Courtesy of Network World by Sandra Henry-Stocker

Getting familiar with your account      
These commands will help new Linux users become familiar with their Linux accounts.      
Command Function Example  
pwd Displays your current location in the file system pwd  
whoami Displays your username – most useful if you switch users with su and need to be reminded what account you’re using currently whoami  
ls Provides a file listing. With -a, it also displays files with names starting with a period (e.g., .bashrc). With -l, it also displays file permissions, sizes and last updated date/time. ls  
ls -a      
ls -l      
env Displays your user environment settings (e.g., search path, history size, home directory, etc.) env  
echo Repeats the text you provide or displays the value of some variable echo hello  
echo $PATH      
history Lists previously issued commands history  
history tail -5    
passwd Changes your password. Note that complexity requirements may be enforced. passwd  
history tail -5    
       
Examining files      
Linux provides several commands for looking at the content and nature of files. These are some of the most useful commands.      
Command Function Example  
cat Displays the entire contents of a text file. cat .bashrc  
more Displays the contents of a text file one screenful at a time. Hit the spacebar to move to each additional chunk. more .bash_history  
less Displays the contents of a text file one screenful at a time, but in a manner that allows you to back up using the up arrow key. less .bash_history  
file Identifies files by type (e.g., ASCII text, executable, image, directory) file myfile  
file ~/.bashrc      
file /bin/echo      
       
Managing files      
These are some Linux commands for changing file attributes as well as renaming, moving and removing files.      
Command Function Example  
chmod Changes file permissions (who can read it, whether it can be executed, etc.) chmod a+x myscript  
chmod 755 myscript      
chown Changes file owner sudo chown jdoe myfile  
cp Makes a copy of a file. cp origfile copyfile  
mv Moves or renames a file – or does both mv oldname newname  
mv file /new/location      
mv file /newloc/newname      
rm Deletes a file or group of files rm file  
rm *.jpg      
rm -r directory      
       
Creating and editing files      
Linux systems provide commands for creating files and directories. Users can choose the text editor they are comfortable using. Some require quite a bit of familiarity before they’ll be easy to use while others are fairly self-explanatory.      
Command Function Example  
nano An easy-to-use text editor that requires you to move around in the file using your arrow keys and provides control sequences to locate text, save your changes, etc. nano myfile  
vi A more sophisticated editor that allows you to enter commands to find and change text, make global changes, etc. vi myfile  
ex A text editor designed for programmers and has both a line-oriented and visual mode ex myfile  
touch Creates a file if it doesn’t exist or updates its timestamp if it does touch newfile  
touch updatedfile      
> Creates files by directing output to them. A single > creates a file while » appends to an existing file. cal > calendar  
ps > myprocs      
date » date.log      
mkdir Creates a directory mkdir mydir  
mkdir ~/mydir      
mkdir /tmp/backup      
       
Moving around the file system      
The command for moving around the Linux file system is ls, but there are many variations.      
Command Function Example  
cd With no arguments, takes you to your home directory. The same thing would happen if you typed cd $HOME or cd ~ cd  
cd .. Moves up (toward /) one directory from your current location cd ..  
cd Takes you to the specified location. If the location begins with a /, it is taken to be relative to the root directory; otherwise it is taken as being relative to your current location. The ~ character represents your home directory. cd /tmp  
cd Documents      
cd ~/Documents      
       
Learning about and identifying commands      
There are a number of Linux commands that can help you learn about other commands, the options they offer and where these commands are are located in the file system. Linux systems also provide a command that can help you to learn what commands are available related to some subject – for example, commands that deal with user accounts.      
Command Function Example  
man Displays the manual (help) page for a specified command and (with -k) provides a list of commands related to a specified keyword man cd  
man -k account      
which Displays the location of the executable that represents the particular command which cd  
apropos Lists commands associated with a particular topic or keyword apropos user  
apropos account      
       
Finding files      
There are two commands that can help you find files on Linux, but they work very differently. One searches the file system while the other looks through a previously built database.      
Command Function Example  
find Locates files based on criteria provided (file name, type, owner, permissions, size, etc.). Unless provided with a location from which to start the search, find only looks in the current directory. find . -name myfile  
find /tmp -type d      
locate Locates files using the contents of the /var/lib/mlocate/mlocate.db which is updated by the updatedb command usually run through cron. No starting location is required. locate somefile  
locate “*.html” -n 20      
       
Viewing running processes      
You can easily view processes that are running on the system – yours, another user’s or all of them.      
Command Function Example  
ps Shows processes that you are running in your current login session ps  
ps -ef Shows all processes that are currently running on the system ps -ef  
ps -ef more    
pstree Shows running processes in a hierarchical (tree-like) display that demonstrates the relationships between processes (-h highlights current process) pstree  
pstree username      
pstree -h      
       
Starting, stopping and listing services      
These commands allow you to display services as well as start and stop them.      
Command Function Example  
systemctl The systemctl command can start, stop, restart and reload services. Privileged access is required. sudo systemctl stop apache2.service  
sudo systemctl restart apache2.service      
sudo systemctl reload apache2.service      
service Lists services and indicates whether they are running service –status-all  
       
Killing processes      
Linux offers a few commands for terminating processes. Privileged access is needed if you did not start the process in question.      
Command Function Example  
kill Terminates a running process provided you have the authority to do so kill 8765  
sudo kill 1234      
kill -9 3456      
killall Terminates all processes with the provided name killall badproc  
pkill Terminates a process based on its name pkill myproc  
       
Identifying your OS release      
The table below lists commands that will display details about the Linux OS that is running on a system.      
Command Function Example  
uname Displays information on OS release in a single line of text uname -a  
uname -r      
lsb_release On Debian-based systems, this command displays information on the OS release including its codename and distributor ID lsb_release -a  
hostnamectl Displays information on the system including hostname, chassis type, OS, kernel and architecture hostnamectl  
       
Gauging system performance      
These are some of the more useful tools for examining system performance.      
Command Function Example  
top Shows running processes along with resource utilization and system performance data. Can show processes for one selected user or all users. Processes can be ordered by various criteria (CPU usage by default) top  
top jdoe      
atop Similar to top command but more oriented toward system performance than individual processes atop  
free Shows memory and swap usage – total, used and free free  
df Display file system disk space usage df  
df -h      
       
Managing users and groups      
Commands for creating and removing user accounts and groups are fairly straightforward.      
Command Function Example  
useradd Adds a new user account to the system. A username is mandatory. Other fields (user description, shell, initial password, etc.) can be specified. Home directory will default to /home/username. useradd -c “John Doe” jdoe  
useradd -c “Jane Doe” -g admin -s /bin/bash jbdoe      
userdel Removes a user account from the system. The -f option runs a more forceful removal, deleting the home and other user files even if the user is still logged in. userdel jbdoe  
userdel -f jbdoe      
groupadd Adds a new user group to the system, updating the /etc/group. groupadd developers  
groupdel Removes a user group from the system groupdel developers  
       
Examining network connections      
The commands below help you view network interfaces and connections.      
Command Function Example  
ip Displays information on network interfaces ip a  
ss Displays information on sockets. The -s option provides summary stats. The -l option shows listening sockets. The -4 or -6 options restrict output to IPv4 or IPv6 connections. ss -s  
ss -l      
ss -4 state listening      
ping Check connectivity to another system ping remhost  
ping 192.168.0.11      
       
Managing security      
There are many aspects to managing security on a Linux system, but there are also a lot of commands that can help. The commands below are some that will get you started. Click on this link to see these and other commands on 22 essential Linux security commands.      
Command Function Example  
visudo The visudo command allows you to configure privileges that will allow select individuals to run certain commands with superuser authority. The command does this by making changes to the /etc/sudoers file. visudo  
sudo The sudo command is used by privileged users (as defined in the /etc/sudoers file to run commands as root. sudo useradd jdoe  
su Switches to another account. This requires that you know the user’s password or can use sudo and provide your own password. Using the - means that you also pick up the user’s environment settings. su (switch to root)  
su - jdoe      
sudo su - jdoe      
who Shows who is logged into the system who  
last Lists last logins for specified user using records from the /var/log/wtmp file. last jdoe  
ufw Manages the firewall on Debian-based systems. sudo ufw status  
sudo ufs allow ssh      
ufw show      
firewall-cmd Manages the firewall (firewalld) on RHEL and related systems. firewall-cmd –list-services  
firewall-cmd –get-zones      
iptables Displays firewall rules. sudo iptables -vL -t security  
       
Setting up and running scheduled processes      
Tasks can be scheduled to run periodically using the command listed below.      
Command Function Example  
crontab Sets up and manages scheduled processes. With the -l option, cron jobs are listed. With the -e option, cron jobs can be set up to run at selected intervals. crontab -l  
crontab -l -u username      
crontab -e      
anacron Allows you to run scheduled jobs on a daily basis only. If the system is powered off when a job is supposed to run, it will run when the system boots. sudo vi /etc/anacrontab  
       
Updating, installing and listing applications      
The commands for installing and updating applications depend on what version of Linux you are using, specifically whether it’s Debian- or RPM-based.      
Command Function Example  
apt update On Debian-based systems, updates the list of available packages and their versions, but does not install or upgrade any packages sudo apt update  
apt upgrade On Debian-based systems, installs newer versions of installed packages sudo apt upgrade  
apt list Lists all packages installed on Debian-based system. With –upgradable option, it shows only those packages for which upgrades are available. apt list  
apt list –installed      
apt list –upgradable      
apt install On Debian-based systems, installs requested package sudo apt install apache2  
yum update On RPM-cased systems, updates all or specified packages sudo yum update  
yum update mysql      
yum list On RPM-based systems, lists package sudo yum update mysql  
yum install On RPM-based systems, installs requested package sudo yum -y install firefox  
yum list On RPM-based systems, lists known and installed packages sudo yum list  
sudo yum list –installed      
       
Shutting down and rebooting      
Commands for shutting down and rebooting Linux systems require privileged access. Options such as +15 refer to the number of minutes that the command will wait before doing the requested shutdown.      
Command Function Example  
shutdown Shuts down the system at the requested time. The -H option halts the system while the -P powers it down as well. sudo shutdown -H now  
shutdown -H +15      
shutdown -P +5      
halt Shuts down the system at the requested time. sudo halt  
sudo halt -p      
sudo halt –reboot      
poweroff Powers down the system at the requested time. sudo shutdown -H now  
sudo shutdown -H +15      
sudo shutdown -P +5      
       
Wait, wait, there’s more!      
Remember to consult the man pages for more details on these commands. A cheat sheet provides only a quick explanation and a handful of command examples to help you get started.