A command line interface (CLI) is a way of interacting with a computer system where the user issues commands in the form of successive lines of text.
Fluency on the command line is a skill which although considered arcane, improves your productivity and flexibility as a computer engineer:
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Powerful Remotes All major organizations use systems that are a lot more powerful than your average personal computer.
These remote systems almost never use a graphical user interface (GUI) and can be operated only with commands.
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Workflow Since command line is the closest you can get to the operating system, the use of direct commands improves your workflow.
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Enlightenment Apart from its demand in the industry, command line expertise brings with it a holistic idea about the operating system.
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Control It provides fine-grained control over a system and various methods to automate boring tasks.
Working on the command line interface is very similar to solving a simple puzzle. With the right approach, anything is possible. You just need to pick the right commands from a large toolbox.
Over the course of this workshop, we will work in a Linux environment.
The workspaces are mounted on a Telnet server, at 172.16.22.5
To connect to it:
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macOS, Linux Open a terminal and run the following command:
telnet 172.16.22.5
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Windows:
- Ensure that the Telnet service is enabled by navigating to
Control Panel>Programs and Features>Turn Windows features on or off> CheckTelnet Client - Hit
Windows+Rand enter the following command:
telnet 172.16.22.5
- Ensure that the Telnet service is enabled by navigating to
If you have followed the above instructions, you would have successfully connected to the telnet server.
Enter your credentials to access your workspace.
Once successfully authenticated, you should see the following prompt:
[username@linuxbpdc1 ~]$
This is where we input commands.
By default we are provided with a Bash terminal.
When we input commands, the bash interpreter figures out what we are trying to do and requests the operating system to get it done.
Use the echo command to print stuff
echo "Hello, world."
echo 'Single quotes also work'You can also use the printf command to print according to a format
printf "Hello, %s!\n" "World"Comments are like 'thoughts' The interpreter ignores them. They are there just for you. So you know what you're doing.
Write single line comments with #
# Anything to the right of the # is ignored by the command line interfaceThe man command gives the manual for different commands
man echo # This gives a manual about "echo" . Press 'q' to exitWhenever you enter a system for the first time, it is a good idea to learn more about it.
The uname command tells you which operating system is running.
unameUse the -a flag to find out a lot more about the system.
uname -aIt shows you:
- hostname: The name given to this system by the administrator
- kernel version: The version of the backbone of the operating system
- date & timezone
- system architecture: This tells us whether the system uses 32 or 64 bit operations
A flag is nothing but an extra piece of information that you specify with the command. This provides additional functionality
There are many people logged in right now.
Use the who command to check them out.
whoCan't find yourself? Try ONE of the following:
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who -m
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who am i
When working with a GUI, we navigate across the filesystem using a file explorer:
- File Explorer (Windows)
- Finder (macOS)
- Dolphin, Nautilus, Thunar and many others! (Linux)
Use the pwd command to find the present working directory
pwd # If you ever get lost,`pwd` will tell you exactly where you areA folder. Simple.
You are currently in your HOME directory which is the equivalent of Desktop on PCs.
All files and directories on a computer system have a unique path that describes their location:
- An absolute path refers to the same location in a file system relative to the root > directory
- A relative path points to a specific location in a file system relative to the present working directory
Some important shorthand notations:
.refers to thePWD..refers to the parent ofPWD/refers to the root directory~refers to theHOMEdirectory
The cd command changes the PWD to a specified path
cd .. # Changes `PWD` to its parent. You can confirm this using `pwd`We can list the contents of a directory using ls
cd ../../.. # in this case, it takes you to the root directoryTo return to your HOME directory, run ONE of the following:
You can format this output using flags
ls -l # formatted as a listls -al # displays all the contents (including hidden ones) in a list formatls -h # sizes automatically adjusted to KB, MB, GBYou can chain .. and switch to a directory that is an ancestor of the PWD
cd ../../.. # in this case , it takes you to the root directoryTo return to your HOME directory, run ONE of the following:
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cd # not specifying a path teleports you back home
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cd $HOME # The $HOME variable stores the absolute path of your HOME directory
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cd ~ # ~ = $HOME
Jump back to root and try all the above options.
cd / # jumps back to root, since "/" is the absolute path for rootOnce you are back HOME, create a directory using the mkdir command
mkdir strangers # creates a directory named 'strangers' in the `PWD`Move into this new directory
cd strangersUse the touch command to create a file
touch description.txt # creates a file named 'description.txt'Files store data
- Files are stored in the computer's secondary memory
- The file's data is stored in the form of bytes
For text files:
- The bytes that represent this data take values in a particular range.
- This range is determined by the encoding used, which may be ASCII, UTF-8 etc.
- When being rendered, specific characters that match the bytes of these files are shown.
- Sometimes there are bytes that do not make sense so they don't get rendered properly.
Most systems have in-built text editors. Vim (the sequel to Vi) and Emacs are the best to use. However, due to their steep learning curves, they are not advisable for beginners.
Let us use the nano editor instead
nano description.txt # opens 'description.txt' with nanoType something in.
When a young boy vanishes
a small town uncovers a mystery
involving secret experiments, terrifying supernatural forces
and one strange little girl.
The text that is shown are not the exact contents of the file. This data is held by the
nanoeditor as a buffer and is residing in the RAM of the system.
To save the contents of the file:
- Hit
Ctrl+Xto write the contents of the buffer into the file - Hit
Yto confirm andEnter nanonow shows the name of the file it is about to write into. You can change this if you like. HitEnterto finish.
Use the cat command to display the contents of the file
cat description.txtUse the head command to display the first few lines of the file
head description.txt -n 2 # prints the first 2 linesSimilarly, you can display the last few lines with tail
tail description.txt -n 2 # prints the last 2 lines of the fileCreate files with the following content:
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will.txtWill is scared! -
demogorgon0.txtThis beast is hungry! -
el.txtEl is powerful! -
hopper.txtHopper is the best!
The workspace is very messy. Let us use directories to organize our files
Create 2 directories:
hawkinsupsidedown
We can move files into specific directories using the mv command
Try one of the following commands to move will.txt into upsidedown:
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mv will.txt upsidedown/ # 'will.txt' is moved into the directory 'upsidedown' -
mv will.txt upsidedown/will.txt # renames 'will.txt' as 'upsidedown/will.txt' and hence moves it into 'upsidedown'
Similarly, move :
el.txtintohawkins/hopper.txtintohawkins/demogorgon0.txtintoupsidedown/
Be very careful when using the mv command:
When / is not specified at the end of the name of a directory, it renames the source to the destination instead of moving it inside.
mv will.txt upsidedown # renames 'will.txt' to 'upsidedown'In this case the the upsidedown directory is unchanged but will.txt is renamed to upsidedown
To better visualize the contents of PWD use the tree command
tree # displays the names of files and directories in the PWD in a recursive manner
You can also use -R with ls
ls -RWe can create copies of files using the cp command:
cp description.txt plot.txt # makes a copy of 'description.txt' and calls it 'plot.txt'Check if the contents of the two files are the same.
Copy demogorgon0.txt multiple times
cp upsidedown/demogorgon0.txt upsidedown/demogorgon1.txt
cp upsidedown/demogorgon0.txt upsidedown/demogorgon2.txt
cp upsidedown/demogorgon0.txt upsidedown/demogorgon3.txt
cp upsidedown/demogorgon0.txt upsidedown/demogorgon4.txtWe can scale these operations to affect multiple files
Let's bring will.txt and the demogorogon0.txt copies from the upsidedown to hawkins
mv upsidedown/*.txt hawkins/Those demogorgons are annoying. Let's kill them all!
Use the rm command to delete files
rm demogorgon0.txtScale this command to delete all of them
rm demogorgon*.txtYou got rid of them from hawkins.
The upsidedown is still bothering you.
Use the rmdir command to delete directories.
You can destroy the upsidedown in ONE of the following ways:
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rmdir upsidedown
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rm -rf upsidedown # -r specifies recursion, -f specifies force. Deletes a directory recursively
Give yourself a pat on the back. You saved hawkins
Right now, there isn't anything important on your system. It is still a good idea to update your login credentials.
Use the passwd command to do so.
View the details of the contents in the PWD
ls -alYou should see the following characters next to each entity in the directory
-rwxr-xr-x
These characters represent the permissions for a particular entity.
Everything on such systems has permissions attached to it. Since files, directories & programs all have permissions of their own, there is a very elaborate permission mechanism.
The first character describes the type of entity
- - for a regular file
- d for directory
- c for character special file
- b for block special file
- l for symbolic links
You can learn more here
The next 9 characters are in fact 3 groups of characters that represent permissions for different users
- The first 3 show the permissions for the owner of the file
- The second 3 show the permissions for the group associated with that file
- The last 3 show the permissions for the rest of the users
Each group of 3 characters represents the 3 major permissions
- r for read and is equivalent to 4
- w for write and is equivalent to 2
- x for execute and is equivalent to 1
A permission of 5 allows only read and execute (4 + 1 = 5).
File permissions can be changed by:
- The owner
rooti.e. any superuser
Use the chmod command to change permissions
chmod 777 hawkins # allows anyone (owner, group and rest) to do anything (read, write and execute) to 'hawkins'The chown command can be used to change the ownership of a file but can only be called by root
Use the wc command to count display:
- The number of lines
- The number of words
- The number of characters
wc plot.txtYou can use flags to limit the output to what you need
wc plot.txt -l # shows the number of lines for 'plot.txt'Use the grep command to search for specific characters in a file
grep "a" plot.txt # shows the occurrences of 'a' in 'plot.txt'An important feature of Bash is the pipe system
It enables the output of one command to be used as the input for another.
You can combine grep and wc to combine their functionalities
grep -o "a" plot.txt | wc -l # shows the number of occurrences of 'a' in 'plot.txt'Use the history command to show your command history
historyYou can combine grep and history to search for a particular command
history | grep "cd"
Use the ps command to look at processes that are running on the system
ps # shows you what you are runningLook at all processes
ps auxThis is a lot. Let us save it into a file
Redirections are very similar to pipes except that they work on files
ps aux > processes.txtWe covered: