1.How do you write the contents of 3 files into a single file?

$ cat file1 file2 file3 > file4


In this way we can use the cat command to concatenate the files and write them into another file.

2.How can I reverse a word like ‘Unix’ to ‘xinU’ ?

We can use rev command to reverse the word.

$ echo “Unix” | rev


Read also: Unix interview questions and answers part – 2

3.How to display the fields in a text file in reverse order?

$ awk 'BEGIN {ORS=""} { for(i=NF;i > 0;i--) print $i," "; print "\n"}' filename


If field delimiter is other than space, you need to define it in BEGIN block using FS variable.

4.Write a command to find the sum of bytes (size of file) of all files in a directory.

$ ls -plL | grep -v / | awk 'BEGIN {sum=0} {sum = sum + $5} END {print sum}'


ls -plL This command lists everything that isn’t a directory and link (files,device files, etc.)

5.Write a command to print the lines which end with the word “unix”?

$ grep 'unix$' filename

The ‘$’ symbol specifies the grep command to search for the pattern at the end of the line.


Read also: Unix interview questions and answers part – 3

6.Write a command to select only those lines containing “unix” as a whole word?

$ grep -w ‘unix’ filename


The ‘-w’ option makes the grep command to search for exact whole words. If the specified pattern is found in a string, then it is not considered as a whole word.

For example: In the string “unix_os”, the pattern “unix” is found. However “unix” is not a whole word in that string.

7.How to remove the first 2 lines from a file?

$ sed -i '1,2 d' filename

If you want to really delete the first 2 lines from the original file then you can make sed command interactive with the -i option.

This doesn’t echo anything, it will just modify the file in-place. If you don’t need to pipe the result to another command, this is easier.


$ tail -n +3 input_file > output_file

Note that it’s +3 to eliminate first 2 lines, because +3 means “start from line 3” and tail numbers lines from 1.


$ awk 'NR &gt; 2 { print }' < file.txt


8.Write a command to duplicate each line in a file?

$ sed 'p' filename


9.What happens if use touch command on existing file?

Nothing will happen to the file and that will update the last modified time of the file.


10.How to extract the username from ‘who am i’ command?

$ who am i | cut -f1 -d' '



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