Unix Interview Questions and Answers V1.1

15 Frequently Asked Unix Interview Questions and Answers

You will get Answers for below mentioned questions, in this post:

#How to print/display the first line of a file?
#How to print/display the last line of a file?
#How to display n-th line of a file?
#How to remove the first line / header from a file?
#How to remove the last line/ trailer from a file in Unix script?
#How to remove certain lines from a file in Unix?
#How to remove the last n-th line from a file?
#How to check the length of any line in a file?
#How to get the nth word of a line in Unix?
#How to reverse a string in unix?
#How to get the last word from a line in Unix file?
#How to get the n-th field from a Unix command output?
#How to replace the n-th line in a file with a new line in Unix?
#How to show the non-printable characters in a file?

#How to print/display the first line of a file?

There are many ways to do this. However the easiest way to display the first line of a file is using the [head] command.

$> head -1 file.txt

If you specify [head -2] then it would print first 2 records of the file.

Another way can be by using [sed] command. [Sed] is a very powerful text editor which can be used for various text manipulation purposes like this.

$> sed '2,$ d' file.txt

#How to print/display the last line of a file?

The easiest way is to use the [tail] command.

$> tail -1 file.txt

If you want to do it using [sed] command, here is what you should write:

$> sed -n '$ p' test

From our previous answer, we already know that '$' stands for the last line of the file. So '$ p' basically prints (p for print) the last line in standard output screen. '-n' switch takes [sed] to silent mode so that [sed] does not print anything else in the output.

#How to display n-th line of a file?

The easiest way to do it will be by using [sed] I guess. Based on what we already know about [sed] from our previous examples, we can quickly deduce this command:

$> sed –n ' p' file.txt

You need to replace with the actual line number. So if you want to print the 4th line, the command will be

$> sed –n '4 p' test

Of course you can do it by using [head] and [tail] command as well like below:

$> head - file.txt | tail -1

You need to replace with the actual line number. So if you want to print the 4th line, the command will be

$> head -4 file.txt | tail -1

#How to remove the first line / header from a file?

We already know how [sed] can be used to delete a certain line from the output – by using the'd' switch. So if we want to delete the first line the command should be:

$> sed '1 d' file.txt

But the issue with the above command is, it just prints out all the lines except the first line of the file on the standard output. It does not really change the file in-place. So if you want to delete the first line from the file itself, you have two options.

Either you can redirect the output of the file to some other file and then rename it back to original file like below:

$> sed '1 d' file.txt > new_file.txt

$> mv new_file.txt file.txt

Or, you can use an inbuilt [sed] switch '–i' which changes the file in-place. See below:

$> sed –i '1 d' file.txt

#How to remove the last line/ trailer from a file in Unix script?

Always remember that [sed] switch '$' refers to the last line. So using this knowledge we can deduce the below command:

$> sed –i '$ d' file.txt

#How to remove certain lines from a file in Unix?

If you want to remove line to line from a given file, you can accomplish the task in the similar method shown above. Here is an example:

$> sed –i '5,7 d' file.txt

The above command will delete line 5 to line 7 from the file file.txt

#How to remove the last n-th line from a file?

This is bit tricky. Suppose your file contains 100 lines and you want to remove the last 5 lines. Now if you know how many lines are there in the file, then you can simply use the above shown method and can remove all the lines from 96 to 100 like below:

$> sed –i '96,100 d' file.txt   # alternative to command [head -95 file.txt]

But not always you will know the number of lines present in the file (the file may be generated dynamically, etc.) In that case there are many different ways to solve the problem. There are some ways which are quite complex and fancy. But let's first do it in a way that we can understand easily and remember easily. Here is how it goes:

$> tt=`wc -l file.txt | cut -f1 -d' '`;sed –i "`expr $tt - 4`,$tt d" test

As you can see there are two commands. The first one (before the semi-colon) calculates the total number of lines present in the file and stores it in a variable called “tt”. The second command (after the semi-colon), uses the variable and works in the exact way as shows in the previous example.

#How to check the length of any line in a file?

We already know how to print one line from a file which is this:

$> sed –n ' p' file.txt

Where is to be replaced by the actual line number that you want to print. Now once you know it, it is easy to print out the length of this line by using [wc] command with '-c' switch.

$> sed –n '35 p' file.txt | wc –c

The above command will print the length of 35th line in the file.txt.

#How to get the nth word of a line in Unix?

Assuming the words in the line are separated by space, we can use the [cut] command. [cut] is a very powerful and useful command and it's real easy. All you have to do to get the n-th word from the line is issue the following command:

cut –f -d' '

'-d' switch tells [cut] about what is the delimiter (or separator) in the file, which is space ' ' in this case. If the separator was comma, we could have written -d',' then. So, suppose I want find the 4th word from the below string: “A quick brown fox jumped over the lazy cat”, we will do something like this:

$> echo “A quick brown fox jumped over the lazy cat” | cut –f4 –d' '

And it will print “fox”

#How to reverse a string in unix?

Pretty easy. Use the [rev] command.

$> echo "unix" | rev

xinu

#How to get the last word from a line in Unix file?

We will make use of two commands that we learnt above to solve this. The commands are [rev] and [cut]. Here we go.

Let's imagine the line is: “C for Cat”. We need “Cat”. First we reverse the line. We get “taC rof C”. Then we cut the first word, we get 'taC'. And then we reverse it again.

$>echo "C for Cat" | rev | cut -f1 -d' ' | rev

Cat

#How to get the n-th field from a Unix command output?

We know we can do it by [cut]. Like below command extracts the first field from the output of [wc –c] command

$>wc -c file.txt | cut -d' ' -f1

109

But I want to introduce one more command to do this here. That is by using [awk] command. [awk] is a very powerful command for text pattern scanning and processing. Here we will see how may we use of [awk] to extract the first field (or first column) from the output of another command. Like above suppose I want to print the first column of the [wc –c] output. Here is how it goes like this:

$>wc -c file.txt | awk ' ''{print $1}'

109

The basic syntax of [awk] is like this:

awk 'pattern space''{action space}'

The pattern space can be left blank or omitted, like below:

$>wc -c file.txt | awk '{print $1}'

109

In the action space, we have asked [awk] to take the action of printing the first column ($1). More on [awk] later.

 

#How to replace the n-th line in a file with a new line in Unix?

This can be done in two steps. The first step is to remove the n-th line. And the second step is to insert a new line in n-th line position. Here we go.

Step 1: remove the n-th line

$>sed -i'' '10 d' file.txt       # d stands for delete

Step 2: insert a new line at n-th line position

$>sed -i'' '10 i This is the new line' file.txt     # i stands for insert

#How to show the non-printable characters in a file?

Open the file in VI editor. Go to VI command mode by pressing [Escape] and then [:]. Then type [set list]. This will show you all the non-printable characters, e.g. Ctrl-M characters (^M) etc., in the file.