Linux Commands

To execute the following Linux commands, open your terminal/shell.

You may use a one of the following to get a Linux shell on your computer:

Linux distribution(distro) like ubuntu
Virtual machine
Windows subsystem for Linux(WSL) to get a Linux shell on Windows 10.

If you don't have any, the easiest to start with will be WSL in my opinion as you can easily download it from Microsoft Store on Windows 10.

Commands

date : prints the system date and time
pwd : Short for the present working directory, it outputs the current working directory.
touch : Creates a new file
```
touch filename
```
Now you have a file named filename in your current folder(directory)
- To create multiple files
```
touch file1 file2
```
ls : displays all the files and folders in our current directory.
- If you followed previous touch command it will show filename, file1, file2.
- Options for ls command:
  - ls -u : show newest files first
  - ls -r : show files in reverse order, oldest first.
  - ls -l : show files and folder in long listing format
    
    Example output:
```
total 2
-rwxrwxrwx 1 root root 11 Aug 16 15:47 file1*
-rwxrwxrwx 1 root root  9 Aug 16 15:47 file2*
-rwxrwxrwx 1 root root 27 Aug 16 15:47 filename*
```
    Explanation of output:
    
    Column 1:
    
    Column 1 shows permission for files and folders for three different users - owner | group | others
    - r = read, w = write, x = execute
    Column 2:
    
    Column 2 shows the number of hard links to that file or folder which is 1 by default.
    
    A link in UNIX is a pointer to a file.
    
    Column 3: Shows file owner
    
    Column 4: Shows file-group
    
    Column 5: Shows file size in bytes
    
    Column 6: Shows file date and time of the creation
    
    Column 7: Shows the filename
- You may use different options in combination like: ls -lr
Terminal Based Editors
- Various different editors exist for editing files in a terminal like vi, vim, nano.
- vi : Used to create a new file or edit previously created once.
```
vi newFileName
```
  this will create a new file named newFileName in your current directory and prompt an editor in the terminal.
  - To start editing the file, press i. Now you are in insert mode. Type whatever you want.
  - To exit the insert mode press esc.
  - To save the file: press : and then type w.
  - To save and exit the vi, press : and then type wq.
  - To quit the vi editor without saving, press : and type !q.
- vim : vim is similar editor to vi but with more features. You can perform the similar commands in vim like vi.
- nano : nano is the easiest between all three. Type nano filename and you will understand the rest yourself.
cat : displays file content on the standard output(console).
```
cat filename
```
wc : displays word count of a file in the following format
- Options
  - -w : prints only the number of words.
  - -l : prints only a number of lines.
  - -c : prints only the number of characters.
```
wc filename
```
Output:

115 538 3320 filename

number-of-lines number-of-words - byte-count filename
clear : clear the terminal screen.

alternatively use control + l.
rm : removes a file
```
rm filename
```
the file with the name 'filename' will be deleted.

To delete a directory(folder) use the option -r which means recursively delete all the files in the folder
```
rm -r foldername
```
mkdir : creates a new directory(folder).
```
mkdir folderName
```
Now if you execute ls it will show folderName.
cd : change the current working directory to the given directory.
```
cd folderName
```
To Move a folder back from current folder type cd ..

.. = previous folder.

. = current folder
cp : copy a file/files to another folder.

Suppose you have two files named file1, file2 and you want to copy them in a folder named new in the current directory.
```
cp file1 file2 new
```
mv : Similar to cp but moves the file instead of copying.
alias: creates a different name for a command.
```
alias create=touch
create file1
```
Now create works the same as touch.

Note : The alias will exist for the current session only and will be forgotten once you restart the terminal.

To make it persistent, add the alias line in ./bashrc as per the following:
```
vi ~/.bashrc
```
Then add a new line in bashrc as you edit in vi

alias create="touch"

save the file.

Now you can use create instead of touch anytime.

ed: ed is a line-oriented text editor, you can use it to edit/create a file like you did with vi.

ed
a    # type a to append content to file
This is a line    # Start appending content to your file
This is another line my file
.    # This marks the end of file
w filename # appends lines to file or create a new file

who: shows who is logged in

who
yuvrajsj18 :0           2020-08-19 23:13 (:0)    # sample output

who am i: show your own system details if you are in a multi-user system.
pr: shows the content of files in print format.
- Total lines per page are 66, 10 for headers, and 56 for content.
- The content is paginated so we can know how much content will be shown per page.
- You can columnate the content using -n option.

  pr filename

  2020-08-19 11:41              file1                       Page 1


  This is a text
  this is more text
  This is more more text

cmp: Compare two files byte by byte. It will print the first-byte number on the first line number where the content of two files differs.
```
cmp file1 file2
file1 file2 differ: byte 1, line 1    # Sample output
```

diff: compare files line by line. It also shows what needs to be changed to make file to make file1 same as file2.

diff file1 file2
1,3c1,4                # Sample output
< This is a text
< this is more text
< This is more more text
---
> ABC
> XYZ
> MNNP
> ABCC

a means append.

3a4
> This is some text
# This means after line 3 you need to append given text to match line 4 of the second file

c means change.

2,3c3
< Uttar Pradesh
< Kolkata
---
> Andhra Pradesh
# This means change line 2 to 3 of file 1 to line 3 of file 2

d means delete.

3d2
< Telangana
# Means delete line 3 of file1 to match line 2 of file2

sort : sort lines of text files.

-n means sort by numeric values.

sort filename    # to sort by ASCII values
sort -n filename # to sort by numeric values

+c means to sort according to a particular column in the file. c starts with 0.

cat filename    # we have a file with following content
word1    word4    word1
word2    word3    word2
word3    word2    word3
word4    word5    word4
word5    word6    word5
word6    word1    word6
word7    word8    word7
word8    word7    word8
word9    word9    word9

sort +1 filename    # sort file according to second column
word6   word1   word6
word3   word2   word3
word2   word3   word2
word1   word4   word1
word4   word5   word4
word5   word6   word5
word8   word7   word8
word7   word8   word7
word9   word9   word9
# Look lines are sorted according to second column

; : semicolon is used as a command separator to execute multiple commands one after another.

date; ls; who
Wednesday 19 August 2020 11:43:37 PM IST        # Sample output
file1*  file3*  file-ed*    mvfile1*  sort*
file2*  file4*  file-nums*  newDir/   yuvraj/
yuvrajsj18 :0           2020-08-19 23:13 (:0)

tail: Output the last part of a file, by default it will output the last 10 lines.

tail filename     # Output last 10 lines
tail -n filename     # Output last n lines
tail +n filename     # Output lines from nth line to end

Redirection Symbols: There are three redirection symbols as follows:
- > : It prints the output of a command to file.
```
ls > filename        # this will add all files and folder to filename
date > filename     # this will change the content of the file to the date
```
- >> : It will append the output of a command to a file.
```
ls >> filename        # This will append the output of ls to file
```
- | : This is used to send the output of one command to another command.
  - ls | sort means first ls is executed and its output is send to sort then it will execute.
```
ls | sort     # this will print the files in sorted order
```
- Exercise
  - How to print the sorted list of users?
    
    who | sort
  - Count the number of files and directories in the current directory:
    
    ls | wc -l
  - Count the number of files and directories in directory user1/networks:
    
    ls user1/networks | wc -l
grep : print lines that match a given pattern.
- Format: grep -options regex filename
- Regex Basic Options:
  - ? : The question mark indicates zero or one occurrences of the preceding element. For example, colou?r matches both "color" and "colour".
  - * : The asterisk indicates zero or more occurrences of the preceding element. For example, ab*c matches "ac", "abc", "abbc", "abbbc", and so on.
  - + : The plus sign indicates one or more occurrences of the preceding element. For example, ab+c matches "abc", "abbc", "abbbc", and so on, but not "ca".
  - {n} : The preceding item is matched exactly n times.
  - {min,} : The preceding item is matched min or more times.
  - {min,max} : The preceding item is matched at least min times, but not more than max times.
  - . : matches any single character.
  - ^ : Specify beginning.
  - $ : Specify end.
  - Read more about regex
  - Build Regex expression easily and visually with this tool - Regex101
- Options for grep
  - -v : print all lines that exludes the given regex.
  - -n : print line numbers.
- Exercise
  - Search for 'is' in a file
```
grep 'is' filename
```
  - Search for lines that don't include 'is'
```
grep -v 'is' filename
```
  - Check if user1 is logged in or not
```
who | grep 'user1'
```
  - Find how many times user1 has logged in
```
who | grep 'user1' | wc -l
```
  - Search all lines that end with 's'.
```
grep 's$' filename
```
  - See all filenames that begin with 'a'
```
# Solution 1 with grep
ls | grep '^a'

# Solution 2 with ls
ls a*
```
  - See all empty lines in a file
```
grep '^$' filename -n
```
  - Find all email address in a file
```
grep '.\+@.\+\..\+' email_files.txt
```
    - Explanation
      
      .\+ means any character 1 or more times
      
      @ means match @ character literally
      
      .\+ means any character 1 or more times
      
      \. means match the . character literally
      
      .\+ means any character 1 or more times
Shell Variables : Shell variables are special variables used by the shell.
- $HOME : Contains path of home directory
- $PATH : Contains executable's directories separated by a colon :
- To Define your own variables:
```
variable_name="value"
```

Thanks For Reading 😊

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