Unix Shell Scripting Interview Questions And Answers
by Venkatesan M
1: What is a shell?
Shell is a interface between user and the kernel. Even though there can be only one kernel ; a system can have many shell running simultaneously . Whenever a user enters a command through keyboard the shell communicates with the kernel to execute it and then display the output to the user.
2: What are the different types of commonly used shells on a typical linux system?
csh,ksh,bash,Bourne . The most commonly used and advanced shell used today is “Bash” .
3:What is the equivalent of a file shortcut that we have on window on a Linux system?
Shortcuts are created using “links” on Linux. There are two types of links that can be used namely “soft link” and “hard link”
4:What is the difference between soft and hard links?
Soft links are link to the file name and can reside on different filesytem as well; however hard links are link to the inode of the file and has to be on the same filesytem as that of the file. Deleting the orginal file makes the soft link inactive (broken link) but does not affect the hard link (Hard link will still access a copy of the file)
5: How will you pass and access arguments to a script in Linux?
Arguments can be passed as:
scriptName “Arg1” “Arg2”….”Argn” and can be accessed inside the script as $1 , $2 .. $n
6: What is the significance of $#?
$# shows the count of the arguments passed to the script.
7: What is the difference between $* and [email protected]?
[email protected] treats each quoted arguments as separate arguments but $* will consider the entire set of positional parameters as a single string.
8: Use sed command to replace the content of the file (emulate tac command)
if cat file1
Then O/p should be
sed ‘1! G; h;$!d’ file1
Here G command appends to the pattern space,
h command copies pattern buffer to hold buffer
and d command deletes the current pattern space.
9: Given a file, replace all occurrence of word “ABC” with “DEF” from 5th line till end in only those lines that contains word “MNO”
sed –n ‘5,$p’ file1|sed ‘/MNO/s/ABC/DEF/’
10: Given a file , write a command sequence to find the count of each word.
tr –s “(backslash)040” <file1|tr –s “(backslash)011”|tr “(backslash)040 (backslash)011” “(backslash)012” |uniq –c
where “(backslash)040” is octal equivalent of “space”
”(backslash)011” is octal equivalent of “tab character” and
“(backslash)012” is octal equivalent of newline character.
11: How will you find the 99th line of a file using only tail and head command?
tail +99 file1|head -1
12: Print the 10th line without using tail and head command.
sed –n ‘10p’ file1
13:In my bash shell I want my prompt to be of format ‘$”Present working directory”:”hostname”> and load a file containing a list of user defined functions as soon as I login , how will you automate this?
In bash shell we can create “.profile” file which automatically gets invoked as soon as I login and write the following syntax into it.
export PS1=’$ `pwd`:`hostname`>’ .File1
Here File1 is the file containing the user defined functions and “.” invokes this file in current shell.
14: Explain about “s” permission bit in a file?
“s” bit is called “set user id” (SUID) bit.
“s” bit on a file causes the process to have the privileges of the owner of the file during the instance of the program.
Eg: Executing “passwd” command to change current password causes the user to writes its new password to shadow file even though it has “root” as its owner.
15: I want to create a directory such that anyone in the group can create a file and access any person’s file in it but none should be able to delete a file other than the one created by himself.
We can create the directory giving read and execute access to everyone in the group and setting its sticky bit “t” on as follows:
chmod g+wx direc1
chmod +t direc1
16: How can you find out how long the system has been running?
17: How can any user find out all information about a specific user like his default shell, real life name, default directory,when and how long he has been using the sytem?
finger “loginName” …where loginName is the login name of the
user whose information is expected.
18: What is the difference between $$ and $!?
$$ gives the process id of the currently executing process whereas $! shows the process id of the process that recently went into background.
19: What are zombie processes?
These are the processes which have died but whose exit status is still not picked by the parent process. These processes even if not functional still have its process id entry in the process table.
20: How will you copy file from one machine to other?
We can use utilities like “ftp” ,”scp” or “rsync” to copy file from one machine to other.
Eg: Using ftp:
Above copies file file1 from local system to destination system whose hostname is specified.
21: I want to monitor a continuously updating log file, what command can be used to most efficiently achieve this?
We can use tail –f filename . This will cause only the default last 10 lines to be displayed on std o/p which continuously shows the updating part of the file.
22: I want to connect to a remote server and execute some commands, how can I achieve this?
We can use telnet to do this:
telnet hostname –l user
>Write the command to execute
23: I have 2 files and I want to print the records which are common to both.
We can use “comm” command as follows:
comm -12 file1 file2 … 12 will suppress the content which are
unique to 1st and 2nd file respectively.
24: Write a script to print the first 10 elemenst of Fibonacci series.
for I in 1 2 3 4 5 6 7 8
September 20, 2019
January 29, 2019
May 26, 2018