A Guide to Essential CLI Commands

The Command Line Interface (CLI) is a tool for interacting with your operating system.This is a guide to some of the most commonly used CLI commands.

Navigating File System

1. pwd - Print the working directory

   • Usage:

     $ pwd
     home/user
     

2. ls - List all files in the current directory

   • Usage:

     $ ls # ls command used in home directory.
     
     Desktop Documents  Downloads
     

   • Flags:

     • -a - List all files including hidden files
     • -S - Sort files in size order
     • -d - List all directories
     • -l - List all files with detailed information
       

     These are some important flags which used with the ls command. You also learn in detail by visiting this documentation.

3. cd - Change directory

   • Usage:

     $ cd directory_name
     

     cd is used to move forward to the child directory.

     $ cd ..
     

     cd .. is used to move backward to the parent directory.

     $ cd ~
     

     cd ~ is used to move to the home directory.

   • Relative paths

     $ cd ./directory_name/subdirectory_name
     

     You can also use relative paths to move to a certain folder.

   • Absolute paths

     $ cd ~/directory_name/subdirectory_name
     

     A sample command using absolute path.

Managing Files and Directories

1. touch - Create a new file

   • Usage:

     $ touch file_name
     

     This is the syntax to create a file.

2. mkdir - Create a new directory

   • Usage:

     $ mkdir directory_name
     

     This is the syntax to create a directory.

   • Flags:

     • -p - Create parent directories if they do not exist

       $ mkdir -p directory_name/subdirectory_name
       

       This is the most commonly used flag . You can learn more about it here.

3. 'rmdir' - Remove a directory

   • Usage:

     $ rmdir directory_name
     

     This is the syntax to remove a directory.

     If you want to remove a directory which contains files, you can use this command.

     $ rm -r directory_name
     

4. 'rm' - Remove a file

   • Usage:

     $ rm file_name
     

     This is the syntax to remove a file. You can learn more about it here.

5. 'cp' - Copy a file

   • Usage:

     $ cp file_name destination
     

     This is the syntax to copy a file.

6. 'mv' - Move a file

   • Usage:

     $ mv file_name destination
     

     This is the syntax to move a file.To rename a file you can also use this command.

Viewing and Editing files

1. cat - View a file

   • Usage:

     $ cat file_name
     

     This is the syntax to view a file.

     $ cat > file_name
     #Create a new file
     
     $ cat file_name1 >> file_name2
     #Append the contents of file_name1 to file_name2
     
     $ cat -n file_name
     #View a file with line numbers
     

     These are some stuffs used with cat commands. You can learn more about it here.

2. vi , nano - Edit a file

   • Usage:

     $ vi file_name
     
     $ nano file_name
     

     There are lot more commands for editing files. You can learn more about it here.

3. head - View the first 10 lines of a file

   • Usage:

     $ head file_name
     #View the first 10 lines of a file
     
     $ head -n 10 file_name
     #You can also specify the number of lines you want to view.
     

4. tail - View the last 10 lines of a file

   • Usage:

     $ tail file_name
     #View the last 10 lines of a file
     $ tail -n 10 file_name
     #You can also specify the number of lines you want to view.
     

5. less , more - View a file in a detailed manner

   • Usage:

     $ less file_name
     
     $ more file_name
     

     • Differences
       • more: forward navigation and limited backward navigation.
         
       • less: both forward and backward navigation and also has search options. You can go to the beginning and the end of a file instantly.

Some Useful Commands

1. man - View a manual page

   • Usage:

     $ man command_name
     #You view the manual page for every commands
     #Example usage
     $ man ls
     

2. wc - Count the number of lines, words, and characters in a file

   • Usage:

     $ wc file_name
     

     • Flags (Frequently used):
     • -c -> Count the number of characters
     • -l -> Count the number of lines
     • -w -> Count the number of words

     You can learn more about it here

3. history - View the command history

   • Usage:

     $ history
     

Network commands

1. ping - Ping an IP address

   • Usage:

     $ ping google.com
     # Checks the network connectivity to specific server.
     

2. ifconfig - View wired network information

   • Usage:

     $ ifconfig
     # Displays the ip address and more stuffs related to network connection.
     

3. iwconfig - View wireless network information

   • Usage:

     $ iwconfig
     

4. ssh - Securely connect to a remote server

   • Usage:

     $ ssh [username]@[hostname or IP address]
     

5. netstat - Network Statistics

   • Usage:

     $ netstat
     

   • Flags:

     • -a -> List all network connections
     • -l -> List all listening ports
     • -at-> List all network connections with TCP ports
     • au -> List all network connections with UDP ports

     You can learn more about it here.

OS,Process Commands

1. ps - List all processes

   • Usage:

     $ ps
     

   • Flags (Commonly Used):
     • -ef -> List all the process of the entire system with parent pid
     • -aux ->List all the process with the CPU and memory usage

2. pstree - List all processes in a tree format

   • Usage:

     $ pstree
     

     This image shows the senior most process with the sub process.

     

3. top - View running processes

   • Usage:

     $ top
     #This command is used to view the running processes.
     

4. df - View disk usage
   • Usage:

     $ df -h
     #This command with -h is used ofter to display in human readable format.
     

5. uname - Unix name

   • Usage:

     $ uname
     #Displays information about the system.
     

     • Flags:

       • -a -> Display all information

       You can learn more about it here

6. free - View memory usage
   • Usage:

     $ free -h
     #Displays memory usage in human readable format.
     

7. lspci - List all the PCI devices

   • Usage:

     $ lspci
     #displaying information about PCI buses in the system and devices connected to them..
     

8. kill - Kill a process

   • Usage:

     $ kill [pid]
     

     • Flags:

       • -9 -> Kill the process forcefully
       • -15 -> Kill the process gracefully

       You can learn more about it here

File permission

1. chmod - Change file permission

   • Usage:

     $ chmod [permission] file_name
     

   • Arguments:
     • ugo -> User, Group and Others

       $ chmod u+rwx g+rwx o+rwx file_name
       #The user,group,others have read,write and execute the file.You can modify the permission.
       

   • Binary Flags

     • Read (4) - Write (2) - Execute (1) the values are represented like this.

     $ chmod 755 file_name
     #The file will be readable,writeable and executable for user and readable, executable for group and other.
     

     

     • You can learn more about here

2. chown - Change file owner

   • Usage:

     $ chown new_owner[:new_group] file_name
     

     • Flags (Frequently used):
       • -R -> Change the owner of the files and its subdirectories

3. sudo - Run a command as root user

   • Usage:

     $ sudo command
     # Gives you the root user privileges.
     
     #Example usage
     $ sudo apt-get update
     

Package manager

1. apt-get - Install a package

   • Usage:

     $ sudo apt-get install package_name
     #root user privileges are used to install a package.
     

Searching Finding and Manipulating data

1. grep - Search and manipulating text patterns in a file

   • Usage:

     $ grep [pattern] file_name
     

   • Flags (Frequently Used):

     • -i -> Case insensitive
     • -l -> List the files that contain the pattern
     • -o -> Print only the matched pattern

     You can learn more about it here

2. sort - Sort a file

   • Usage:

     $ sort file_name
     

     • Flags (Frequently used):
       • -r -> Reverse the order of the sort
       • -u -> Remove duplicate lines

3. find - Find files

   • Usage:

     $ find [path] [type]-name pattern
     #Example command . Use type if necessary. 
     $ find . -name '*.txt'
     

     • Flags (Frequently used):

       • -name -> Find files with the pattern

       You can learn more about it here

Bash Related Commands

1. Pipe Operator | - Combines the list of commands

   • Usage:

     #The first command output is passed to the second command.
     $ command1 | command2
     

     • Example usage:

       $ cat filename | sort
       

2. printenv - Print environment variables

   • Usage:

     $ printenv
     #Print all the environment variables.
     

3. xargs - Execute commands from a input

   • Usage:

     $ xargs command
     

• Example usage

  $ pgrep firefox | xargs kill -9
  
  $ find . -name "*.txt" | xargs rm
  
  #xargs get input from previous command and execute it.
  

  4. sed - Search replace filtering text in a file and much more stuffs

• Usage:

  $ sed 's/searchString/replaceString/' file_name
  # s-String 
  
  $ sed '1,2p' file_name
  # Print the first two lines.
  

• It can do much more other stuffs .You can learn from here.

5. awk - Search and manipulate text patterns in a file

   • Usage:

     $ awk 'pattern' file_name
     

   • Arguments (Frequently used):

     • NR -> Get the line number
     • NF -> Get the number of fields

   • Example usage

     $ awk '{print $0,$1}' file_name
     # Print the first and second columns.
     

     It is similar to sed . There are lot more useful patterns.You can learn from here.

Conclusion

• These are some useful commands for a Software Engineer who just started with linux.

• I hope you like this article.