Linux Interview Questions: 2015

Tuesday, 27 October 2015

Write a command sequence to find all the files modified in less than 2 days and print the record count of each.

find . –mtime -2 –exec wc –l {} \;

What is the significance of $? ?

$? gives the exit status of the last command that was executed.

Write the syntax for “if” conditionals in linux?

Syntax
If condition is successful
then
execute commands
else
execute commands
fi

How will you find the total disk space used by a specific user?

du -s /home/user1 ….where user1 is the user for whom the total disk
space needs to be found.

Write down the syntax of “for “ loop

Syntax:
for iterator in (elements)
do
execute commands
don

How to set an array in Linux?

In bash
A=(element1 element2 element3 …. elementn)

How will you print the login names of all users on a system?

/etc/shadow file has all the users listed.
awk –F ‘:’ ‘{print $1} /etc/shadow’|uniq -u

What is the difference between grep and egrep?

egrep is Extended grep that supports added grep features like “+” (1 or more occurrence of previous character),”?”(0 or 1 occurrence of previous character) and “|” (alternate matching)

Given a file find the count of lines containing word “ABC”.

grep –c “ABC” file1

What are the 3 standard streams in Linux?

0 – Standard Input
1 – Standard Output
2 – Standard Error

How will you connect to a database server from linux?

We can use isql utility that comes with open client driver as follows:
isql –S serverName –U username –P password

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 1^st and 2^nd file respectively.

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
>Enter password
>Write the command to execute
>quit

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.

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.

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.

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.

How can you find out how long the system has been running?

Command “uptime”

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:

mkdir direc1

chmod g+wx direc1

chmod +t direc1

Print the 10th line without using tail and head command.

sed –n ‘10p’ file1

How will you find the 99th line of a file using only tail and head command?

tail +99 file1|head -1

What is the difference between $* and $@?

$@ treats each quoted arguments as separate arguments but $* will consider the entire set of positional parameters as a single string.

What is the significance of $#?

$# shows the count of the arguments passed to the script.

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

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)

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”

hat 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” .

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.

Wednesday, 15 April 2015

How to Extract (untar) an archive

Extract a *.tar file using option xvf

Extract a tar file using option x as shown below:

tar xvf archive_name.tar

x – extract files from archive

Extract a gzipped tar archive ( *.tar.gz ) using option xvzf

Use the option z for uncompressing a gzip tar archive.

tar xvfz archive_name.tar.gz

Extracting a bzipped tar archive ( *.tar.bz2 ) using option xvjf

Use the option j for uncompressing a bzip2 tar archive.

tar xvfj archive_name.tar.bz2

Note: In all the above commands v is optional, which lists the file being processed.

How to create a bzipped tar archive

tar cvfj archive_name.tar.bz2 dirname/

j – filter the archive through bzip2

gzip vs bzip2: bzip2 takes more time to compress and decompress than gzip. bzip2 archival size is less than gzip.
Note: .tbz and .tb2 is same as .tar.bz2

How to create a tar gzipped archive

tar cvzf archive_name.tar.gz dirname/

z – filter the archive through gzip

gzip vs bzip2: bzip2 takes more time to compress and decompress than gzip. bzip2 archival size is less than gzip.
Note: .tbz and .tb2 is same as .tar.bz2

How to Create an uncompressed tar archive

tar cvf archive_name.tar dirname/

In the above command:

c – create a new archive
v – verbosely list files which are processed.
f – following is the archive file name