line and place cursor at the start in terminal
ctrl + u

# Comment out the currently types line of text in the terminal
shift + 3

# Add sudo before the currently entered statement
ctrl + 3, ctrl + 3

# Remove all characters on the right of the cursor
ctrl + k

# Remove word before the cursor
ctrl + w

# Undo deleted characters
ctrl + y

# Switch to Bash shell
bash

# Switch to Zsh Shell
zsh

# Compute and show MD5 checksum
md5sum <file>

# Check whether MD5 checksum (in the checksum file) matches the actual file's checksum
md5sum -c <checksum-file>

# Compute and show SHA1 checksum
sha1sum <file>

# Check whether SHA1 checksum (in the checksum file) matches the actual file's checksum
sha1sum -c <checksum-file>

# Set default editor
sudo update-alternatives --config editor # if it still doesn't work, set the value of 'EDITOR' environment variable to 'vim' in the user profile

openssl.sh

# Show version
openssl version

# Show built date for the installed version
openssl version -b

# Create a self signed certificate and a private key
sudo openssl req -x509 -nodes -newkey rsa:2048 -days 365 -keyout <key-file>.key -out <certificate-file>.crt

# Create a certificate signing request (CSR) and a private key
sudo openssl req -new -nodes -newkey rsa:2048 -keyout <key-file>.key -out <csr-file>.csr

package-management.sh

## Package Management commands in Ubuntu

# Remove a package along with all of its configuration - note: this only removes system-wide configuration data, and not user-specific ones
sudo apt-get remove --purge <package>
# or simply
sudo apt-get purge <package>

# Check if a package is installed
dpkg -l <package>
# or, show similarly named packages
dpkg -l | grep <package>

# Show detailed package information
dpkg -s <package>

# Search for a package in the cached repository index (searches for the package text in the title as well as description text)
apt-cache search <package>

# Search for a package by title only
apt-cache search --names-only <package>

# Search for a package using Regex and by title only
apt-cache search --names-only '<package-regex>'

# Show package information
apt-cache show <package>

# Show extended package dependency information (e.g. reverse dependencies)
apt-cache showpkg <package>

# Show dependencies for a package
apt-cache depends <package>

# Show reverse dependencies for a package (i.e. list of packages that depend on a given package)
apt-cache rdepends <package>

# Show policy information regarding a package (basically shows installed and available versions)
apt-cache policy <package>

# Add an apt key (e.g. GPG)
sudo apt-key add <key>

# Download an apt key and add it
wget -0 - <key-url> | sudo apt-key add -

# List all keys
apt-key list

# Delete a key (the key ID is the idenfitier after the '/' and can be seen by listing all keys)
apt-key del <key-id>

# Remove a package's dependencies - note: this should only be used for dependencies of a meta package as it's got the potential to remove everything if used otherwise
$(apt-cache depends --no-recommends --no-suggests <package> | awk '{print "sudo apt-get remove "$NF}')
# optionally (but safer and, hence, recommended), save the output to a script which could be edited before running
apt-cache depends --no-recommends --no-suggests <package> | awk '{print "sudo apt-get remove "$NF}' > remove.sh

# Remove orphan packages no longer needed (e.g. as a result of removing a meta-package)
sudo apt-get autoremove
# or to also purge configuration data - note: this only removes system-wide configuration data, and not user-specific ones
sudo apt-get autoremove --purge

# List orphan packages no longer needed
deborphan

# Remove a package, purge its configuration data, along with any consequent orphan packages and their own respective configuration data
sudo apt-get autoremove --purge <package>
sudo apt-get autoremove --purge `deborphan`

postfix.sh

# Check whether postfix is running or not
sudo postfix status

# If it is not running, start it
sudo postfix start

# Send a test email (note that '.' at the end to indicate end of message)
telnet localhost 25

ehlo localhost
mail from: root@<sender-host>
rcpt to: <recipient-email>
data
Subject: <subject>

<multi-line-message>
.

process-management.sh

## Process Management

# Show process ID based on name (e.g. httpd, nginx, mysql, etc.)
pgrep <process-name>

# Show process ID of a process (e.g. httpd, nginx, mysql, etc.) 
pidof <process-name>

# Kill a process using its process ID
sudo kill <process-id>

# Edit crontab for the current user
crontab -e

# List crontab for the current user
crontab -l

# List crontab for a given user
sudo crontab -u <user> -l

# Show all running processes and files opened by them (e.g. this is useful for checking which processes depend on Openssl - sudo lsof | grep libssl)
sudo lsof
# or, for cleaner output
lsof | grep libssl | awk '{print $1}' | sort | uniq

processor.sh

# Show the number of processing units available
nproc

# Show processor information (e.g. architecture, cores, etc.)
lscpu

security.sh

# Show all IP table rules
sudo iptables -L

# Show all firewall rules
sudo ufw status
# or, for more verbose output 
sudo ufw status verbose

# Enable firewall
sudo ufw enable

# Disable firewall
sudo ufw disable

# Deny all connections by default
sudo ufw default deny

# Allow all connections on a port
sudo ufw allow <port>

# Allow connection from an IP address to any protocol on a certain port
sudo ufw allow from <IP/CIDR> to any port <port>

ssh.sh

# Specify private key to use
ssh -i <path-to-private-key> <remote-host>

# Show detailed connection process
ssh -vv <remote-host>

# Generate a pair of SSH keys
ssh-keyget -t rsa -C "<comments>"

# Copy the public key(s) to remote host, where <remote-host> is in SSH format
ssh-copy-id <remote-host>

# To copy only a specific public key
ssh-copy-id -i <path-to-public-key> <remote-host>
# or if ssh-copy-id command doesn't exist (e.g. on Mac OSX)
cat <path-to-public-key> | ssh <user>@<remote-host> "mkdir ~/.ssh; cat >> ~/.ssh/authorized_keys"

# Add an existing key to ssh-agent (for cases where the key was restored from backup)
ssh-add <path-to-private-key>

# List all keys being managed by the ssh-agent
ssh-add -l

# Restart SSH daemon
sudo service ssh restart

# Authorise the public key on the remote host
cat <path-to-public-key> | ssh <remote-host> "cat >> ~/.ssh/authorized_keys"

# Kill an active SSH session
sudo pkill -9 -t <tty>

# Check current session's TTY
tty

system.sh

# Print low level system information (e.g. OS, kernel, machine, processor)
uname -a
# or
cat /proc/version

# Print hight level system information (e.g. distro name, version, pretty name)
cat /etc/*-release

ubuntu-ui.sh

# Restart Unity without logging out and in
unity # Note: Run it in dash (i.e. Alt + F2) instead of terminal so that it doesn't bind to the terminal

users-and-groups.sh

## User and group management commands in Ubuntu

# Show the name of the current user
whoami

# Create a new user
adduser

# Add an existing user to an existing group
sudo usermod -a -G <group> <user>

# Refresh user permissions without restart or logging out and in
su <user>

# Change to another user
sudo -i -u <user>
# or
su - <user>

# Simulate root login shell (this is the current best practice if root access is really needed; see https://help.ubuntu.com/community/RootSudo#Logging_in_as_another_user)
sudo -i

# Run a command as another user
sudo -u <user> <command>

# Edit user priviledges (e.g. giving a user sudo priviledges)
sudo visudo

# Delete a user, but keep its home directory
sudo deluser username

# Delete a user, including his home directory
sudo deluser --remove-home username

# Change user account password
passwd

# See a list of all logged in users
who
# or simply
w

# Show a list of all users
cat /etc/passwd
# or
cut -d : -f 1 /etc/passwd

# Show a list of all groups
cat /etc/groups
# or
cut -d : -f 1 /etc/groups