As Linux is a command-line-based operating system, the use of commands is crucial for interacting with the system, performing tasks, and managing files and applications.
Here is a list of essential and useful commands:
Create a bash instance
bash
Exit from a bash instance
exit
See the manual/documentation or find a command
man $command
man -k $keyword #Make a search of command by a keyword
apropos $keyword #Find a command by description
Clean up the console content
clear
Get information about users logged in
whoami #Shows the currently logged-in user
who #Shows all logged-in users
Get the name of the computer
hostname
Get information about the machine and system
uname
uname -a #Print all information
uname -r #Print kernel version
uname -m #Print machine hardware name
Information comes in the following order separated by spaces:
kernel-name
nodename
kernel release
kernel version
machine architecture
processor architecture
hardware platform architecture
operating system
Executes a command with superuser permissions
sudo $command
sudo -l #See what commands can use the user with sudo
See user and group information
id
id $username #Can be used with any user
List files, directories, devices, and more
ls #In the actual directory
ls $path #In path directory
ls -a #Include hidden files
ls -1 #List each file in a single line
ls -l #File permissions, associated user and groups, creation date and hour
ls -l -lr #List and order in z-a order
ls -l -lt #List and order by last modified
ls -i #List with index/inode number
lsof #List open files
lsusb #List USB devices
lsblk #List disk partitions
Hidden files start with a dot, for example .hiddenfile
Change permissions of a file
sudo chmod +$permision $filename #Add permission
sudo chmod -$permision $filename #Remove permission
sudo chmod -R 777 $filename #Give all permission to everyone
sudo chown $owner:$group $filename #Change owner and group of a file
Shows Actual Working directory
pwd
Change the directory to the specified path
cd #Redirect to the home directory
cd ~ #Redirect to the home directory
cd $path #Redirect to the specified file
cd ../ #Redirect to the upper directory
cd - #Redirect to the last directory
Get the path of the file by searching in directories defined at the $PATH environment variable
which $filename
Get the path of a file by searching inlocate.db
locate $filename
sudo updatedb #Update locate.db
locate.db is a built-in database of system files
Update locate.db manually
sudo updatedb
List sockets/services statistics
ss
Use enviroment variables
env #List current variables
$variablename=$value #Created only in the current shell
export $variablename=$value #Created globally
List command history
history
export HISTCONTROL=$value #Set output format for history
export HISTIGNORE='$regex' #Set ignore values for history
export HISTTIMEFORMAT='$options' #Set time format for history
Repeat a command based on the history
!$number #Repeat the command with that number
!! #Repeat the last command used
Open a file, usually graphic files such as images and PDFs
open $filename
display $imagefile
Assign a name for simplified a command or redefine a command
alias $name=$command
alias #See all defined aliases
unalias $name #Delete alias
Output any text that we provide
echo $text
Create a new directory
mkdir $dirname
mkdir ${dirname1,dirname2} #Create various folders
mkdir -p $dirname/{$subdir1,$subdir2} #Create a folder with subfolders
Shows file content in command-line
cat $file
cat $path #File in the specified file path
head $filename #Shows 10 initial lines
head -n$number $filename #Shows the first $number lines
tail $filename #Shows 10 final lines
tail -n$number $filename #Shows the last $number lines
tail -f $filename #Shows appended lines as data grows
more $file #Charge one page of content at the time
less $file #Same as more but have more features
Find a file or directory
find . #All files and directories from the actual directory
find / #All files and directories from root directory
find $path #All files and directories from specific directory
find . -name $filename #Find by exact name
find . -name "*.$extension" #All files with the specified extension
find . -d $dir #Only search directories
find . -f $dir #Only search files
find . -perm $permissions #Find by specified permissions
find . -group $groupname #Find by group
find . -user $username #Find by owner
find . -size $size$unit #Find by exact size, c for bytes, M for MB
find . -size +$size$unit #Find by larger size than specified
find . -size -$size$unit #Find by smaller size than specified
find . -mtime $n #Files modified in the last n days
find . -atime $n #Files accessed in the last n days
find . -cmin $n #Files changed in the last n minutes
find . -amin $n #Files accessed in the last n minutes
find . -newermt $date #Files newer than the specified date
find . 2>/dev/null #Exclude files with standard errors
The -permoption can receive permission such as 777 or a=x
Count the number of entries in a file or output
wc $filename #Count the number of lines, words, and bytes
wc -l $filename #Count number of lines
wc -m $filename #Count number of characters
wc -c $filename #Count number of bytes
wc -w $filename #Count number of words
wc $filename $filename #Count various files and show the total for each one
Search through a file and show any entries with the specified value
grep "$text" $filename
grep -i "$text" $filename #Ignore the value given
grep -v "$text" $filename #Returns non-matching results
grep -e "$ReGex" $filename #Use regular expressions
egrep "$ReGex" $filename #Alternative for use of regular expressions
Remembering the use of RegEx:
(a) -> Close and expression
[a-z] -> Indicates a class of characters (In this case letters)
{1,10} -> Indicates how many times a pattern must be repeated (In this case 1 to 10 times)
| -> Works as the OR logical operator
.* -> Works as the AND logical operator
Edit text and streams on output
sed 's/$objetive/$replacement/g' $filename
Extract a section of text from the file
cut -f $fieldnumber -d "$delimiter" $filename
awk '{print $field, $field}' #By default space as separator, fields are columns
awk '{print $1, $NF}' #With $1 get first line result, with $NF last line result
awk -F "$delimiter" '{print $field, $field}' #Spicify delimiter
Sort lines of a file
sort $filename #Order by alphabet using the first character
sort -f $filename #Ignore Case sensitive
sort -u $filename #Count and not repeat if the same
sort -r $filename #Reverse the result
sort -n $filename #Compare as numerical value
sort -k $field$delimitertor$position $filename #Order by field and position
comm $file1 $file2
comm -$number $file1 $file2 #Supress specified lines
diff $file1 $file2
diff -c $file1 $file2 #Ouput in context format
diff -u $file1 $file2 #Ouput in unified format
In comm output first the columns. Represents unique lines for the first file, the second column the unique lines for the second file, and the third column the lines both share
Shows the content of a file with replacements on the output
tr $filename $expression $replacement
tr $filename -c $expression $replacement #What not match is replaced
tr $filename -d $expression #Delete what coincides with expression
Format the output in columns
column $file -t #By default use space as a separator
column $file -t -s $sep #Use set of characters as separator
watch
watch -n$number $command #Execute command every $number seconds
Change to the specified user
su $username #Change user and stay in the same place
su -l $username #Redirects to the home directory of the other user
Create or edit a file through the command line
nano $filename
nano -B $filename #Make a backup when opening a file with nano
vi $filename #More powerful Text Editor
vim $filename #Improved version of vi
vimtutor #Enter tutor mode for practicing
In Unix the ^ symbol represents the Ctrl key
Shows estimated disk usage of a file
du $filename
du -h $filename #Shows output in human-readable mode
du -b $filename #Shows output in bytes mode
Make petitions to websites
curl $URL #Make a get petition
curl -L $URL #Follow redirections
curl -O $URL/$file #Save a file with the same name
curl -o $name $URL/$file #Save a file with a different name
curl -o $URL/$file1 -o $URL/$file2 #Get various files
curl $URL -d "$param=$value" #Specificate values of body
curl -X $PETITION $URL #Make an specific type of petition
curl $firstpetition --next $secondpetition #Make various petition
curl $URL -H "$header:$value" #Specific header in a petition
curl $URL -v #Get verbose output
curl -IL $URL #Catch header, useful for banner grabbing
wget $URL
wget -O $URL/file #Save a file with the same name
wget -o $name $URL/file #Save a file with a different name
axel $URL #Dowload through various connections
axel -n$number $URL #Specify the number of connections to use
axel -O $name $URL #Save with a different name
Verify MD5 encryption for a file
md5sum $filename
Get and Send files via SCP
scp $filename $user@$remoteIP:$remotepath #For sending a file
scp $user@$remoteIP:$remotepath $filename #For getting a file
scp -r $dirname $user@$remoteIP:$remotepath #For copying a directory
Execute python utilities
python3 $filename #Execute a python file
python3 -m $modulename #Start a python module
Get information about the system running processes
ps #Show process run in the user session
ps aux #Shows other user and complete system processes
ps -A #Shows all processes
ps -f #Display full format listing
ps -C $commandname #Shows process with a specified command
ps axjf #Show as a tree
top #Real-time information about the system running processes
Kill a system process, we can send some signals to specify the type of killing
kill -9 $PID
kill -s SIGTERM $PID #Kill the process, but allow to do some cleanup
kill -s SIGKILL $PID #Kill the process but doesn't do any cleanup
kill -s SIGSTOP $PID #Stop/suspend a process
kill -s SIGINT $PID #Interrupt a process (same as CTRL+C)
kill -s SIGQUIT $PID #Quit process (same as CTRL+D)
kill -s SIGTSTP $PID #Suspend process but can still be handled (same as CTRL+Z)
kill -l #See all possible signals that can be sent
The process with a PID equal to 0 is a process that starts when the system boots
Manage system processes and services
systemctl $option $process
systemctl start $process #For starting a process
systemctl stop $process #For killing a process
systemctl enable $process #Set process to start at startup
systemctl disable $process #Set process to not start at startup
systemctl status $Process #View the status of a process
systemctl list-unit-files #List the status of all processes
systemctl list-units --type=service #List status of all services
journalctl -u $service --no-pager #See logs of a service
Handle background processes
jobs #List all actual background processes
bg #Resume stopped background processes
bg $PID #Send specified process to background
fg $PID #Foreground a background process
fg %$jobID #Foreground specified list ID process
fg %$String #Refers to the beginning of the suspended process
fg %+ #Refers to the current job
fg %- #Refers to the previous job
bg can only be used after stopping a process with ^z
Schedule to repeat a process as a crontab
crontab -e #Edit actual crontab
crontab -l #Show actual crontab
crontab -r #Delete actual crontab
crontab -i #Prompt actual crontab and delete it
Every user has only one crontab
Manage packages, repositories, and digital signatures from APT
apt install $packagename #Install a package
apt reinstall $packagename #Reinstall a package from scratch
apt remove $packagename #Remove a package
apt remove --purge $packagename #Delete package and its configuration
apt update $packagename #Update a package
apt update #Update all packages
apt upgrade #Update Linux system and applications
apt-get #Use the first version for actions
apt-cache search $toolname #Search for a tool in apt repositories
apt show $packagename #Show information about a package
add-apt-repository #Add repositories from developers to your apt lists
apt-key add $filename #Add key file to trusted keys list
apt-key del $keyID #Remove key trusted keys list
apt-key list #List all trusted keys
apt list --installed | grep "installed" | wc -l #Check number of installed packages
Manage packages via Debian
dpkg -i $packagename #Install a package
dpkg -r $packagename #Remove a package
dpkg -P $packagename #Delete package and its configuration
Test if the connection to a remote resource is possible
ping $IPaddress
ping $URLdomain #Use domain name
ping $IPaddress -4 #Limit to only IPv4 requests
ping $IPaddress -6 #Limit to only IPv6 requests
ping $IPaddress -i $seconds #Set time interval to send each packet
ping $IPaddress -v #Show a verbose output
ping $IPaddress -c $number #Send an exact number of packet
ping $IPaddress -s $size #Specify the number of bytes of the data
Show the path a request takes as it heads to the target machine
traceroute $IPaddress
traceroute $URLdomain
traceroute $IPaddress -i $interface #Specify interface for send packets
traceroute $IPaddress -T #Use TCP SYN for connection probes
A * symbol on the response represents that a package hasn't received an ICMP message from the router.
Gather information about with DNS lookups
whois $URLdomain #Get domain register information
host 72.163.10.1 #Do reserve lookup to get a host names
Obtain IP and record information from a domain
nslookup $URLdomain
nslookup $URLdomain $serverIP #Change server used to perform lookups
nslookup -type=$record $URLdomain #Specify record type
nslookup #Enter interactive mode
> Exit #Quit interactive mode
Query recursive DNS servers for domain's information
dig $URLdomain $record
dig $URLdomain $serverIP #Change server used to perform lookups
dig @$DNSIPaddress $URLdomain #Look up DNS records
dig @$DNSIPaddress $URLdomain $record #Specify record type on DNS records
dig -x $IP #Perform reverse DNS lookups
dig CH TXT $URLdomain @$DNSIPaddress #Check DNS version
dig axfr $URLdomain @$DNSIPaddress #Check zone transfer
Manage disk partition table
fdisk $device
fdisk -l #List all current disk partitions
Network Utilities
ifconfig #Check network interfaces
ifconfig $interface up #Activate a network interface
ifconfig $interface $IP #Assign IP Address to an Interface
ifconfig $interface netmask $mask #Assign a Netmask to an Interface
route add default gw $IP $interface
iwconfig #Check only wireless Interfaces
ip route #Check existing network routes
ip link set $interface up #Activate a network interface
netstat #Check ports and active connections
netstat -a #List all ports and connection
netstat -l #List listening ports
netstat -t #List only TCP ports
netstat -u #List only UDP ports
netstat -s #List network usage statistics by protocol
netstat -p #List with PID and program name
netstat -ano #Most common use, n to no resolve names, o to display timers
netstat -tunap #Nice version
When using netstat -p if the PID and name aren't shown, it's because the process is owned by another user. Running the command with sudo could show this information
List capabilities of all binary files
getcap -r / 2>/dev/null #Is good practice to redirect errors if not run with sudo
Modify users, groups and passwords
useradd #Creates a new user or update default new user information
userdel #Deletes a user account and related files
usermod #Modifies a user account
addgroup #Adds a group to the system
delgroup #Deletes a group of the system
passwd #Changes user password
