Interview Questions : PART 5 - Shell Script Basic

Interview Questions : Shell Script Basic: PART 5

The below Questions will help for basic shell scripting interview.


1. Construct pipes to execute the following jobs.

1. Output of who should be displayed on the screen with value of total number of users who have logged in
displayed at the bottom of the list.

2. Output of ls should be displayed on the screen and from this output the lines containing the word ‘poem’
should be counted and the count should be stored in a file.

3. Contents of file1 and file2 should be displayed on the screen and this output should be appended in a file
.
From output of ls the lines containing ‘poem’ should be displayed on the screen along with the count.

4. Name of cities should be accepted from the keyboard . This list should be combined with the list present
in a file. This combined list should be sorted and the sorted list should be stored in a file ‘newcity’.

5. All files present in a directory dir1 should be deleted any error while deleting should be stored in a file
‘errorlog’.


2. Explain the following commands.

$ ls > file1
$ banner hi-fi > message
$ cat par.3 par.4 par.5 >> report
$ cat file1>file1
$ date ; who
$ date ; who > logfile
$ (date ; who) > logfile


3. What is the significance of the “tee” command?

It reads the standard input and sends it to the standard output while redirecting a copy of what it has read to
the file specified by the user.


4. What does the command “ $who | sort –logfile > newfile” do?
The input from a pipe can be combined with the input from a file . The trick is to use the special symbol “-“
(a hyphen) for those commands that recognize the hyphen as std input.
In the above command the output from who becomes the std input to sort , meanwhile sort opens the file
logfile, the contents of this file is sorted together with the output of who (rep by the hyphen) and the sorted
output is redirected to the file newfile.



5. What does the command “$ls | wc –l > file1” do?

ls becomes the input to wc which counts the number of lines it receives as input and instead of displaying
this count , the value is stored in file1.



6. Which of the following commands is not a filter man , (b) cat , (c) pg , (d) head
Ans: man

A filter is a program which can receive a flow of data from std input, process (or filter) it and send the result
to the std output.

7. How is the command “$cat file2 “ different from “$cat >file2 and >> redirection operators ?

is the output redirection operator when used it overwrites while >> operator appends into the file.


8.What difference between cmp and diff commands?

cmp - Compares two files byte by byte and displays the first mismatch
diff - tells the changes to be made to make the files identical

9.Write a command to kill the last background job?

Kill $!

10. Explain the steps that a shell follows while processing a command.

After the command line is terminated by the key, the shel goes ahead with processing the command line in
one or more passes. The sequence is well defined and assumes the following order.

Parsing: The shell first breaks up the command line into words, using spaces and the delimiters, unless
quoted. All consecutive occurrences of a space or tab are replaced here with a single space.

Variable evaluation: All words preceded by a $ are avaluated as variables, unless quoted or escaped.
Command substitution: Any command surrounded by backquotes is executed by the shell which then
replaces the standard output of the command into the command line.

Wild-card interpretation: The shell finally scans the command line for wild-cards (the characters *, ?, [, ]).
Any word containing a wild-card is replaced by a sorted list of
filenames that match the pattern. The list of these filenames then forms the arguments to the command.

PATH evaluation: It finally looks for the PATH variable to determine the sequence of directories it has to
search in order to hunt for the command.

11.Write a command to display a file’s contents in various formats?

Answer :

$od -cbd file_name

12. What will the following command do?

 $ echo *

Ans : It is similar to 'ls' command and displays all the files in the current directory

13.What is the use of the command "ls -x chapter[1-5]"

ls stands for list; so it displays the list of the files that starts with 'chapter' with suffix '1' to '5', chapter1,
chapter2, and so on.


c - character, b - binary (octal), d-decimal, od=Octal Dump.


14. How does the kernel differentiate device files and ordinary files?

Answer :  Kernel checks 'type' field in the file's inode structure.

15. What are shell variables?

Shell variables are special variables, a name-value pair created and maintained by the shell.


16.How to terminate a process which is running and the specialty on command kill 0?

Answer :
With the help of kill command we can terminate the process.
Syntax: kill pid
Kill 0 - kills all processes in your system except the login shell.
Example: PATH, HOME, MAIL and TERM