The Command Line

What is Command Line?

A user interface where one can enter text prompts to execute commands or automate tasks.

The terms: Command Line, Bash, Shell, Terminal, Command Prompt, are used interchangeably but they all generally mean the same thing. Below are some quick descriptions for each term.

Command Line Interface (CLI) is a user interface where users type in commands and view outputs printed on the screen.
Bash is a shell and command language in Linux.
Shell is a software that interprets and executes the various commands that we type in the CLI.
Terminal, is a CLI application in Mac OS.
Command Prompt, is a CLI application in Windows OS.

Benefits of using the Command Line

While many users are accustomed to using a graphical user interface (GUI), the command line offers the following benefits:

Direct interaction with the computer, with no overhead processing from the GUI, allowing faster and more efficient execution of tasks.
Automation of commands and tasks by running scripts.
Makes you feel like a hacker (sort of)

What can we use it for?

In my day-to-day, I use the command line to:

Inspect details of my localhost/servers/applications/processes
Navigate and manipulate files and folders (Filesystem Management)
Access servers over networks
Run applications
Debug logs
Write and run scripts to automate tasks/applications
Version Control (Git)

Note: The above list is not exhaustive, use cases will differ between individuals

Cheatsheet

The below cheatsheet is a compilation of commands that I either frequently used or found useful along the way when I approach the command line.

General Usage

 date                  Print current date and time          
 exit                  Close current session                
 history               Check the history of used commands   
 hostname              Print name of host system            
 sudo                  Run command as an root/administrator 
 sudo shutdown -h now  Shutdown computer                    
 sudo shutdown -r now  Restart computer                     
 sudo su -             Log in as an root/adminstrator      
 whoami                Prints current user

Filesystem management

cat `filename`                      Read and print content from file (Fast for reading small contents). Use "less `filename`" for viewing large contents page by page. For Windows, use "`filename`"                                                     
cd                                  Change directory to home                                                                                    
cd -                                Change to previous directory                                                                                
cd ..                               Change to parent directory                                                                                  
cd `dirname`                        Change directory to specific directory                                                                      
cd ~                                Change to home directory                                                                                    
clear                               Clear the screen/Remove previously typed commands. For Windows, use "cls"                                             
cp -r `dirname1` `dirname2`         Copy contents recursively from directory 1 to directory 2. For Windows, use "robocopy `dirname1` `dirname2`"                                                     
cp `filename` `dirname`             Copy file to a directory. For Windows, use "copy `filename` `dirname`"                                                                                    
ditto -V `dirname1` `dirname2`      Copy all contents of one folder to another folder                                                           
echo "Hello World"                  Display message "Hello World"                                                                               
echo "Hello World" > `filename`     Create a file with "Hello World" or appends "Hello World" to file                                           
find `dirtosearch` -name `filename` Find location of a program                                                                                  
grep `regex` `dirname`              Search for text patterns in directory                                                                       
grep `regex` `filename`             Search for text patterns in filename                                                                        
head -100f `filename`               Show the first 100 lines of file in real-time                                                               
head -n 5 `filename`                Show first 5 lines of file                                                                                  
ls                                  List directory contents..For Windows, use "dir"                                                                       
ls -a                               List contents including hidden files (Files that begin with a dot)                                          
ls -l                               List contents with more info including permissions (long listing)                                           
ls -r                               List contents reverse order                                                                                 
mkdir `dirname`                     Make directory    
mv `dirname1` `dirname2`            Move directory from directory 1 to directory 2.For Windows, use "move" instead of "mv"                                                              
mv `filename` `dirname`             Move file to directory                                                                                      
mv `filename1` `filename2`          Rename file or folder from file 1 to file 2.For Windows, use "ren `filename1` `filename2`"                                                                  
mv `filename1` `filename2` -v       Rename Verbose - print source/destination directory                                                         
open `dirname`                      Open a file/folder..For Windows, use "start `filename`". For Linux, use "xdg-open `filename`"
ps -ef >> `filename`                Append the piped output of ps -ef to file ">" : Overwrite existing file or creates a file if the file is not present in directory ">>": Append existing file or creates a file if the file is not present in directory.                                                           
pwd                                 List path to the working directory.For Windows, use "cd"                                                                          
rm -i `filename`                    Remove directory  
rm -r `dirname`                     Remove directory with contents recursively                                                                  
rm -rf `dirname`                    Remove directory with contents recursively and ignore non-existent files and arguments, no confirmation prompt                                                                                     
rm ./\*                             Remove everything in the current folder                                                                     
rm `filename`                       Remove file. For Windows, use "del `filename`"       
rmdir `dirname`                     Remove empty directory                                                                                      
tail -10 `input_file` 
grep "Hello World" 
>> `output_file`                    Pipe last 10 lines of `input_file` and grep texts matching "Hello World" to `output_file`.                                                               
tail -100f `filename`               Show the last 100 lines of file in real-time. Useful for inspecting running logs
tail -n 5 `filename`                Show last 5 lines of file
tar -czvf `dirname`.tar.gz          Create tarball                                     
tar -tzvf `dirname`                 See what is in the tarball
tar xzvf `dirname`.tar.gz           Extract tarball         
tar flags                           c : Creates Archive, x : Extract the archive, f : creates archive with given filename, t : displays or lists files in archived file, u : archives and adds to an existing archive file, v : Displays Verbose Information, A : Concatenates the archive files, z : Zip - tellstar command that creates tar file using gzip, j : filter archive tar file using tbzip, W : Verify a archive file, r : update or add file or directory in already existed .tar file 
touch `filename`                    Create a file                                                                            
vi `filename`                       Create/Edit a file in VIM

Server and Network Management

curl -o `path to file` Download file                    
nc -v `serverName`     Initiate a telnet connection to server                                                                                     
netstat                Show network status and protocol statistics                                                                                
ping/curl `serverName` Ping or curl a server            
scp `dest1` `dest2`    Remote copy filepath from dest 1 to dest 2 where dest1 and dest2 can be written as `user`@`remote_server`:`filepath` 
ssh `serverName`       Initiate SSH connection to server

Disk Management

df -h    Check free disk space           
du       List subdirectory memory usage  
du -s    Check storage for specific file

Process Management

kill -9 `PID`                   Kill processes using PID                        
ps -ax                          Show all running processes                      
ps -ax | grep `name/pid/regex`  Search process using name or PID or regex             
top                             Display live process statistics                 
top -o rsize                    Display processes with the highest memory usage 
nohup `file` > `logFile` &      Run process in the background (&) and pipes the standard output to log file 
nohup `file` > `logFile` 2>&1 & Run process in the background (&) and pipes both standard output and error (2>&1) to log file

Conclusion

The command line is the bread and butter application for all computer systems. Not only will it automate and speed up your day-to-day development work, it serves as a starting point for you to run, inspect and monitor the applications and filesystems of servers directly. For many who are initially starting out, this application looks and feels like a "black box". But rest assured, with sufficient exposure and practise, you will eventually master it and enjoy the many benefits this application brings to your everyday workflow.