Commands

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

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• Exit from a bash instance

exit

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• See the manual/documentation or find a command

man $command
man -k $keyword        #Make a search of a command by a keyword
apropos $keyword       #Find a command by description

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• Clean up the console content

clear

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• Get information about users logged in

whoami #Shows the currently logged-in user
who    #Shows all logged-in users

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• Get the name of the computer

hostname

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• 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

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• Executes a command with superuser permissions

sudo $command
sudo -l #See what commands the user can use with sudo

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• See user and group information

id
id $username #Can be used with any user

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• 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 time
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

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• Change the 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

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• Shows the actual working directory

pwd

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• 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

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• Get the path of the file by searching in the directories defined at the $PATH environment variable

which $filename

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• Get the path of a file by searching in locate.db

locate $filename
sudo updatedb    #Update locate.db

locate.db is a built-in database of system files

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• Update locate.db manually

sudo updatedb

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• List sockets/services statistics

ss

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• Use enviroment variables

env                           #List current variables
$variablename=$value          #Created only in the current shell
export $variablename=$value   #Created globally

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• 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

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• Repeat a command based on the history

!$number   #Repeat the command with that number
!!         #Repeat the last command used

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• Open a file, usually graphic files such as images and PDFs

open $filename
display $imagefile

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• Assign a name for a simplified command or redefine a command

alias $name=$command
alias          #See all defined aliases
unalias $name  #Delete alias

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• Output any text that we provide

echo $text

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• Create a new directory

mkdir $dirname
mkdir ${dirname1,dirname2}            #Create various folders
mkdir -p $dirname/{$subdir1,$subdir2} #Create a folder with subfolders

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• Show file content in the 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 a time
less $file #Same as more, but has more features

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• Find a file or directory

find .                      #All files and directories from the actual directory
find /                      #All files and directories from the root directory
find $path                  #All files and directories from a 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 a larger size than specified
find . -size  -$size$unit   #Find by a 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

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• 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

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• 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

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• Edit text and streams on output

sed 's/$objetive/$replacement/g' $filename

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• 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

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• 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 do 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

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• Find unique occurrences in a file

sort $filename | uniq 
sort $filename | uniq -c #Count number of occurrences
sort $filename | uniq -b #Get repeated lines

uniq needs a sort file as input to work correctly

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• Compare the content of files

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

The first column of comm output represents unique lines for the first file, the second column represents the unique lines for the second file, and the third column represents the lines that both share

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• Shows the content of a file with replacements in 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

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• 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

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• Encode or decode base64

base64 $filename -e        #Encode
base64 $filename -d        #Decode

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• Transform hex dump

xdd $filename        #Show hex dump of the file
xdd -r $filename     #Reverse hex dump to a file

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• Compress or extract files

gzip $filename                 #Compress
gzip -d $filename              #Extract
bzip2 $filename                #Compress
bzip2 -d $filename             #Extract
tar -f $filename               #Compress
tar -cvz $filename               #Compress
tar -xf $filename              #Extract
7z a $namecompressed $filename #Compress
7z e $filename                 #Extract

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• Use data encryption

gpg -e $filename               #Encrypt data
gpg -d $filename               #Decrypt data

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• Create a file with the specified name

touch $filename

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• Delete a file or directory

rm $filename       #To remove a file
rm -R $dirname     #To remove a directory

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• Copy the content of a file into a new one

cp $filename $newfilename

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• Move or rename a file

mv $filename $dirname
mv $filename $newname  #Rename file

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• Shows the type of file

file $filename

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• Execute a command at regular intervals

watch
watch -n$number $command #Execute command every $number seconds

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• 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

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• 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

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• Shows the 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

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• 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
curl -H "header:value" $URL # Set headers for the request
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 $URL                  #Download through various connections
axel -n$number $URL        #Specify the number of connections to use
axel -O $name $URL         #Save with a different name

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• Verify MD5 encryption for a file

md5sum $filename

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• 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

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• Execute Python utilities

python3 $filename         #Execute a Python file
python3 -m $modulename    #Start a python module

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• Get information about the system running processes

ps                  #Show process run in the user session
ps aux              #Shows other users 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

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• To 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 allows it 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

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• 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

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• 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

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• 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

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• 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 from trusted keys list
apt-key list               #List all trusted keys
apt list --installed | grep "installed" | wc -l #Check number of installed packages

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• Manage packages via Debian

dpkg -i $packagename   #Install a package
dpkg -r $packagename   #Remove a package 
dpkg -P $packagename   #Delete package and its configuration

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• 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

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• 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.

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• Gather information about with DNS lookups

whois $URLdomain     #Get domain register information
host 72.163.10.1     #Do reverse lookup to get a host name

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• Obtain the 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

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• Query recursive DNS servers for the 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

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• Manage disk partition table

fdisk $device
fdisk -l       #List all current disk partitions

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• 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 connections
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

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• List the capabilities of all binary files

getcap -r / 2>/dev/null

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• Modify users, groups, and passwords

useradd    #Creates a new user or updates the default 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 from the system
passwd     #Changes user password