Cat Command

The cat command is a Unix and Linux utility that stands for "concatenate." It is used to display the contents of one or more files to the terminal, and it can also be used to combine files or create new files.

Display the contents of a single file:

  $ cat file1.txt
  This is the contents of file1.txt.

Display the contents of multiple files:

  $ cat file1.txt file2.txt file3.txt
  This is the contents of file1.txt.
  This is the contents of file2.txt.
  This is the contents of file3.txt.

Combine two or more files and display the output:

  $ cat file1.txt file2.txt > combined.txt
  $ cat combined.txt
  This is the contents of file1.txt.
  This is the contents of file2.txt.

Add the contents of one file to the end of another file:

  $ cat file1.txt >> file2.txt
  $ cat file2.txt
  This is the contents of file1.txt.
  This is the contents of file1.txt.

Display the contents of a file with line numbers:

  $ cat -n file1.txt
  1  This is the contents of file1.txt.

Display the contents of a file in reverse order:

  $ tac file1.txt
  .txt1 elif fo stnetnoc eht si sihT

chmod Command

The chmod command is a command-line utility used in Unix-based operating systems to change the access permissions of files and directories. It stands for "change mode" and is used to modify the read, write, and execute permissions for a file or directory.

chmod 777 foldername

History Command

The history command is used to display a list of previously executed commands from the user's command history.

history: Display the list of previously executed commands along with their line numbers

  $ history
     1  ls
     2  cd ..
     3  mkdir test
     4  cd test
     5  touch file.txt
     6  ls
     7  rm file.txt
     8  ls
     9  history

history n: Display the last n commands from the command history.

  $ history 3
     7  rm file.txt
     8  ls
     9  history 3

!n: Execute the command with line number n.
```
  $ !5
  touch file.txt
```
!!: Execute the last command in the command history.

  $ ls -l
  total 0
  -rw-r--r-- 1 user user 0 Feb 24 10:42 file.txt

  $ !!
  ls -l
  total 0
  -rw-r--r-- 1 user user 0 Feb 24 10:42 file.txt

!string: Execute the most recent command that starts with string.

  $ ls -l
  total 0
  -rw-r--r-- 1 user user 0 Feb 24 10:42 file.txt

  $ !ls
  ls -l
  total 0
  -rw-r--r-- 1 user user 0 Feb 24 10:42 file.txt

Remove Command

rm file.txt: Remove a single file named file.txt.

  $ ls
  file.txt

  $ rm file.txt

  $ ls
  (no output)

rm -r directory: Remove a directory and its contents recursively.

  $ ls -R test/
  test/:
  subdir  file.txt

  test/subdir:
  file.txt

  $ rm -r test/

  $ ls -R test/
  ls: cannot access 'test/': No such file or directory

rm -i file.txt: Remove a file and prompt for confirmation before removing it.

  $ ls
  file.txt

  $ rm -i file.txt
  rm: remove regular file 'file.txt'? y

  $ ls
  (no output)

rm -f file.txt: Remove a file without prompting for confirmation.

  $ ls
  file.txt

  $ rm -f file.txt

  $ ls
  (no output)

Touch Command

touch file.txt: Create a new file named file.txt.

  $ ls
  (no output)

  $ touch file.txt

  $ ls
  file.txt

touch *.txt: Update the modification timestamp of all files in the current directory that end with the .txt extension.

  $ ls -l
  -rw-r--r-- 1 user user 0 Feb 24 11:00 file1.txt
  -rw-r--r-- 1 user user 0 Feb 24 11:00 file2.txt
  -rw-r--r-- 1 user user 0 Feb 24 10:58 file3.txt

  $ touch *.txt

  $ ls -l
  -rw-r--r-- 1 user user 0 Feb 24 11:01 file1.txt
  -rw-r--r-- 1 user user 0 Feb 24 11:01 file2.txt
  -rw-r--r-- 1 user user 0 Feb 24 11:01 file3.txt

Head Command

head file.txt: Display the first 10 lines of the file file.txt.

head -n 5 file.txt: Display the first 5 lines of the file file.txt.

  $ cat file.txt
  line 1
  line 2
  line 3
  line 4
  line 5
  line 6
  line 7
  line 8
  line 9
  line 10
  line 11
  line 12

  $ head -n 5 file.txt
  line 1
  line 2
  line 3
  line 4
  line 5

Tail Command

tail file.txt: Display the last 10 lines of the file file.txt.
tail -n 5 file.txt: Display the last 5 lines of the file file.txt.
tail -f file.txt: Display the last few lines of the file file.txt and then monitor the file for changes, displaying any new lines as they are added.The -f option is particularly useful for monitoring log files that are constantly being updated.

  $ tail -f file.txt
  line 11
  line 12
  line 13
  line 14
  line 15

  $ echo "line 16" >> file.txt  # Add a new line to the file.

  $ tail -f file.txt
  line 12
  line 13
  line 14
  line 15
  line 16

vi Command

The vi command is a Unix-based text editor that is commonly used on the command line.

vi file.txt: Open the file file.txt in the vi editor.
```
  $ vi Colors.txt
```
Once the file is open in the vi editor, you can enter editing mode by pressing the i key. In editing mode, you can make changes to the file.
To save your changes and exit the editor, first press the Esc key to exit editing mode. Then, type :wq and press Enter. This will write your changes to the file and quit the editor.

diff Command

diff file1.txt file2.txt: Compare the files file1.txt and file2.txt and display the differences between them.

  $ cat file1.txt
  hello world
  this is a line
  this is another line
  this is a fourth line

  $ cat file2.txt
  hello world
  this is a line
  this is a new line
  this is another line
  this is a fourth line

  $ diff file1.txt file2.txt
  3a4
  > this is a new line

Thank you for reading! Hope you find this article helpful.

~Kunal

Day 3 Task: Basic Linux Commands