Unix/Linux Interview Questions and Answers

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.

UNIX is a powerful multitasking, multiuser computer Operating System initially developed by Ken Thompson, Dennis Ritchie at AT&T Bell laboratories in 1970.

The shell is the Unix command line interpreter. It provides an interface between the user and the kernel and executes programs called commands.

There are following different Shells in unix operating system:

Bsh- Bourne shell, It is developed by Stephen Bourne.
ksh – This is called as Korn shell. This is developed by David Korn.
Bash- Bourne Again shell, It is most used for shell scripting in Unix operating system.
csh – This is also called as C-shell

A shell script is a list of commands in a computer program that is run by the Unix shell which is a command line interpreter.

Example:

$cat testHello.sh
#!/bin/sh
echo "-------------" 
echo Hello World  

$./testHello.sh
-------------
Hello World 

There are multiple ways to create a file in Unix-
cat command
$cat file.txt

touch command
$touch file.txt

echo and printf command
$echo 'New file.' > file.txt
$printf 'New file. ' > file.txt

Using Vi,emacs text editors
$vi file.txt

rm command is used to delete file

To delete a file
$rm file.txt

To delete a file with confirmation
$rm -i file.txt

To deletes a file forcefully
$ rm -f file.txt

rm command to delete the specified files and directories.

To delete all files and subdirectories in the current directory
$rm -rf *

To delete the files with name
$rm file1.txt file2.txt file3.txt

Unix commands alone are powerful, but when you combine them together. The symbol | is the Unix pipe symbol that is used in command line to combine them together.

The output of the command to the left of the pipe gets sent as input of the command to the right of the pipe.

Example:

$cat apple.txt | wc

ls - | grep "page" | wc -l

"Cat" displays file contents. If the file is large the contents scroll off the screen before we view it. "More" command is like a pager which displays the contents page by page.

$cat bigfile.txt
$more bigfile.txt
$cat bigfile.txt | more

You make a new directory via mkdir newdirectoryname ,and can remove a directory using rmdir directoryname. To remove a directory, you must first remove all the files it contains.

$mkdir Directory_name
$rmdir Directory_name

Nohup stands for no hang up. It is a Unix command that is used to run another command. It tells Unix not to stop passed command once it has started, that will keep running until done.

When you execute a Unix job in the background( &, bg command), and logout from the session, then that process will get killed. It can avoid this by executing the job with nohup. Nohup prevents the command from being aborted automatically when you log out or exit the shell.

# nohup sample.sh &

By default, standard output will be redirected to nohup.out file in the current directory. You can also redirect the output to an other file using the normal shell redirection.

# nohup sh sample.sh > sample-out.log &

Once a job is started or executed using the nohup command, stdin will not be available to the user.

An inode is a unique entry which by default gets created on a disk section for a file system. It contains the following information:

Location on the disk
User of the file
Size of the file
Group of the file
Device ID
File size
Date of creation
Permission
Owner of the file

ls and stat command used to display the inode numbers of the files-
$ls -i file.txt
$stat file.txt

Absolute path : Exact path from root directory.
Relative path : Relative to the current path.

Some basic Unix commands -
man – view manual pages for Unix commands
clear – clear screen
date – show current date and time
cal - shows a calendar of the current month
whoami – show your username
who – find out who is logged into the system
cd – change directory
pwd – show current directory
mkdir – make new directory
rmdir – remove directory
cat – create file and show contents to the standard output
more – basic pagination when viewing text files
less – improved pagination tool for viewing text files
ls – list files and directories
cp – copy files
rm – remove files and directories
mv – rename or move files and directories to another location
head – show the specified lines from first line of text file
tail – show the specified lines from last lines of text file
wc - tells you how many lines, words, and characters are in a file
grep – search for patterns in text files
chmod – change file/directory access permissions
chown – change file/directory ownership
ps – list processes
top – show tasks and system status
kill – kill a process (stop application running)
gzip - compresses files
gunzip- uncompresses files
du - shows the disk usage of the files and directories

du command is short for disk usage, with the help of this command you can find the disk capacity and free space of the disk.

$du -h /home/demo/dict

Output:
44K /home/demo/dict/data
2.0M /home/demo/dict/system design
28K /home/demo/dict/table/sample_table
32K /home/demo/dict/table
98M /home/demo/dict

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

grep (Global Regular Expression Print) is a pattern search command.
It searches for the pattern, specified in the command line with appropriate option, in a given file(s).
Syntax : grep [options] pattern [files]
Example :
$grep 99mx mcafile
$grep “coml” file.txt
$grep “me” file1.txt file2.txt file3.txt
$grep “me” *

$grep -i "Search" Filename --to use the option -i for case insensitive search

Combining the two commands head and tail can give you the result -

$ head -12 <filename> | tail -3
Ex -

$ head -12 test.txt | tail -3

Combining the two commands head and tail can give you the result -

$ tail +99 test.txt|head -1

by using wc-stands for word count.
$wc -c for counting number of characters in a file
$wc -l for counting lines in a file
$grep -c 'me' file.txt for count the number of lines which is having that specific string

“chmod” command which stands for “change mode” is used to add, remove, or modify file or directory permissions.

Ex-
$chmod 477 file.txt
provides Read Permission to User, Read/Write/Execute Permissions to Group & Others.
$chmod 777 file.txt
provides full read write and execute permission (code=7) to all the group.

$ls -l *
-rw-r--r-- 1 users 777 Dec 18 lookup.icn
-rwxr-x--x 1 users 562 Jan 10 hello
drwxr-xr--- 1 users 1024 Nov 2 mydir

- rw- r-- r--
type of file owner group others
the first character specifies the file type either a ‘-‘ means it’s a file, or a ‘d’ means it’s a directory.

To get the latest file or directory in the current working directory

$ls -lrt |tail -1  

-t switch sorts the listing by 'time' so the output is displayed from newest to oldest, -r switch reverses so the newest file is at the end of the listing, And -l gives you the long listing.