Unix Interview Questions

To list only files and avoid all directories

The following command will only list files and will exclude directories, special files, pipes, symbolic links etc:

$ find . -type f -name '*.pl'

find / -type f -name hosts 2>/dev/null

Search all directories

Search file called httpd.conf in all directories:

$ find /
-type f -name httpd.conf

Execute command on all files

Run ls -l command on all *.c files to get extended information :

$ find .
-name "*.c" -type f -exec ls -l {} \;

Write a unix command to display the every character in the word "AMAZON" in new line.

Ex : Input - AMAZON
output: A
M
A
Z
O
N

~ $ echo AMAZON | fold -w 1

UNIX / Linux pipes and grep command

grep command often used with shell pipes. In this example, show the name of the hard disk devices:

# dmesg |
egrep '(s|h)d[a-z]'

Display cpu model name:

# cat
/proc/cpuinfo | grep -i 'Model'

However, above command can be also used as follows without shell pipe:

# grep -i
'Model' /proc/cpuinfo

Sample outputs:

model                            : 30

model name               : Intel(R) Core(TM) i7 CPU       Q 820  @ 1.73GHz

model                            : 30

model name               : Intel(R) Core(TM) i7 CPU       Q 820  @ 1.73GHz

Use grep to search words only

When you search for boo, grep will match fooboo, boo123, barfoo35 and more. You can force the grep command to select only those lines containing matches that form whole words i.e. match only boo word:

$ grep -w
"boo" file

Use grep to search 2 different words

Use the egrep command as follows:

$ egrep
-w 'word1|word2' /path/to/file

Count line when words has been matched

The grep can report the number of times that the pattern has been matched for each file using -c (count) option:

$ grep -c
'word' /path/to/file

Pass the -n option to precede each line of output with the number of the line in the text file from which it was obtained:

$ grep -n
'root' /etc/passwd

Sample outputs:

1:root:x:0:0:root:/root:/bin/bash

1042:rootdoor:x:0:0:rootdoor:/home/rootdoor:/bin/csh

3319:initrootapp:x:0:0:initrootapp:/home/initroot:/bin/ksh

Write a regular expression for a phone number in unix?

123-456-7890

(123)456-7890

1234567890

123.456.7890

+91(123)456-7890

find missing number in 1 to 1000 random digits. ?

[/u03/appl/farbat1]$ bannersuresh

#### # # ##### ###### #### # #

# # # # # # # # #

#### # # # # ##### #### ######

# # # ##### # # # #

# # # # # # # # # # #

#### #### # # ###### #### # #

[/u03/appl/farbat1/suresh]$ echo "This is a file." > file1.txt

[/u03/appl/farbat1/suresh]$ link file1.txt file2.txt

[/u03/appl/farbat1/suresh]$ ls -alrt

total 10

drwxr-xr-x 23 farbat1 arboradm 3072 Sep 28 12:06 ..

-rw-rw-r-- 2 farbat1 arboradm 16 Sep 28 12:06 file2.txt

-rw-rw-r-- 2 farbat1 arboradm 16 Sep 28 12:06 file1.txt

drwxrwxr-x 2 farbat1 arboradm 96 Sep 28 12:06 .

[/u03/appl/farbat1/suresh]$ more file2.txt

This is a file.

[/u03/appl/farbat1/suresh]$ echo "It points to data on the disk." >> file1.txt

[/u03/appl/farbat1/suresh]$ more file2.txt

This is a file.

It points to data on the disk.

[/u03/appl/farbat1/suresh]$ more file1.txt.txt

file1.txt.txt: No such file or directory

[/u03/appl/farbat1/suresh]$ more file1.txt

This is a file.

It points to data on the disk.

[/u03/appl/farbat1/suresh]$

About Symbolic Links

Symbolic links, sometimes called "soft" links, are different than "hard" links. Instead of linking to the data of a file, they link to another link. So in the example above, file2.txtpoints to the link file1.txt, which in turn points to the data of the file.

This has several potential benefits. For one thing, symbolic links (also called "symlinks" for short) can link to directories. Also, symbolic links can cross file system boundaries, so a symbolic link to data on one drive or partition can exist on another drive or partition.

You should also be aware that, unlike hard links, removing the file (or directory) that a symlink points to will break the link. So if we create file1.txt:

echo "This is a file." > file1.txt

...and create a symbolic link to it:

ln -s file1.txt file2.txt

...we can cat either one of these to see the contents:

cat file1.txt

This is a file.

cat file2.txt

This is a file.

...but if we remove file1.txt:

rm file1.txt

...we can no longer access the data it contained with our symlink:

cat file2.txt

cat: file2.txt: No such file or directory

[/u03/appl/farbat1/suresh]$ ls -alrt

total 10

drwxr-xr-x 23 farbat1 arboradm 3072 Sep 28 12:06 ..

drwxrwxr-x 2 farbat1 arboradm 96 Sep 28 12:06 .

-rw-rw-r-- 2 farbat1 arboradm 47 Sep 28 12:07 file2.txt

-rw-rw-r-- 2 farbat1 arboradm 47 Sep 28 12:07 file1.txt

[/u03/appl/farbat1/suresh]$ unlink file2.txt

[/u03/appl/farbat1/suresh]$ ls -alrt

total 8

drwxr-xr-x 23 farbat1 arboradm 3072 Sep 28 12:06 ..

-rw-rw-r-- 1 farbat1 arboradm 47 Sep 28 12:07 file1.txt

drwxrwxr-x 2 farbat1 arboradm 96 Sep 28 12:13 .

[/u03/appl/farbat1/suresh]$

Tr command Examples:

1. Convert lower case letters to upper case

The following tr command translates the lower case letters to capital letters in the give string:

>echo "linux dedicated server" | tr "[:lower:]" "[:upper:]"

LINUX DEDICATED SERVER

>echo "linux dedicated server" | tr "[a-z]" "[A-Z]"

LINUX DEDICATED SERVER

2. Transform upper case letters to lower case.

Similar to the above example, you can translate the uppercase letters to small letters.

> echo "UNIX DEDICATED SERVER" | tr "[:upper:]" "[:lower:]"

unix dedicated server

> echo "UNIX DEDICATED SERVER" | tr "[A-Z]" "[a-z]"

unix dedicated server

3. Replace non-matching characters.

The -c option is used to replace the non-matching characters with another set of characters.

> echo "unix" | tr -c "u" "a"

uaaa

In the above example, except the character "c" other characters are replaced with "a"

4. Delete non-printable characters

The -d option can be used to delete characters. The following example deletes all the non-printable characters from a file.

>tr -cd "[:print:]" < filename

5. Squeezing characters

You can squeeze more than one occurrence of continuous characters with single occurrence. The following example squeezes two or more successive blank spaces into a single space.

>echo "linux server" | tr -s " "

linux server

Here you can replace the space character with any other character by specifying in set2.

> "linux server" | tr -s " " ","

linux,server

6. Delete characters

The following example removes the word linux from the string.

> echo "linuxserver" | tr -d "linux"

server

tee examples

ls -1 *.txt | wc -l | tee count.txt

In the above example, the ls command lists all files in the current directory that have the filename extension .txt, one file per line; this output is piped to wc, which counts the lines and outputs the number; this output is piped to tee, which writes the output to the terminal, and writes the same information to the file count.txt. If count.txt already exists, it is overwritten.

cat list.txt

apples

oranges

potatoes

lemons

garlic

nl list.txt

     1             apples

     2             oranges

     3             potatoes

     4             lemons

     5             garlic

In the above example, we use the cat command to display the contents of list.txt. Then we use nl to number each line and display the result to standard output.

pg examples

pg myfile.txt

Displays the first screenful of the contents of text file myfile.txt, and a prompt (":"). Pressing the RETURN key displays the next page, or any of the command listed above may be entered to otherwise navigate the file.

Paste Command Examples:

Create the following three files in your unix or linux servers to practice to practice the examples:

>cat file1

Unix

Linux

Windows

>cat file2

Dedicated server

Virtual server

>cat file3

Hosting

Machine

Operating system

1. Merging files in parallel

By default, the paste command merges the files in parallel. The paste command writes corresponding lines from the files as a tab delimited on the terminal.

> paste file1 file2

Unix Dedicated server

Linux Virtual server

Windows

> paste file2 file1

Dedicated server Unix

Virtual server Linux

Windows

2. Specifying the delimiter

Paste command uses the tab delimiter by default for merging the files. You can change the delimiter to any other character by using the -d option.

>paste -d"|" file1 file2

Unix|Dedicated server

Linux|Virtual server

Windows|

In the above example, pipe delimiter is specified

3. Merging files in sequentially.

You can merge the files in sequentially using the -s option. The paste command reads each file in sequentially. It reads all the lines from a single file and merges all these lines into a single line.

>paste -s file1 file2

Unix Linux Windows

Dedicated server Virtual server

The following example shows how to specify a delimiter for sequential merging of files:

> paste -s -d"," file1 file2

Unix,Linux,Windows

Dedicated server,Virtual server

4. Specifying multiple delimiters.

Multiple delimiters come in handy when you want to merge more than two files with different delimiters. For example I want to merge file1, file2 with pipe delimiter and file2, file3 with comma delimiter. In this case multiple delimiters will be helpful.

>paste -d"|," file1 file2 file3

Unix|Dedicatedserver,Hosting

Linux|Virtualserver,Machine

Windows|,Operating system

5. Combining N consecutive lines

The paste command can also be used to merge N consecutive lines from a file into a single line. The following example merges 2 consecutive lines into a single line

>cat file1 | paste - -

Unix Linux

Windows

CUT

$ cat test.txt

cat command for file oriented operations.

cp command for copy files or directories.

ls command to list out files and directories with its attributes.

1. Select Column of Characters

To extract only a desired column from a file use -c option. The following example displays 2nd character from each line of a file test.txt

$ cut -c2 test.txt

As seen above, the characters a, p, s are the second character from each line of the test.txt file.

2. Select Column of Characters using Range

Range of characters can also be extracted from a file by specifying start and end position delimited with -. The following example extracts first 3 characters of each line from a file called test.txt

$ cut -c1-3 test.txt

cat

cp

ls

3. Select Column of Characters using either Start or End Position

Either start position or end position can be passed to cut command with -c option.

The following specifies only the start position before the ‘-‘. This example extracts from 3rd character to end of each line from test.txt file.

$ cut -c3- test.txt

t command for file oriented operations.

command for copy files or directories.

command to list out files and directories with its attributes.

The following specifies only the end position after the ‘-‘. This example extracts 8 characters from the beginning of each line from test.txt file.

$ cut -c-8 test.txt

catcomm

cp comma

ls comma

The entire line would get printed when you don’t specify a number before or after the ‘-‘ as shown below.

$ cut -c- test.txt

cat command for file oriented operations.

cp command for copy files or directories.

ls command to list out files and directories with its attributes.

4. Select a Specific Field from a File

Instead of selecting x number of characters, if you like to extract a whole field, you can combine option -f and -d. The option -f specifies which field you want to extract, and the option -d specifies what is the field delimiter that is used in the input file.

The following example displays only first field of each lines from /etc/passwd file using the field delimiter : (colon). In this case, the 1st field is the username. The file

$ cut -d':' -f1 /etc/passwd

root

daemon

bin

sys

sync

games

bala

5. Select Multiple Fields from a File

You can also extract more than one fields from a file or stdout. Below example displays username and home directory of users who has the login shell as “/bin/bash”.

$ grep "/bin/bash" /etc/passwd | cut -d':' -f1,6

root:/root

bala:/home/bala

To display the range of fields specify start field and end field as shown below. In this example, we are selecting field 1 through 4, 6 and 7

$ grep "/bin/bash" /etc/passwd | cut -d':' -f1-4,6,7

root:x:0:0:/root:/bin/bash

bala:x:1000:1000:/home/bala:/bin/bash

6. Select Fields Only When a Line Contains the Delimiter

In our /etc/passwd example, if you pass a different delimiter other than : (colon), cut will just display the whole line.

In the following example, we’ve specified the delimiter as | (pipe), and cut command simply displays the whole line, even when it doesn’t find any line that has | (pipe) as delimiter.

$ grep "/bin/bash" /etc/passwd | cut -d'|'  -f1

root:x:0:0:root:/root:/bin/bash

bala:x:1000:1000:bala,,,:/home/bala:/bin/bash

But, it is possible to filter and display only the lines that contains the specified delimiter using -s option.

The following example doesn’t display any output, as the cut command didn’t find any lines that has | (pipe) as delimiter in the /etc/passwd file.

$ grep "/bin/bash" /etc/passwd | cut -d'|' -s -f1

7. Select All Fields Except the Specified Fields

In order to complement the selection field list use option –complement.

The following example displays all the fields from /etc/passwd file except field 7

$ grep "/bin/bash" /etc/passwd | cut -d':' --complement -s -f7

root:x:0:0:root:/root

bala:x:1000:1000:bala,,,:/home/bala

8. Change Output Delimiter for Display

By default the output delimiter is same as input delimiter that we specify in the cut -d option.

To change the output delimiter use the option –output-delimiter as shown below. In this example, the input delimiter is : (colon), but the output delimiter is # (hash).

$ grep "/bin/bash" /etc/passwd | cut -d':'  -s -f1,6,7 --output-delimiter='#'

root#/root#/bin/bash

bala#/home/bala#/bin/bash

9. Change Output Delimiter to Newline

In this example, each and every field of the cut command output is displayed in a separate line. We still used –output-delimiter, but the value is $’\n’ which indicates that we should add a newline as the output delimiter.

$ grep bala /etc/passwd | cut -d':' -f1,6,7 --output-delimiter=$'\n'

bala

/home/bala

/bin/bash

Install the mailx command

On Ubuntu/Debian based systems the mailx command is available from 2 different packages -

1. heirloom-mailx
2. bsd-mailx

We shall be using the heirloom-mailx package because it has more features and options.
On CentOS/Fedora based systems, there is only one package named "mailx" which is the heirloom package.

To find out what mailx package is installed on your system, check the "man mailx" output and scroll down to the end and you should see some useful information.

# ubuntu/debian

$ sudo apt-get install heirloom-mailx

# fedora/centos

$ sudo yum install mailx

Using the mailx command

Once installed, the mailx command can be directly referenced with the name mail, so you just type in that in the command line.

1. Simple mail

Run the following command, and then mailx would wait for you to enter the message of the email. You can hit enter for new lines. When done typing the message, press Ctrl+D and mailx would display EOT.

After than mailx automatically delivers the email to the destination.

$ mail -s "This is the subject" someone@example.com

Hi someone

How are you

I am fine

Bye

EOT

2. Take message from a file

The message body of the email can be taken from a file as well.

$ mail -s "This is Subject" someone@example.com < /path/to/file

The message can also be piped using the echo command -

$ echo "This is message body" | mail -s "This is Subject" someone@example.com

3. Multiple recipients

To send the mail to multiple recipients, specify all the emails separated by a comma

$ echo "This is message body" | mail -s "This is Subject" someone@example.com,someone2@example.com

4. CC and BCC

The "-c" and "-b" options can be used to add CC and BCC addresses respectively.

$ echo "This is message body" | mail -s "This is Subject" -c ccuser@example.com someone@example.com

5. Specify From name and address

To specify a "FROM" name and address, use the "-r" option. The name should be followed by the address wrapped in "<>".

$ echo "This is message body" | mail -s "This is Subject" -r "Harry<harry@gmail.com>" someone@example.com

6. Specify "Reply-To" address

The reply to address is set with the internal option variable "replyto" using the "-S" option.

# replyto email

$ echo "This is message" | mail -s "Testing replyto" -S replyto="mark@gmail.com" someone@example.com

# replyto email with a name

$ echo "This is message" | mail -s "Testing replyto" -S replyto="Mark<mark@gmail.com>" someone@example.com

7. Attachments

Attachments can be added with the "-a" option.

$ echo "This is message body" | mail -s "This is Subject" -r "Harry<harry@gmail.com>" -a /path/to/file someone@example.com

[/u03/appl/farbat1/suresh]$ finger -p ch

Directory: /home_ldap/ankloe Shell: /bin/bash

Never logged in.

No unread mail

Directory: /home_ldap/bjholt Shell: /bin/bash

Never logged in.

No unread mail

Directory: /home/roleem Shell: /bin/bash

Never logged in.

No unread mail

[/u03/appl/farbat1/suresh]$ finger -p suresh

Directory: /home_ldap/skunku Shell: /bin/bash

On since Sep 28 12:00:30 on pts/2 from nlamsp6xen152.upcit.ds.upc.biz

No unread mail

[/u03/appl/farbat1/suresh]$

NextGen4IT

Unix Interview Questions

Search all directories

Execute command on all files

UNIX / Linux pipes and grep command

Use grep to search words only

Use grep to search 2 different words

Count line when words has been matched

About Symbolic Links

tee examples

pg examples

CUT

1. Select Column of Characters

2. Select Column of Characters using Range

3. Select Column of Characters using either Start or End Position

4. Select a Specific Field from a File

5. Select Multiple Fields from a File

6. Select Fields Only When a Line Contains the Delimiter

7. Select All Fields Except the Specified Fields

8. Change Output Delimiter for Display

9. Change Output Delimiter to Newline

Install the mailx command

Using the mailx command

1. Simple mail

2. Take message from a file

3. Multiple recipients

4. CC and BCC

5. Specify From name and address

6. Specify "Reply-To" address

7. Attachments