August 2012 | Linux Blog

Perl Script: How to find if variable is defined or not?

Below is a simple perl script which explains how to find if any given variable is defined or not.

Feel free to copy and use this code.

Source
: cat undefined.pl
#!/usr/bin/perl

use strict;
use warnings;

my $Undefined;
my $Defined = "linuxpoison";

print "The value of Undefined is ", defined $Undefined, "\n";
print "The value of Defined is ", defined $Defined, "\n";

print "------------------------------\n";

$Undefined = $Defined;
$Defined = undef;

print "The value of Undefined is ", defined $Undefined, "\n";
print "The value of Defined is ", defined $Defined, "\n";

Output: perl undefined.pl
The value of Undefined is
The value of Defined is 1
------------------------------
The value of Undefined is 1
The value of Defined is

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Bash Script: How to read password without dispalying on the terminal

Below is the simple script which reads the user's password without displaying on the terminal.

Feel free to copy and use this code.

Source: cat passwd.sh
#!/bin/bash

# Here the input passwd string will get display on the terminal
echo -n "Enter your passwd: "
read passwd
echo
echo "Ok, your passwd is: $passwd"
echo

# Now, lets turn off the display of the input field.
stty -echo
echo -n "Again, please enter your passwd: "
read passwd
echo
echo "Ok, your passwd is: $passwd"
echo
stty echo

Output: ./passwd.sh
Enter your passwd: linuxpoison

Ok, your passwd is: linuxpoison

Again, please enter your passwd:
Ok, your passwd is: linuxpoison.blog@gmail.com

As you can see above in the scrip, with the use of stty we can enable or disable the screen echo.

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Bash Script: Calculate the total time taken by script execution

There is a bash predefined parameter SECONDS

Each time this parameter is referenced, the number of seconds since shell invocation is returned. If a value is assigned to SECONDS, the value returned upon subsequent references is the number of seconds since the assignment plus the value assigned. If SECONDS is unset, it loses its special properties, even if it is subsequently reset.

Below is simple bash script which demonstrate the usage of SECOND parameter

Source: cat time_taken.sh
#!/bin/bash

for i in {1..10}; do
sleep 1
echo $i
done

echo "Total time taken by this script: $SECONDS"

Output: ./time_taken.sh
1
2
3
4
5
6
7
8
9
10
Total time taken by this script: 10

Bash Script: Execute loop in background withing a script

Below is simple bash script which shows the way to make any loop execute in background, with this approach one can make the script to perform multiple things at the same time .... somewhat similar to multi threading.

In the example below, the '&' at the end of the first loop which makes this loop to go into background and at the same time second loop also gets started which makes both the loop to operate simultaneously.

Source: cat background-loop.sh
#!/bin/bash

for i in 1 2 3 4 5 6 7 8 9 10; do
echo -n $i
done &

for i in a b c d e f g h i j k; do
echo -n $i
done
echo

Output: ./background-loop.sh
abcdef1gh23ij4k5

Practical example: you can use above approach to copy some set of files to some other server or storage (backup) and at the same time you can perform the ftp job to copy other set of files to ftp server.

Bash Script: How read file line by line (best and worst way)

There are many-many way to read file in bash script, look at the first section where I used while loop along with pipe (|) (cat $FILE | while read line; do ... ) and also incremented the value of (i) inside the loop and at the end I am getting the wrong value of i, the main reason is that the usage of pipe (|) will create a new sub-shell to read the file and any operation you do withing this while loop (example - i++) will get lost when this sub-shell finishes the operation.

In second and the worst method, One of the most common errors when reading a file line by line is by using a for loop (for fileline in $(cat $FILE);do ..), which prints the every word in a file on separate line, because, for loop uses the default value of IFS (space).

In third perfect method, The while loop (while read line;do .... done < $FILE) is the most appropriate and easiest way to read a file line by line, see the example below.

Input: $ cat sample.txt
This is sample file
This is normal text file

Source: $ cat readfile.sh
#!/bin/bash

i=1;
FILE=sample.txt

# Wrong way to read the file.

# This may cause problem, check the value of 'i' at the end of the loop

echo "###############################"
cat $FILE | while read line; do
echo "Line # $i: $line"
((i++))
done
echo "Total number of lines in file: $i"

# The worst way to read file.

echo "###############################"
for fileline in $(cat $FILE);do
echo $fileline
done

# This is correct way to read file.

echo "################################"
k=1
while read line;do
echo "Line # $k: $line"
((k++))
done < $FILE
echo "Total number of lines in file: $k"

Bash Script: Using IFS to split the strings into tokens

IFS (Internal Field Separator) is one of Bash's internal variables. It determines how Bash recognizes fields, or word boundaries, when it interprets character strings.

$IFS defaults to whitespace (space, tab, and newline), but can be changed, below example show you how to change the IFS value to split the strings into tokens based on any delimiter, in our case we are using ':' to split the string into tokens.

Source: cat ifs.sh
#!/bin/bash

var="google:yahoo:microsoft:apple:oracle:hp:dell:toshiba:sun:redhat"
echo "Original value of IFS is: $IFS"

# Saving the original value of IFS
OLD_IFS=$IFS
IFS=:
echo "New value of IFS is: $IFS"
echo "================="

for word in $var;do
echo -e $word
done <<< $var

echo "================"

# Restore the value of IFS back to the script.
IFS=$OLD_IFS
echo "Back to original value of IFS: $IFS"

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Bash Script: String manipulation (Find & Cut)

${parameter#word}
${parameter##word}
Remove matching prefix pattern. If the pattern matches the beginning of the value of parameter, then the result of the expansion is the expanded value of parameter with the shortest matching pattern (the ``#'' case) or the longest matching pattern (the ``##'' case) deleted.

${parameter%word}
${parameter%%word}
Remove matching suffix pattern. If the pattern matches a trailing portion of the expanded value of parameter, then the result of the expansion is the expanded value of parameter with the shortest matching pattern (the ``%'' case) or the longest matching pattern (the ``%%'' case) deleted.

Below bash script explains the concepts of find and cut the string within the string, feel free to copy and use this script.

Source: cat string_cut.sh
#!/bin/bash

var="Linuxpoison.Linuxpoison.Linuxpoison"
echo
echo -e "Value of var is: $var"
echo "--------------------------------------------"
echo 'Find all *nux from the start of the string and cut ${var##*nux} :'
echo ${var##*nux}

echo "-------------------------------------------"
echo 'Find the first *nux from the start of the string and cut ${var#*nux} :'
echo ${var#*nux}

echo "-------------------------------------------"
echo 'Find all .* from the back of the string and cut ${var%%.*} :'
echo ${var%%.*}

echo "------------------------------------------"
echo 'Find first .* from the back of the string and cut ${var%.*} :'
echo ${var%.*}

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NTM (Network Traffic Monitor) is a monitor of the network and internet traffic for Linux. NTM is useful for the people that have a internet plan with a limit, and moreover the exceed traffic is expensive.

NTM (Network Traffic Monitor) features:
* Choice of the interface to monitoring.
* Period to monitoring: Day, Week, Month, Year or Custom Days. With auto-update.
* Threshold: Auto-disconnection if a limit is reached (by Network Manager).
* Traffic Monitoring: Inbound, outbound and total traffic; Show the traffic speed.
* Time Monitoring: Total time of connections in the period.
* Time Slot Monitoring: Number of sessions used.
* Reports: Show of average values and daily traffic of a configurable period.
* Online checking with Network-manager or by "Ping Mode".
* The traffic is attributed to the day when the session began.
* Not need root privilege.
* Not invasive, use a system try icon.

NTM is write in python and is a open source software, the license is the GNU GPL v2.

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* Free (as in freedom , also free of charge ) and Open Source.
* Simple , easy-to-use and lightweight.
* Support resume download , so if your connection lost you don’t need to start from first.
* Classify downloads , and every category has independent configuration and queue.
* Queue download.
* Integrate with Firefox through Flashgot plugin.
* Monitoring clipboard.
* Import downloads from HTML file.
* Batch download , you can download many files has same arrange , like file_1 file_2 …. file_20 , you don’t need to add all links , just one link and changeable character.
* Can be used from command line.

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