Question

Write a linux command that will add a "#" character to the start of every line that does not already start with a "#" character.

Write a linux command that will append all lines of the /etc/passwd file that contain the word "STUDENT" to the end of the file.

Write a linux command that will move the last word of each line to the start of the line.

Write a linux command that will add the line number to the beginning of each line. Note that vim will display line numbers - what you must do is insert the line number at the start of each line.

Answer 1

1. Write a linux command that will add '#' charracter to the start of every line that does not already start with '.#' charracter

Ans: You can do this with sed command shown below
      
     e.g sed 's/^./#/' inputfile.txt

     Explaination: sed   s/find string /replace with string/   

     s -- to search and replace
     ^ -- search at begining
     . -- It will avoid adding '#' to those line already starting with '#'
     # -- It is the charracter to be added. You can change it to any other

2.Write a linux command that will append all lines of the /etc/passwd file that contain the word "STUDENT" to the end of the file.

Sol: cat /etc/passwd | grep -i "STUDENT" >> /etc/passwd

    Command break and explanation:

    cat /etc/passwd :

    It will display the content of the file to screen.

    grep -i "STUDENT" :

    It will find the lines containing "STUDENT". -i is to ignore case.
    You should use it if lowercase "student" and uppercase "STUDENT" are same for u. Other wise simply use
    grep "STUDENT".

    >> /etc/passwd :
  
    >> filename . It is to append the output to a file at the end.
  
      

3.

Write a linux command that will move the last word of each line to the start of the line.

Ans: cat input.txt | awk '{lastword=$NF;NF=NF-1;print lastword,$0}'

     Logic and explanation:
  
     find the last word : In awk $NF stores the last word automatically.
     Store the last word in a variable : lastword = $NF
     Reassign the last word to the second last word. : $NF = NF -1
     print the lastword and remaining : print lastword, $0).

     e.g
  
input.txt:
---------
Hi I m neeraj
My last name is mishra
I m born in year 1992

Output:

/home/neeraj> cat input.txt | awk '{lastword=$NF;NF=NF-1;print lastword,$0}'
neeraj Hi I m
mishra My last name is
1992 I m born in year

Sorry for not answering the last one. I tried my best to answer. I hope you understood.