Comand Line, Linux vs Windows

If your not a computer geek you probably have never heard of Linux or have been told that its command line is powerful but difficult to use. However, I dont know much about Windows either, like did you know windows has a command line?. So without any further ado, let's get to the bottom of this.

Linux and Windows are 2 very different operating systems(OS). (I know there is mac too but I've heard its command line is similar to Linux so we'll leave that for another day)

• Linux is free and open source while Windows is not.
• Windows is more GUI(Graphical User Interface) oriented(hence the name windows) while Linux has mastered the command line.
• Windows was originally designed to have one user per OS and device while Linux right from the start was made such that many users could have separate sessions while using the same OS.
  This meant that in the past Windows was a lot less secure since it did not have the concept of file permissions. Now technically it still doesn't because it manages permissions on a per user/process level rather than on a per file level. This is why when file permissions where introduced in windows they where called access privileges instead.
• The 2 started off as arch rivals due to Microsoft hating the idea of open source. But they have come a long way since, both have now incorporated some of the syntax and functionality of the other.

Brief History

Microsoft Windows

First there was MS-DOS(Microsoft Disk Operating System) released in 1981. Then in 1983 Windows added a GUI(but left the COMMAND.COM command line for backwards compatibility).
DOS was set up to have one user per OS which proved to be less secure since it did not have the concept of access privileges. This was changed in the early 2000s with the Windows NT versions who abandoned parts of the DOS implementation and replaced it with a system that was more suited for multi user/session applications. However, initially it made no difference because the default user was the administrative(aka root) user which has all permissions. The Windows NT and later versions also had an improved command line called CMD(Command Prompt). But because it was still quite primitive, Microsoft eventually made a completely new shell released in 2006 called PowerShell. Nowadays(2025) windows comes with both CMD and PowerShell.

Linux

First there was Unix, first released in 1970 and later rewritten in C. Then The GNU Project developed a collection of free and open source software including a kernel base on Unix. The kernel was disliked but many of its other software packages live on to this day(the C compiler called GCC(GNU Compiler Collection), the bash shell, etc...). Then in the 1990s Linus Torvald made a better kernel called Linux(technically Linux is not an OS but only a kernel but commonly the word Linux refers to the Linux kernel plus GNU and other features). From there other companies and organizations made their own free and open source Linux distributions, among them: Debian, Red Hat, Ubuntu(based on Debian), Fedora(related to Red Hat), Oracle(based on Red Hat), etc...

Android

It's an OS based on Linux. It was developed primarily by Google and first released in 2008. This is why android phones come with Google play(nowadays however there are alternatives to the Google Play store like for example F-Droid)

macOS

It's a closed source OS based on Unix. First released in 1984 by NeXT, founded by Steve Jobs and later bought by Apple. Before this Apple devices were running Apple DOS.

Linux and Windows Main Differences

Versions and Updates

Windows is managed by one company which makes regular releases and then forces you to update next time you boot up your computer so that everything is pretty much the same for every user on the planet.
Linux on the other hand is made and updated by many different organizations, so you chose when and what you update. You chose what distribution you want(Debian, Ubuntu, etc...), what GUI you want(Xfce, GNOME, etc...) and what command line(aka shell) you want(Bash(usually the default), dash(sometimes aliased to sh), zsh, csh(seashell :) sorry lol C shell as in the programming language), fish, etc...) because there are several to chose from. This is why on the surface Linux seems more complicated because it's focused on accessibility and customizability instead of standardization for ease of use.

Overall Organization

On Linux:
Files and directories are organized in one big tree structure. There is a single root and everything branches off from there. When a USB or disk is mounted to the tree it just gets attached as a branch to whatever directory it is mounted in. You can mount the same device in several places at the same time. Learn more about how to mount USBs below

On Windows:
Files and directories are organized per drive and inside each drive you have a tree structure like in Linux. The C drive(denoted as C: ) is the main disk and it contains default programs, applications, system files and the OS itself. If you have extra storage disks(internal or external) or you plug in a USB or memory card on your computer then they will be named D:, E:, etc...
This means that you can not mount the same device in several places at the same time.

Files and File Types

On Linux:
Linux does not have a strict concept of file types. Everything is just a file, like even hardware(ex: keyboard, mouse, sound cards, etc...). And a file can have almost any name, you dont need to have the .something like in Windows. What the file is/does/is allowed to do is determined in the file permissions which is independent from the file extension.(To view file permissions of all files in the current directory type ls -l)(To change file permissions use the command chmod). The .somthing is only for better readability.
So often text files have .txt, executable files have .elf(Executable and Linkable Format), etc... but its not necessary.

Also, file names are case sensitive and you must NOT include spaces(cus otherwise you always need to use quotes which is annnnnoooooyyyyying)

On Windows:
In Windows instead of everything is a file its more like everything is an object. An object is a name plus an extension denoting what type of object it is. This is why Windows has strict file types. A file has a name and then a second field(separated by a dot) to denote the type of file.
here is a list of some common file type name extensions:
.txt is for text
.exe is for executable files
.bat is for batch files, a type of script file(a batch of instructions executed in order)

The file names can have spaces and are NOT case sensitive.

File Permissions and Ownership or Access Privileges

• normal user
• privileged/super user: called root in Linux and administrator in Windows

On Linux:
Each file has file permissions which specify what the file can do depending on what user is using the file.

• read(r):look at file
• write(w):change or delete file
• execute(x):run the file/program(usually directories have executable permissions meaning you can cd into the directory to see what is inside)
• special permission:sticky bit(t):can be used in directories that are shared among all users(ex: /bin, /tmp, etc...) to only allow the owner of the file to delete it.
• special permission:setuid/setgid(s):runs an executable as the owner(setuid) or group(setgid) with the owner's permissions instead of the current user. This is what the command sudo uses to run programs as root.

• User owner: the user that owns the file, that user is usually also the creator of the file unless the ownership was changed.
• Group owner, if you want only some other users on the device to have more rights/permissions over a specific file. A file can only have one group but a user can belong to many groups

On Windows:
On Windows, for a long time this concept did not exist. But as mentioned earlier this security flaw has been fixed, sortof. Windows calls it access privileges because what a file can do is managed globally per user. To see the access privileges type whoami /priv in either CMD or PowerShell.
To see and or change the permissions of a specific file right click on the file in the file manager then go on properties and then to the security tab.
Notice a simple text file has all permissions(read, write, execute) both for the creator of the file as well as system wide! This is very different from Linux where files by default are never executable for security reasons.

Hardware Management and Audio Stuff

On Linux:
Note: Nowadays, some Linux distributions do like Windows and by default configure and mount everything for you but you can always configure it to be done manually because in Linux everything is just one tree which is helpful in server networks if you replace a server with its backup no one will notice and things go on smoothly(unlike windows where the discs are lettered by plugging in order so you would need an extra step to not have name mismatches). So for some practical and otherwise historical reasons Linux typically manually mounts its external hardware.

General hardware:
As mentioned earlier, in Linux everything is a file. When you plug in a device the device and your computer talk to each other. The device tells your computer what type of device it is so that the correct type of driver file can be created. Device files are stored in the /dev directory. You plug in a device via USB and a file is created, meaning the device and the computer talk even before you mount anything. If you plug in a mouse via USB, a file called mcelog is created. While if you plug in a USB stick, 2 files called sdb and sdb1 respectively are created(side note: sda is the internal storage hard drive of your computer and sda1, sda2, etc... are its different partitions. So sdb is the second detected storage device. Meaning if you plug in a second USB stick it would get the files sdc and sdc1).
Now pluging in a device just creates one or a few files in /dev. For some devices thats enough but for others like a USB stick you need to mount it. Mounting is the process of attaching the file tree structure of the usb stick to the computers file tree structure and establishing all the right permissions associated with that. This is why you dont need to mount a mouse in order to use it. Learn more about how to mount USBs below

Audio Specific:
The loud speakers and headset jack are internally connected so when you plug headphones in it automatically switches.
If you want to plug in a headset via USB with a USB adapter you have to go in your settings under audio mixer and change the settings there under playback. However, a video or other sound needs to be playing otherwise it does not show the needed settings.
Unlike Windows, Linux does not necessarily switch the audio card if you plug in a new device, taking a more configure things manually approach.

On the command line the commands pactl and pacmd, amixer can be used.

On Windows:
It's very similar to Linux just that the OS does all the configuration and mounting automatically for you, and instead of creating/loading a driver file it creates/loads a driver object. A driver is a program that acts as the middle man between hardware and the kernel.(So Windows is actually a self driving car lol and Linux is a manually controlled car, thats why it's called windows not window(windshield+ side and back windows) :) and your a passenger enjoying the view)
To see all your devices/currently active drivers use the device manager.

Unlike Linux, you just plug in your device via USB or headset jack and it automatically configures it for you and if necessary also mounts everything for you.

Several Simultaneous Logins

On Windows:
You can switch user without loging out the current user by doing Ctrl+Alt+Del.(VERY IMPORTANT: in Linux Ctrl+Alt+Delete is often configured to log out or in some cases even reboot the computer(if you are in a non-graphical interface(I talk about them just below) Ctrl+Alt+Delete will cause a reboot) so dont mix up the key combinations of Linux and Windows). But otherwise simultaneous logins are not easily doable in Windows, you need special software(Remote Desktop Protocol) to do remote login. Now PowerShell and CMD both support the ssh and scp Linux commands but they only work one way. You can ssh from Windows into a Linux device but you can not easily ssh from a Linux device into a Windows device without installing/configuring an ssh server on the Windows computer. Same goes for scp, you can run the scp command on Windows and copy from or to Linux but you can not run the command on Linux to copy to or from Windows.

On Linux:
In Linux on the other hand its easy because it was made to be multiuser right from the start. I explain how to use ssh and scp to login remotely into your computer from a different computer below.
But you can also login several times from your computer into your computer at the same time. This is done by doing:ctrl+alt+fn+f1(or ctrl+alt+f1 depending on your keyboard layout)
It is not a graphical user interface but otherwise its exactly like your computer. To switch back to your graphical user interface do: alt+fn+f7(or alt+f7 depending on your keyboard layout)
Now there are actually 5 more of these. So, there is f1 through f6 which are non-graphical(also called tty1-tty6) plus f7 being the graphical one). To switch between them just do ctrl+alt+fn+f(put nb here) if you are currently in the graphical UI and alt+fn+f(put nb here) if you are in a non-graphical UI.
The graphical interface has the added ctrl to avoid conflicting with user or application set key bindings. But in a non-graphical environment you wont have an application using the alt fn keys for something else so adding the ctrl to avoid confusion is not necessary.

Cross Compatibility Issue and Attempted Resolution

Compiled programs only run on the OS they have been compiled in because different OS have different system call conventions. As we have seen the whole setup of everything is totally different in these 2 operating systems so if you want cross compatibility you need some sort of translator.

To run Windows programs on Linux you serve wine :). lol no really a program called Wine will do the trick. Wine is not a virtual machine, it simply translates the system calls.

To have a bash shell(Linux shell) in Windows you can install wsl(Windows Subsystem for Linux).

To run Linux Programs on Windows that have a gui, I have not tried it but I heard that you can install wslg(Windows Subsystem for Linux GUI).

Command Line Commands and Basic Usage

In Linux open a shell(also called terminal)(the default is usually bash). In windows open CMD(Command Prompt) or PowerShell. (note: putty is not a shell. It's a way to ssh into a Linux device)

IMPORTANT: In Linux things are case sensitive unless specified. While in Windows things are not case sensitive unless specified.

Also, when wanting to specify a filename, if the file is not in the current directory then include path(can be relative or absolute).

If you are lost and confused about some of the concepts mentioned in the table(like aliases, command options, and how to get non mentioned details about specific commands) scroll down below the table for more explanation.

Shells and How to Access Documentation

The words terminal, command line and shell are used interchangeably because for the user they are pretty much the same thing. Technically the shell is the software and the terminal is the thing displaying the output of the shell to the user but nowadays you cant really open a terminal without a shell running because the terminal automatically launches a shell(for Linux usually bash cus it's the default). And command line is the Windows term for both terminal and shell.
The name shell comes from the idea that you the user is on the outside, the kernel is on the inside and to access the kernel you have to go through the protective layer around the kernel, aka the shell.
The name terminal comes from the idea that the cables start at the computer and end aka terminate at the screen.

On Linux:
Linux has many different shells, but the most commonly used one is bash. Bash stands for Bourne again shell because it is a newer version of the Bourne shell. Also at the time the bourne shell was not open source so to have a shell that had the same/similar style and syntax as the bourne shell but have the same open source licence as Linux, bash was created.
To see what shell is your default type: echo $SHELL
reminder Linux commands are case sensitive so can NOT do: echo $shell
In this article we will focus on bash only.

In Linux the command man is used to access the manual page of a given command. However, some commands only have a help page(accessed by writing help command-name). The difference between help and man is that help is built into the specific shell you are using(so all built in commands like cd and alias have a help page and not a man page) while man is for getting documentation on programs which have each their own executable, meaning they also run exactly the same way in different shells.

In Linux the commands more and less are used to display man pages. So to learn how to navigate within a man page type: man less
If you type an incomplete command and then do tab twice it will display all commands that start with the letters you entered.

On Windows:
In Windows CMD the command help is used to access the manual page of a given command. Typing just help alone gives a list of all commands with a one line description.

Can I rant about PowerShell for a sec, who the hell came up with the idea of having commands be 2 words long?! Yes some of them have aliases but not enough of them. Also, the help files(accessed by typing help or get-help) give no good information, lots of pseudo examples but no word on what the command exactly does or what each option does. There is some documentation online(https://learn.microsoft.com/en-us/powershell/module/microsoft.powershell.utility/) but I prefer having it locally in the shell.

alias

Alias is a command that allows the user to give a command a custom name. This is very practical if commands are long.
Also fun fact some common commands are aliased by default. Linux ex: ls is aliased to alias ls='ls --color=auto' because ls by default does not color code its output.
Aliases are only for the current session, if want them to be permanent need to add them to an alias file. Here is how to make temporary ones:

In Linux to create an alias do: alias aliasname='command' or alias aliasname="command"
to simply print exisiting aliases write alias without any arguments

In windows CMD:doskey aliasname=command
In windows PowerShell:set-alias -name aliasname -value command
to simply list currently existing aliases:
CMD: doskey /macros
CMD by default does not have any aliases.
PowerShell: type alias or get-alias

Command Options

To run a slightly different version of a command, command options are used.

WARNING: command options are not standardized, so they can vary from command to command. You can look up what the options for a specific command are in the manual (for Linux, in shell write:man commandname. for Windows, in shell write:help commandname).

• In Linux options have the format: -singleletter (unix style) or --fullword (GNU style).
>In unix style the letter options do not need to be separated by spaces(ex: ls -la is the same as ls -l -a (but you can not do ls -l-a)). Generally speaking, Order does not matter if the option does not require input.
>There is also BSD style which does not have any / or - just letters and like unix style the letters can be grouped and generally speaking order does not matter.


• In Windows CMD they start with / and are either a word, letter, or several letters together
• In PowerShell they are -fullword or -lettercombination(even though in some cases it can be shortened, but I cant find good generalized documentation on this)

Commonly Used Symbols and Operations Performed on Commands

&
Used in Linux to run process or command or application you are about to launch in the background. This allows you to keep using the command line for other things while the process or command or application is still running.
ex: nedit &
Note: when launching a browser from the command line it will look like the & did not work because lots of stuff is written on the screen and you dont get the prompt line back. However the & did work, you are able to type the next command you want or simply hit enter to get your usual prompt line back. The reason this happened is because the browser application still prints its standard output (stdout) and standard error (stderr) streams to the terminal even when backgrounded.

In Windows it is not necessary to specify that a process should run in the background because windows automatically reparents processes to make them independent.
However the & is used in CMD instead of a semicolon. And in PowerShell it is the call operator, used to execute a command that is stored in a string or a variable, or to execute a script or command with a path that contains spaces or other special characters.

Wild Cards

These characters can have different meaning depending on context(ex: * is used for both multiplication and as a wild card). For *, . and ? we will only cover what they mean in the context of specifying a search pattern.

*
It is used for filename expansion and means include all
It is the same for both Linux and Windows. ex: ls XXX*
here ls lists all files and directories inside current directory that start with XXX. If the star(aka asterisk) was not included(ls XXX) then it would look for exactly XXX and would not find XXXYZ.
ex2: ls XXX*.txt
this looks for files that start in XXX and end in .txt


. or ?
The particular character can be any character(like * but for a single character).
On Linux:
However, ? is used for filename extension since filenames often have .txt or .zip for example and we would not want to confuse that dot with a wild card. While . is used for grep and other commands where the search pattern is not necessarily a file(I have tried both . and ? for grep and so far they seem to both work).
ex:grep X.X filename
ex2: ls X?X.txt
On Windows:
For commands like dir only the ? works not the . on both PowerShell and CMD
for CMD findstr only the . seems to work
and for PowerShell where-object only ? seems to work

Redirection( > and >> )

In Linux bash and windows CMD:

commands have 3 output streams:
• 0 is stdin, the standard input stream
• 1 is stdout, the standard output stream
• 2 is stderr, the standard error stream

ex: ls > myfile.txt
this will put the output of ls into the file called myfile.txt. If this file already exists it will overwrite.
ex2: ls >> myfile.txt
this appends to the file instead of overwriting.
Here we printed to the stdin stream of the command echo to a file. Putting 1 or 2 infront of > or >> will print the stdout or stderr respectively of the command to file.
For windows CMD its exactly the same.
For PowerShell the > and >> are the same aswell but instead of 3 output streams it has 6.

• 1 is the success stream (the primary output of the command)
• 2 is the error stream
• 3 is the warning stream
• 4 is the verbose stream
• 5 is the debug stream
• 6 is the information stream(user-friendly, general information)

Pipeline( | )

Pipelines are used to be able to use different commands together.
They work from left to right, left most command is executed and its output is feed to the next command as input and so on.

example:
 ls | grep -i xxx
  here -i is a command option of grep that means not case sensitive.
  The command lists all file and directory names in the current directory that contain xxx in the filename(NOT case sensitive due to -i)
  here ls lists all files and directories in the current directory and feeds it as input to grep who then filters out all names that do not contain xxx
In Windows CMD and PowerShell the above example will work the same way, just replace ls with dir and grep with findstr for CMD and where-object for PowerShell
Both Linux and Windows PowerShell are built such that pretty much all commands either support pipe or can be used with other comands (like for Linux grep, xargs, sed, awk, regex etc... and for PowerShell where-object, select-object, select-string, regex, etc...) to support pipe. These commands convert the formating of other commands to allow pipelining but they can also be used as stand alone. The only difference in pipeing in Linux vs PowerShell is that Linux pipes pass text(through input output streams) while PowerShell passes object.
This is very different from CMD where only commands that support pipe can be used in pipelining(there are no conversion commands)

Scripts:Programming in the shell

We will only scratch the surface here because there is lots of amazing stuff to be learned here but this article would be to long.

Variables

Variables are denoted as: $myvariable, but only after declaration.
ex: i=5; echo $i
this will print: 5

• Make sure there is no space between the variable name, the =, and the value


• Like in most programming languages variables can not start with a number but they can contain them. Special characters are also not allowed except for _


• Predefined variables like $SHELL or $PATH are all caps by convention but there is no need to capitalize your variables.


• These variables only exist within the current shell. If you open a new shell or tab the variables do not exist.


• A variable does not have a strict type so you can assign it a string and then reassign it to an integer value later on.
  You can use the command declare(read its docs by typing help declare) to make a variable integer only like so:
  declare -i PRICE=5
  doing echo $PRICE will give 5
  now if you assign a string to $PRICE like so:PRICE=notavailable; echo $PRICE
  you get 0 not notavailable


• To assign a value from a math operation to the variable i: i=$((2+3)) or i=$(expr 2 + 3), where expr is the command to do math operations. The space between 2, + and 3 in expr 2 + 3 is super important because the command expr needs its input arguments to be separated by spaces.
  Note: bash does not natively support decimal/floating point arithmetics and neither does expr. So need to use other commands if want to do that.


• You can make a variable constant(aka read only) like so:NAME="penguin"; readonly NAME;
  if you now do NAME="bear" it will give you an error


• Reserved words and predefined variables that should not be overwritten: to check if a name is a predefined variable just use echo to print its value(ex: echo $ABC). If the value is blank then it's probably safe to use that name. Another way to print all(I think all) predefined variables type the command env.
  For reserved words like if for example you can overwrite them but it probably wont do to much harm since to get the value you gave to if you have to write $if not if, but it makes the code super confusing so this should be avoided.


• To append to a variable: ex: i=hi
  i="$i hello"
  the quotes are important because otherwise it will think hello is a command

If Statements

if [ condition ];
then
statement;
elif [ condition ];
then
statement;
else
statement;
fi

ex:i="4";j="4"; if [ "$i" == "$j" ]; then echo "bonjour"; else echo "hola"; fi
The space between if and [ and the spaces between [ and "$i" == "$j" and ] are important, if not included get syntax error. Also has to be [] not (). The quotes are not necessary here,
i=4;j=4; if [ $i == $j ]; then echo bonjour; else echo hola; fi
would also work

to have several conditions inside the same if do:
i=4;j=4;k=5; if [[ $i == $j && $j < $k ]]; then echo "bonjour"; else echo "hola"; fi

for numerical condition checks you should use two letter abbreviations instead of the traditional == or >=, etc...
ex: a=3; b=1; if [ $b -le $a ]; then echo yes; else echo no; fi
if you use == bash will let it slide but doing <= will give an error

If you really want to use <= etc... for numbers, you can either use (()) instead of [] or [[]]
ex:a=3; b=1; if (( $b <= $a )); then echo yes; else echo no; fi
Or you can use the command let
ex: a=5; b=10; let "c= $a <= $b"; echo $c
here c will be 1(aka true)

For string comparision, == and != are used. < and > converts the strings to ASCII and comparess letter by letter. and <= and >= do not exist.

Loops

For Loops:
ex1:for i in {1..5}; do echo "Iteration $i"; done
this counts up from 1 to 5
ex2:for i in {5..1}; do echo "Iteration $i"; done
this will count down.

While Loops:
ex:cnt=1; while [ $cnt -le 5 ]; do echo "count is $cnt"; ((cnt++)); done
VERY IMPORTANT: need to do ((cnt++)) NOT cnt++ because otherwise cnt will not be incremented resulting in an infinite loop. (stop the infinite loop using ctrl+c).

Until Loops:
Same but opposite of while. While runs as long as condition is true, until runs as long as condition is false.
ex:cnt=1; until [ $cnt -gt 5 ]; do echo "Count is $cnt"; ((cnt++)); done

Break and Continue:
Its just like most other programming languages
continue ex:for i in {1..10}; do if [ $i -eq 5 ]; then echo "half way"; continue; fi; echo "iteration $i";done
break ex:for i in {1..10}; do if [ $i -eq 5 ]; then echo "half way, lets take a break"; break; fi; echo "iteration $i";done

Comments and other useful things to know

If you start typing an if statement but dont type fi and hit enter, bash will show >
and let you continue typing until you end the if statement. Same goes for loops that dont have done at the end, or any other code that bash deams as unfinished.
This is useful because breaking your code into different lines can make your code more readable.

Comments are denoted as #
bash will ignore everything after that line even if you put ;

If you want to write your code/script in a separate file and then execute that file on the command line simply start your file with: #!/bin/bash
And once you are finished writing the script set the file to be executable using the chmod command.
If you want to use a different shell that is not bash then you would specify a different path, ex: #!/usr/bin/ctsh
To run the script simply write its name in the shell and hit enter.

Variables

CMD ex:
set myvar=hello world & echo %myvar%
here the & acts as a semicolon.

PowerShell ex:
$myvar="hello world"; echo $myvar
In CMD the quotes would be included inside the string but in PowerShell they are needed here to not get an error.

• for CMD: make sure there is no space between the variable name and the =, you can have a space between the = and the value if its a string that should start with a space as the first character.
  
  for PowerShell: powershell is not sensitive to spaces between the variable name and the = so $myvar="hello world" is the same as $myvar ="hello world" or $myvar = "hello world" or $myvar= "hello world"


• Your variables can start with a number.


• Variables are not case sensitive like most of Windows.


• CMD has no memory of previous sessions so it does not store the variables.
  In PowerShell the variables are only for the current session.


• for CMD: the content of a variable is treated like a string but if you do set /a then you can do arithmetics
  ex:set /a myr=4+5
  echo %result%
  ex2: set /a myr1=2 & set /a myr2=4 & set /a myr3=%myr1%+%myr2%
  echo %myr3%
  I write the echo after hitting enter because the /a prints the result everytime so if I did set /a myr1=2 & set /a myr2=4 & set /a myr3=%myr1%+%myr2% echo %myr3% I would get 2466 instead of just 6
  Like in bash assigning a string to a variable when using /a will result in the variable being equal to 0.
  Like Linux bash, CMD does not support decimal/floating point arithmetics.
  
  for PowerShell: variables are not type sensitive
  to do both integer or floating point arithmetics its really easy
  ex:$myvar=1+2.5; echo $myvar
  ex2:$a=$b=2; $c=$a+$b; echo $a; echo $b; echo $c
  If you want to restrict your variable to a specific type simply put the type in square brackets infront. ex: [int]$myint=8 if you now try to do $myint="hello" it will throw an error saying it cannot convert.
  However if you try to constrain a variable to be string only its difficult because powershell does arithmetic first before automatically type converting. ex: [string]$mystring=5+4 this gives 9 instead of the string "5+4"


• In CMD there is no direct way to make a variable constant.
  For PowerShell: ex: new-variable -name "myconst" -value 123.4 -option constant


• Reserved words and predefined variables that should not be overwritten: to check if a name is a predefined variable just use echo to print its value(ex: echo %ABC% for CMD and echo $ABC for PowerShell). If the value is blank then its probably safe to use that name.
  For reserved words like if for example you can overwrite them but it probably wont do to much harm since to get the value you gave to if you have to write %if% in CMD and $if in PowerShell not if, but it makes the code super confusing so this should be avoided


• to append to a variable:
  CMD ex: set i=hi
  set i=%i% hello
  PowerShell ex:$i="hi"
  $i+=" hello"

If Statements

For CMD:
if condition (statement) else if condition (statement) else (statement)
If you do not want to use else if and else but just if alone then the statement does not need to be surrounded by ()
ex:set a=5
set b=5
if %a% equ 5 (echo great) else if %a%==%b% (echo amazing) else (echo ok)

CMD does not support arithmetics within the condition of the if statement so need to put the arithmetic into a variable first and then compare.
To have several conditions inside the same if do: if condition1 (if condition2 (statement)) else (statement)
ex: set myvar=hi
set c=8
if %myvar% equ hi (if %c%==8 (echo great)) else (echo ok)

For PowerShell:
if(condition){statment}elseif(condition){statment}else{statment}
There can't be a space between else and if, it needs to be elseif
ex:$myvar=123; $a=5
if($myvar -eq 123){echo "yippee"}elseif($a -ne 6){echo "yea"}else{echo "nope"}
you can easily do arithmetics in the if condition:
$a=1; $b=5;
if($a+$b -eq 6){echo "amazing"}
Note: it is not necessary to put a space before or after the -eq, I just like to do it for readability.
The ==, !=, etc... can not be used, only -eq and -ne respectively.
To have several conditions inside the same if do:
if($a -eq 1 -and $b -eq 5){echo "cool"}

Loops

For CMD:
The command for is used to do all the available loops.
ex: for /l %i in (1,1,5) do echo %i
here it counts up from 1 to 5.
ex2: for /l %i in (10,-2,1) do echo %i
here i counts down from 10 to 1 in steps of 2.
the /l tells it that (from, incrementor, to)
without the /l in ex1 i would have gotten the values 1,1 and 5.
for also has other command options like for example /f for file reading and writing where each file to iterate through is specified in the ()
write help for to see all the possible command options
break and continue: CMD does not have these features however goto can be used instead but it only works in batch files and not on CMD directly, see below.

For PowerShell:

• for: its just like any normal for loop in other programming languages
  ex:for($i=1;$i -lt 5; $i++){echo $i}


• while: ex: $i=1; while($i -lt 5){echo $i; $i++}


• do: there are 2 types, do while and do until. Until is the opposite logic of while. While runs as long as condition is true, until runs as long as condition is false.
  ex:$i=1; do{echo $i; i++}while($i -lt 5)
  ex:$i=1; do{echo $i; i++}until($i -gt 5)


• foreach:ex:$things="i1","i2","i3"; foreach($thing in $things){echo $thing}
  There is no need to have singular and plural. Its just for readability
  ex:$cat="i1","i2","i3"; foreach($car in $cat){echo $car}


• foreach-object:Like for each but used in pipelining
  ex:$cnt=0
  hit enter
  Dir d* | foreach-object{$cnt++;echo $cnt}


• break and continue:
  continue ex:for ($i=1; $i -lt 6; $i++){
  if($i -eq 3){echo "half way"; continue}
  echo $i
  }
  
  break ex:for ($i=1; $i -lt 6; $i++){
  if($i -eq 3){echo "half way lets take a break"; break}
  echo $i
  }

Comments and other useful things to know

If you want to write your code/script in a separate file and then execute that file on the command line, write your code in a text editor like notepad. When saving the file call it mybatchfile.bat and select all files.
To run it simply cd to the directory where it is saved and write mybatchfile.bat
Important: since batch files can be run from both CMD and PowerShell, CMD syntax needs to be used.
ex using labels and an if statement:
@echo off
set i=1
:mylabel
echo hi from batch
echo wow
if %i%==1 (
set /a i+1
goto mylabel
)

The @echo off is so that it does not print every command as it executes it.

If you want to run commands that only exist in PowerShell inside a batch file need to add powershell.exe infront

comments:
CMD comments are rem or ::
ex:Echo hi &rem mycomment
Ex2: echo hi &:: mycomment
PowerShell comments are #
And multiline comments are<#mycomment#>

Other Useful Computer Things to Know

Commands Used When Several Devices Are Involved

Inorder for these commands to work the two devices need to be on the same network.

• Login remotely:
  ssh user@deviceIPaddress
  to log out again simply write exit
  If you do not specify the user name it will assume you want to login as the same user as the one you are currently on your current computer even if that user does not exist on the the computer you want to connect to.
  Note: you can only ssh into other Linux devices. If you want to ssh into a Windows device you first have to set up an ssh server.


• copy files from one device to another:
  scp username@device1IPaddress:/directory-to-copy-from/file-to-copy username@device2IPaddress:/directory-to-copy-to/file-name-to-copy-into
  Its like the normal cp command to copy just that its between different devices, scp=ssh copy.
  So, you only have to specify the file-name-to-copy-into if you want to rename the copied file. So if you want the copy to have the same name as the original, simply do not specify the file-name-to-copy-into
  And if at the destination the file name already exits it will overwrite without asking.
  
  To copy everything inside the current directory to another device:
  scp * device2address:/directory-to-copy-to/
  This does not copy the directory, only the files inside of it. Also if the directory contains another directory this directory will not be copied. scp only copies files.

On Windows:
In the past CMD was not able to do ssh so PuTTY, a graphical ssh application had to be used.
When powershell came along Windows included ssh so nowadays both PowerShell and CMD can do it.
ssh and scp commands have the same syntax as in Linux. Just be careful: in Windows directory slashes are the other way around!!!

How to Use a USB in the Shell

On Linux:
Unless you have previously told your file manager not to, the file manager will do everything for you. But if you want to do it on the shell you can:
1)plug in USB
2)in shell write: dmesg
this will list the message buffer of the kernel. The last entry should be the device you just pluged in.
3)sudo mount device-name where-to-mount-it
ex:sudo mount /dev/sdb1 /mnt/usbka
The directory where to mount it should be an empty directory because if its not then you can not access the directories content until you unmount.
Sudo is used because you need to run the mount command as root.
dev is the directory where device files are stored. So when you plug the USB in, a file(called sdb1) is created for it in /dev
4)cd where-its-mounted
5)use the USB
6)cd out of the directory where the USB is mounted(otherwsie it will say USB is buissy so cant unmount)
7)sudo umount device-name
you can also do: sudo umount where-to-mount-it or sudo umount device-name where-to-mount-it
VERY IMPORTANT: always unmount USBs before unplugging because your computer does write caching to minimize delay. Meaning the computer does not necessarily fully write everything to the USB right away but only once it is not buissy doing other commands or if you unmount. It only guarantees that apon unmounting it will have writen everything.

Fun fact, you can mount a USB device in different directories at the same time. If you then do sudo umount device-name it will only unmount in one of the 2 directories so you have to do the command again to unmount in the other directory.
To see if device is still mounted do: mount | grep sdb

If you do not want to mount devices as root you can configure it such that you just write mount /mnt/usbka and it will know that you want to mount device /dev/sdb1. This configuration is done in a file called /etc/fstab

On Windows:
In windows there is no need to mount anything on the command line or otherwise because the OS should do it for you automatically.

How to compress(zip) files

Archiving is when you combine several files into one file(this is what the command tar does without command options).
Usually you also want to compress this archived file so it takes less space when you send it to someone(in Linux this is done with gzip(GNU zip, often abbreviated gz). The Linux tar command can use gzip to compress as part of its command options but tar and gzip are 2 different commands. While in Windows PowerShell the command compress-archive does both archiving and compression.
What happens when a file is compressed? Compression seems counterintuitive because you add more info(to decompress) to save space, like how should that work?. If you have a file where every single piece of both its data and metadata is totally different then compression wont work cus there is nothing that can be removed without losing info. But chances are there are patterns and repetition. ex: hello hello hello can be rewritten as XXX; X="hello"

Keyboard Shortcuts

On Linux:
To see a list of all bash keyboard shortcuts: stty -a
↪ stty is a command that handles changing and printing terminal line settings
↪ To understand what the abbreviated meanings of the shortcuts mean in the stty -a printout, look in man stty(Note: interrupt means terminate process, and stop means suspend aka its still there but in the background)

There are also other shortcuts not on the stty -a list because they are dependent on the line editing style used. The default is emacs style but vim style also exists. The below table does not include these shortcuts.

On Windows:
I was not able to find a good way to view CMD keyboard short cuts. PowerShell on the other hand for once has good documentation.
To see a list of all PowerShell keyboard shortcuts with a small description: Get-PSReadLineKeyHandler

References

• http://www.catb.org/~esr/writings/taoup/html/ch10s05.html
  about command options


• History
  https://en.wikipedia.org/wiki/History_of_Linux
  https://en.wikipedia.org/wiki/Microsoft_Windows
  https://en.wikipedia.org/wiki/Android_(operating_system)#
  https://en.wikipedia.org/wiki/MacOS


• https://ss64.com/bash/
  List of Linux, CMD and PowerShell commands


• https://learn.microsoft.com/en-us/powershell/module/microsoft.powershell.utility/
  online PowerShell documentation


• https://www.redhat.com/en/blog/top-10-shortcuts
  terminal keyboard shortcuts


• https://www.edureka.co/blog/linux-vs-windows/
  general Linux vs Windows architecture, advantages and disadvantages

Copyright © 2024-2025 Jessica Socher ()