Bash shell provides two kind of parameters.

Positional Parameter and Special Parameter

Bash Positional Parameter – $0, $1, $2 ..

Positional parameters are the arguments given to your scripts when it is invoked. It could be from $1 to $N. When N consists of more than a single digit, it must be enclosed in a braces like ${N}.

The variable $0 is the basename of the program as it was called.

The following example gets two arguments and provides arithmetic operations result between those two integers.

First, create the arithmetic.sh shell script as shown below.

$ cat arithmetic.sh

#!/bin/bash

echo -e "\$1=$1"

echo -e "\$2=$2"

let add=$1+$2

let sub=$1-$2

let mul=$1*$2

let div=$1/$2

echo -e "Addition=$add\nSubtraction=$sub\nMultiplication=$mul\nDivision=$div\n"

Next, execute the arithmetic.sh with proper parameters as shown below.

$ ./arithmetic.sh 12 10

$1=12

$2=10

Addition=22

Subtraction=2

Multiplication=120

Division=1

In the above output $1 has the value 12, and $2 has 10.

Shell built-in ‘let’ allows arithmetic operation to be performed on shell variables. The above script does the arithmetic operations such as addition, subtraction, multiplication and division on the given parameters.

Set / Unset Bash Positional Parameters

The built in set command is used to set and unset the positional parameter.

First, create the positional.sh shell script as shown below.

$ cat positional.sh

#!/bin/bash

# From command line

echo -e "Basename=$0"

echo -e "\$1=$1"

echo -e "\$2=$2"

echo -e "\$3=$3"

# From Set builtin

set First Second Third

echo -e "\$1=$1"

echo -e "\$2=$2"

echo -e "\$3=$3"

# Store positional parameters with -(hyphen)

set - -f -s -t

echo -e "\$1=$1"

echo -e "\$2=$2"

echo -e "\$3=$3"

# Unset positional parameter

set --

echo -e "\$1=$1"

echo -e "\$2=$2"

echo -e "\$3=$3"

The above script prints the command line arguments first, then set command sets the positional parameter explicitly. Set with the – refers end of options, all following arguments are positional parameter even they can begin with ‘-’. Set with ‘–’ without any other arguments unset all the positional parameters.

Next, execute the positional.sh as shown below.

$ ./positional.sh

Basename=positional.sh

$1=12

$2=10

$3=

$1=First

$2=Second

$3=Third

$1=-f

$2=-s

$3=-t

$1=

$2=

$3=

Use Bash $* and $@ to Expand Positional Parameters

This example shows the value available in $* and $@.

First, create the expan.sh as shown below.

$ cat expan.sh

#!/bin/bash

export IFS='-'

cnt=1

# Printing the data available in $*

echo "Values of \"\$*\":"

for arg in "$*"

do

echo "Arg #$cnt= $arg"

let "cnt+=1"

done

cnt=1

# Printing the data available in $@

echo "Values of \"\$@\":"

for arg in "$@"

do

echo "Arg #$cnt= $arg"

let "cnt+=1"

done

Next, execute the expan.sh as shown below to see how $* and $@ works.

$ ./expan.sh "This is" 2 3

Values of "$*":

Arg #1= This is-2-3

Values of "$@":

Arg #1= This is

Arg #2= 2

Arg #3= 3

· The above script exported the value of IFS (Internal Field Separator) with the ‘-’.

· There are three parameter passed to the script expan.sh $1=”This is”,$2=”2″ and $3=”3″.

· When printing the each value of special parameter “$*”, it gives only one value which is the whole positional parameter delimited by IFS.

· Whereas “$@” gives you each parameter as a separate word.

Use $# to Count Positional Parameters

$# is the special parameter in bash which gives you the number of positional parameter in decimal.

First, create the arithmetic.sh as shown below.

$ cat arithmetic.sh

#!/bin/bash

if [ $# -lt 2 ]

then

echo "Usage: $0 arg1 arg2"

exit

fi

echo -e "\$1=$1"

echo -e "\$2=$2"

let add=$1+$2

let sub=$1-$2

let mul=$1*$2

let div=$1/$2

echo -e "Addition=$add\nSubtraction=$sub\nMultiplication=$mul\nDivision=$div\n"

If the number of positional parameters is less than 2, it will throw the usage information as shown below,

$ ./arithemetic.sh 10

Usage: ./arithemetic.sh arg1 arg2

Process related Parameters – $$ and $!

The special parameter $$ will give the process ID of the shell. $! gives you the process id of the most recently executed background process.

The following script prints the process id of the shell and last execute background process ID.

$ cat proc.sh

#!/bin/bash

echo -e "Process ID=$$"

sleep 1000 &

echo -e "Background Process ID=$!"

Now, execute the above script, and check the process id which its printing.

$ ./proc.sh

Process ID=9502

Background Process ID=9503

$ ps

PID TTY TIME CMD

5970 pts/1 00:00:00 bash

9503 pts/1 00:00:00 sleep

9504 pts/1 00:00:00 ps

$

Other Bash Special Parameters – $?, $-, $_

· $? Gives the exit status of the most recently executed command.

· $- Options set using set builtin command

· $_ Gives the last argument to the previous command. At the shell startup, it gives the absolute filename of the shell script being executed.

$ cat others.sh

#!/bin/bash

echo -e "$_"; ## Absolute name of the file which is being executed

/usr/local/bin/dbhome # execute the command.

#check the exit status of dbhome

if [ "$?" -ne "0" ]; then

echo "Sorry, Command execution failed !"

fi

echo -e "$-"; #Set options - hB

echo -e $_ # Last argument of the previous command.

In the above script, the last echo statement “echo -e $_” ($ underscore) also prints hB which is the value of last argument of the previous command. So $_ will give the value after expansion

$ ./others.sh

./others.sh

/home/oracle

Sorry, Command execution failed !

hB

hB

UNIX SED Introduction

· Sed is a “non-interactive” stream-oriented editor. Since its an “non-interactive” it can be used to automate editing.

· This allows you to edit multiple files, or to perform common editing operations without opening VI or emacs.

· Sed reads from a file or from its standard input and outputs to its standard output.

· Sed has two buffers which are called pattern buffer and hold buffer. Both are initially empty.

UNIX Sed Working Method

This is called as one execution cycle. Cycle continues till end of file/input is reached.

1. Read a entire line from stdin/file.

2. Removes any trailing newline.

3. Places the line, in its pattern buffer.

4. Modify the pattern buffer according to the supplied commands

5. Print the pattern buffer to stdout.

Printing Operation in Sed

Linux Sed command allows you to print only specific lines based on the line number or pattern matches. “p” is a command for printing the data from the pattern buffer.

Syntax:

# sed -n 'ADDRESS'p filename

# sed -n '/PATTERN/p' filename

Create the txt file sample.txt that will be used in all examples below.

# cat sample.txt

1. Linux - Sysadmin, Scripting etc.

2. Databases - Oracle, mySQL etc.

3. Hardware

4. Security (Firewall, Network, Online Security etc)

5. Storage

6. Cool gadgets and websites

7. Productivity (Too many technologies to explore)

8. Website Design

9. Software Development

10.Windows- Sysadmin, reboot etc.

Sed Address Format 1: NUMBER

This will only match Nth line in the input.

#sed –n ‘N’p filename

For example, 3p prints 3^rd line of the input file sample.txt as below

# sed -n '3'p thegeekstuff.txt

3. Hardware

Sed Address Format 2: NUMBER1~NUMBER2

M~N with “p” command prints every Nth line starting from line M.

# sed -n ‘M~N’p filename

For example, 3~2p prints every 2nd line starting from 3rd line as shown below.

# sed -n '3~2'p thegeekstuff.txt

3. Hardware

5. Storage

7. Productivity (Too many technologies to explore)

9. Software Development

Sed Address Format 3: START,END

M,N with “p” command prints Mth line to Nth line.

# sed -n ‘M,N’p filename

For example, 4,8p prints from 4th line to 8th line from input file thegeekstuff.txt

# sed -n '4,8'p thegeekstuff.txt

4. Security (Firewall, Network, Online Security etc)

5. Storage

6. Cool gadgets and websites

7. Productivity (Too many technologies to explore)

8. Website Design

Sed Address Format 4: ‘$’ Last Line

$ with “p” command matches only the last line from the input.

# sed -n ‘$’p filename

For example, $p prints only the last line as shown below.

# sed -n '$'p thegeekstuff.txt

10.Windows- Sysadmin, reboot etc.

Sed Address Format 5: NUMBER,$

N,$ with “p” command prints from Nth line to end of file.

# sed -n ‘N,$p’ filename

For example 4,$p prints from 4th line to end of file.

# sed -n '4,$p' thegeekstuff.txt

4. Security (Firewall, Network, Online Security etc)

5. Storage

6. Cool gadgets and websites

7. Productivity (Too many technologies to explore)

8. Website Design

9. Software Development

10.Windows- Sysadmin, reboot etc.

Sed Pattern Format 1: PATTERN

PATTERN could be unix regular expression. The below command prints only the line which matches the given pattern.

# sed -n /PATTERN/p filename

For example, following prints the line only which matches the pattern “Sysadmin”.

# sed -n /Sysadmin/p thegeekstuff.txt

1. Linux - Sysadmin, Scripting etc.

10.Windows- Sysadmin, reboot etc.

Sed Pattern Format 2: /PATTERN/,ADDRESS

# sed -n ‘/PATTERN/,Np’ filename

For example, following prints lines which matches the pattern to Nth line, from input. 3rd line matches the pattern “Hardware”, so it prints from 3rd line to 6th line.

# sed -n '/Hardware/,6p' thegeekstuff.txt

3. Hardware

4. Security (Firewall, Network, Online Security etc)

5. Storage

6. Cool gadgets and websites

Sed Pattern Format 3: ADDRESS,/PATTERN/

It prints from the Nth line of the input, to the line which matches the pattern. If the pattern doesnt match, it prints upto end of the input.

# sed -n ‘N,/PATTERN/p’ filename

For example, 4th line matches the pattern “Security”, so it prints from 3rd line to 4th line.

# sed -n '3,/Security/p' thegeekstuff.txt

3. Hardware

4. Security (Firewall, Network, Online Security etc)

Sed Pattern Format 4: /PATTERN/,$

It prints from the line matches the given pattern to end of file.

# sed -n ‘/PATTERN/,$p’ filename

# sed -n '/Website/,$p' thegeekstuff.txt

8. Website Design

9. Software Development

10.Windows- Sysadmin, reboot etc.

Sed Pattern Format 5: /PATTERN/,+N

It prints the lines which matches the pattern and next N lines following the matched line.

# sed -n ‘/PATTERN/,+Np’ filename

For example, following prints the 5th line which matches the pattern /Storage/ and next two lines following /Storage/.

# sed -n '/Storage/,+2p' thegeekstuff.txt

5. Storage

6. Cool gadgets and websites

7. Productivity (Too many technologies to explore)

Sed Pattern Format 6: /PATTERN/,/PATTERN/

Prints the section of file between two regular expression (including the matched line ).

# sed -n ‘/P1/,/P2/p’ filename

For example, 5th line matches “Storage” and 8th line matches “Design”, so it prints 5th to 8th.

# sed -n '/Storage/,/Design/p' thegeekstuff.txt

5. Storage

6. Cool gadgets and websites

7. Productivity (Too many technologies to explore)

8. Website Design

Delete File Lines Using Address and pattern in Sed

“p” command prints the buffer (Remember to use “-n” option with “p”)

“d” command is just opposite, it’s for deletion. “d” will delete the pattern space buffer and immediately starts the next cycle.

Syntax:

# sed 'ADDRESS'd filename

# sed /PATTERN/d filename

Delete the Nth line.

“Nd” delete the Nth line and prints the other lines.

For example, 3d deletes 3^rd line and prints other lines.

$sed 3d sample.txt

Delete Starting from 3^rd line and every 2^nd line from there.

$sed ‘3~2d’ sample.txt

Delete from 4ht to 8^th line from file

$sed ‘4,8d’ sample.txt

Delete the last line from input

$sed ‘$d’ sample.txt

Delete the line which matches the given pattern from input

$sed /Sysadmin/d sample.txt

Delete the line from which matches the given pattern to end of the file

$sed ‘/Website/ ,$d’ sample.txt

Delete the line from which matches the given pattern and 2 lines next to that

$sed ‘Storage/ ,+2d’ sample.txt

Delete blank line from a file using sed.

$sed ‘/^$/d’ sample.txt

Can any one help on these questions which i couldnt answer in my interview,because iam not sure on perl scripting iam learning the things in perl now.

1.What standard Perl modules have you made use of, and why did you use them instead of the equivalent built-in functions?

2.Write a statement that checks to make sure there are at least two command line arguments and exits with an error if not.

3.How do you find the number of elements in an array?

4.Write a regular expression that would match only the last director of a full path filename (and you don't know the path ahead of time).  Example: for "/fee/fi/fo/fum.txt" you would match "fo" with your regular expression

5.Write a Perl script to get the machine host name and print it. No system command should be used; this script should work on Windows and Linux OS without any change.
6.Write a Perl script to check a directory exists or not and print proper message. The directory location will be passed as parameter while executing the script.

Please help me out for the above questions.